Astronomy

Could the Moon sit on the Earth?

Could the Moon sit on the Earth?

Let say that the Earth's Moon was knocked out of orbit and landed on the Earth with some damage but not world ending. Would Earth's atmosphere envelop to include the Moon? Would the moon sink into the Earth? What would happen?


This is a highly improbable scenario, since the Moon has a lot of momentum. However, let's say you teleport the Moon onto the surface of the Earth. Then Moon can't just sit on the Earth, and it will collapse under its own weight, probably killing everyone on Earth. The two would probably merge into a spherical shape (as most massive objects do).


It can't happen. The moon is quite big and can't get knocked out of orbit easily.

If it were somehow knocked out of orbit and collided with the Earth, the energy released would melt the entire crust. Huge amounts of matter would be knocked into space. After everything had settled down, you would get a lifeless world, still spherical but now slightly larger.

The moon can't get knocked out of orbit. If it did, It can't "land". If it did "land" it would be crushed by its own weight, and all life would end in one way or another.


At the scale of the Earth or the Moon there are no solid objects. The Earth does not behave like a solid, nor does the Moon.

You have to think of these objects as more like piles of sand, made round by their own gravity. So if one round pile of sand touches another round pile of sand, they just merge and make a bigger pile of sand, also round.

the Earth's Moon was knocked out of orbit and landed on the Earth with some damage but not world ending

That's essentially impossible. I can't think of any mechanism that would allow the Moon to gently touch the Earth. Regardless, both the Earth and the Moon would be severely bent out of shape before they even begin to touch - they would look more like eggs, or a teardrop shape, then they merge, and that's the end of everyone and everything on this planet.


Could the Moon sit on the Earth? - Astronomy

At the North Pole during summer when there is permanent daylight, does the Moon rise and set, and if not, does that mean that during winter with no sunrise the Moon is always up?

The Moon does rise and and set during both summer and winter on the North Pole (or South Pole). The exact movement is complicated, but can be understood the combination of two separate movements:

1) Rotation of the Earth on its axis, which results in movements that change over the course of one day.

2) Orbit of the Moon around the earth, which results in movements that change over the course of one lunar month (about 29 days).

While the Moon does rise during the summer at the North Pole, since the Sun is always up, you generally can't see it, so I'll focus on the movement of the Moon during the winter.

The daily movement from Earth's rotation causes the Moon to circle once around the sky. If you spent the entire day staring at it, you'd have to turn around exactly once. This movement is also the same that the Sun makes during the summer. To give you a better idea of how this looks, here is a video showing how the Sun moves in the sky at the North Pole: Arctic Midnight Sun

The second movement caused by the Moon's orbit around the Earth is analogous to the movement of the Sun over the course of a year only it repeats over the course of a lunar month. Near the new Moon phase, the Moon is near the Sun and therefore never rises during the winter. As the Moon approaches full, it will start to pop up above the horizon. Eventually near the full Moon phase it will be high enough in the sky to stay up all day and circle like the Sun in the video above. The elevation of the circle will rise as the Moon becomes completely full and then start to decrease until it begins to dip below the horizon. Eventually the Moon will stop rising at all as it gets close enough to the new phase. The cycle then repeats.

This page was last updated on July 18, 2015.

About the Author

Laura Spitler

Laura Spitler was a graduate student working with Prof. Jim Cordes. After graduating in 2013, she went on to a postdoctoral fellowship at the Max Planck Institute in Bonn, Germany. She works on a range of projects involving the time variability of radio sources, including pulsars, binary white dwarfs and ETI. In particular she is interested in building digital instruments and developing signal processing techniques that allow one to more easily identify and classify transient sources.


If We Had No Moon

The Earth has a large moon, making it unique in the inner solar system. Mercury and Venus have no moons, and Mars has only two small asteroid-sized objects orbiting it. In this essay, the father of the SMART-1 lunar mission, Bernard Foing of the European Space Agency, looks at the effect the Moon has had on the Earth, and explores how different our world would be if we had no planetary companion. Would life have evolved differently, or even appeared on Earth without the Moon?

If We Had No Moon

An essay by Bernard Foing

If the time of Earth&rsquos existence was condensed into a 24-hour clock, the moon formation event occurred just 10 minutes after the Earth was born. The Earth formed 4.56 billion years ago, and the Moon formed about 30 million years later. At that time, the Earth was a magma ocean. An impactor about the size of Mars struck the Earth at an oblique angle, and removed some of the magmatic mantle. This mantle was put in orbit around the Earth, together with some of the debris from the impactor itself, and this material eventually formed the Moon.

Artist&rsquos representation of the moon formation event. Copyright Fahad Sulehria, 2005, www.novacelestia.com

When the Moon first formed, it was very close to the Earth. It was possibly only 20 to 30 thousands of kilometers away, and it would have looked extremely large in the sky, at least 20 to 10 times bigger. But there were no living creatures on the Earth at that time to witness this beautiful scene.

The tidal effect of a body increases as a cube of the distance, so the effect of the Moon&rsquos tidal forcing on the Earth was extremely high at this time, to the point that the early magma ocean was affected. This provided some additional energy to the heating from radioactive elements present, but after the radiogenic heating decayed, the Moon still was a source of heating that may have had some geological effect, keeping the Earth&rsquos magma hot and perhaps forcing additional convection in the Earth&rsquos mantle.

After the Earth started to cool, the first crust started to float on top of the magma. During this period the Earth was subjected to increased meteor bombardment. The bombardment had been very intense at the beginning of the solar system and then had started to decline, but about 500 million years after the birth of the Earth, or about 2 hours and 40 minutes into our clock of 24 hours, there was a burst of impactors. This lasted for about hundred million years, and we call this &ldquothe late heavy bombardment.&rdquo Many of the large basins on the Moon are evidence of this late heavy bombardment period. In this way, the Moon is a history book for the inner solar system and the Earth. We have studied these basins with the SMART-1 mission.

The Moon&rsquos heavily cratered surface is evidence of the many meteorite impacts that occurred in the inner solar system during the late heavy bombardment period.
Credit: ESA

The Earth was hit more often than the Moon, however, because Earth is larger and has more gravity. This increased gravity also caused the impactors to be accelerated to higher velocities towards the Earth. That must have been a catastrophic time to be here. So many bombardments would have sterilized the planet. If life had appeared before this period, it would have been extinguished unless it found a way to retreat into niches where it could be protected from these global catastrophes.

When some of these impactors hit the Earth, the explosion caused rocks and dirt from Earth to shoot up and away from our planet. Some of that projected material flew all over the solar system, and some of it landed on the Moon. There could be a few hundred kilograms of Earth material per square kilometer of the Moon&rsquos surface, buried under a few meters of lunar soil. It would be interesting to retrieve those rocks and bring back samples of the early Earth. Almost nothing from this time period has survived on the Earth because of tectonic recycling of the crust plates or because of atmospheric weathering. We would try to detect some organics within those rocks, and that could tell us about the history of organic chemistry on Earth. Some of these rocks could even have preserved fossils of life. Such rocks could help us look further back into the fossil record, which now stops at 3.5 billion years ago. This way, we could possibly learn about the emergence of life on Earth.

By exploring the Moon, we also can get clues on how the Earth has evolved. We can study processes on the Moon that have also shaped the Earth, like volcanism and tectonics. Because the Moon is smaller than the Earth, the Moon&rsquos radiogenic heating dissipated much faster. After about one billion years, the interior of the Moon didn&rsquot evolve much, and surface changes mostly were due to impacts. There was a brief period of magmatic activity from the subsurface — a few plumes of magma made their way up to the surface and filled newly formed impact basins with basalt, creating what we call the Maria. This happened up to about 2 billion years ago. Because the Moon offers different conditions than the Earth, we can better understand how physical processes work generally by studying a larger range of parameters than just the Earth&rsquos.

During its flight, the Galileo spacecraft returned images of the Earth and the Moon. The separate images were combined to generate this view.
Credit: NASA

The Moon affects the liquid envelope of the Earth, and the oceanic tides in particular. The Moon affects the ocean tides more in some areas than others. For instance, in the channel between the British Isles and the European continent, the tidal range can be 10 meters, compared to what you see in the Pacific, where it is below a meter.

The crust of the Earth is also affected. The Moon&rsquos tidal forcing causes significant heating and dissipation of energy to take place. Part of this energy is heating the Earth, and part of it is dissipated by forcing the Moon to recede from the Earth over time. There are people who propose that the tidal effect of the Moon may have helped trigger the convection on the Earth that led to the multi-plate tectonics. The other planets don&rsquot have the same tectonic cycle. For most of them, the crust is like a lid that doesn&rsquot move much horizontally, and the magma and heat are blocked by this lid on the surface. The Earth instead has rolling convective motion that drags the crust, and then the crust plunges back down into the mantle and gets recycled.

There are some very subtle effects of the Moon in the climate and the oceans. One pattern that has been found recently is related to the Pacific Ocean&rsquos El Niño phenomenon. You have a cold undersea current coming from the Antarctic sea, and that creates the Humboldt stream which keeps the sea around the South American coast near Peru and Chile quite cold. Because of this, there are fewer clouds and less precipitation there. Sometimes this current drifts away from the coast, and then you have much more cloud formation and a period of very bad weather over South America. Satellites have monitored this stream over the Pacific Ocean and they have found some streams which were not known before. They can connect some of these streams with how the Moon&rsquos tidal effect influences the mixing of the deep ocean. There was a French-American mission called TOPEX/Poseidon that accurately measured the altitude of the sea and detected a little stream a few centimeters high. That doesn&rsquot seem like a lot, but over the whole area of the Pacific Ocean it represents a huge amount of water transferred from one place to another.

Map showing tidal variations across the globe. Red areas represent large variations in water level, purple areas represent zero or very low tidal variation. Click image for larger view.
Image Credit: Legos/CNRS.

If you would take away the Moon suddenly, it would change the global altitude of the ocean. Right now there is a distortion which is elongated around the equator, so if we didn&rsquot have this effect, suddenly a lot of water would be redistributed toward the polar regions.

The Moon has been a stabilizing factor for the axis of rotation of the Earth. If you look at Mars, for instance, that planet has wobbled quite dramatically on its axis over time due to the gravitational influence of all the other planets in the solar system. Because of this obliquity change, the ice that is now at the poles on Mars would sometimes drift to the equator. But the Earth&rsquos moon has helped stabilize our planet so that its axis of rotation stays in the same direction. For this reason, we had much less climatic change than if the Earth had been alone. And this has changed the way life evolved on Earth, allowing for the emergence of more complex multi-cellular organisms compared to a planet where drastic climatic change would allow only small, robust organisms to survive.

The Moon has influenced biology in other ways as well. For species living near the coast, the tide is an important factor. When you look at the shorelines, you can recognize different layers of organisms that have adapted to the salt water conditions based on the ebb and flow of the tide.

The eyesight of many mammals is sensitive to moonlight. The level of adaptation of night vision would be very different without the Moon. Many of these species have evolved in such a way that their night vision could work in even partial lunar illumination, because that&rsquos when they are most active. But they can be more subjected to predators, too, so there is a balance between your ability to see and your ability not to be seen. The Moon has completely changed evolution in that aspect.

The various phases of the Moon. As the Moon orbits the Earth, the amount of sunlight reflecting off the lunar surface changes its appearance. When the Earth is between the Sun and the Moon, we see a full moon when the Moon is between the Sun and the Earth, we see a new moon. Click image for larger view.

Human vision is so sensitive that we are almost able to work by the light of the Milky Way. The full Moon has more light than we need to see at night. For most of our history, we were hunting and fishing or doing agriculture, and we organized our lives by using the Moon. It determined the time for hunting, or the time where we could harvest. That&rsquos why most of our calendars are based on the Moon.

In a recent workshop called &ldquoEarth-Moon Relationships,&rdquo psychologists discussed the relation between the lunar phases and several aspects of life. There was a very interesting correlation, not with the birth of children, but with the time of conception. Perhaps that is due to some social or sentimental value of the Moon. We tend to forget the impact the Moon has on our lives because we use electric lights, but for most of our history we had to adapt our behavior to the lunar phases.

Finally, the Moon had a key role in the emergence of science, and in our understanding of our place in the universe. We saw the repetition of the phenomena of lunar phases, and we observed solar and lunar eclipses. These were big challenges to our understanding of nature, and a few astronomers were put to death because they weren&rsquot able to predict the eclipses. This challenged us to develop accurate predictions for the motion of the sun and the motion of the Moon.

Studying the Moon helped us determine distances in the solar system and the size of celestial objects. By studying lunar phases, for example, people were able to determine how far the Moon is from the Earth, the size of the Earth, and our distance from the sun. More recently, the Moon was the terrain where the space race took place between two political systems, allowing for great technical and scientific achievements. The Moon has inspired humankind to learn how to travel to space, and to bring life beyond Earth&rsquos cradle.


2. Moon Rabbit (China/Korea/Japan)

This is an interesting myth because it crosses across several different cultures. The moon rabbit or jade rabbit is said to be one of the companions that Chang&apose eventually was allowed to have with her on the moon. However, it is also a symbol that shows up in myths about the moon in Korea and in Japan.

A statue of Luna, the Roman equivalent of Selene, holding a torch with the crescent symbol of the moon on her forehead.


Contents

A major lunar standstill occurs when the Moon's declination reaches a maximum monthly limit, stopping at 28.725° north or south. An eclipse season near the March equinox has solar and lunar eclipses at an odd-numbered saros, while another eclipse season near the September equinox has solar and lunar eclipses at an even-numbered saros.

A minor lunar standstill occurs when the Moon's declination reaches a minimum monthly limit, stopping at 18.134° north or south. An eclipse season near the March equinox has solar and lunar eclipses at an even-numbered saros, while another eclipse season near the September equinox has solar and lunar eclipses at an odd-numbered saros.

The term lunar standstill was apparently first used by archeologist Alexander Thom in his 1971 book Megalithic Lunar Observatories. [1] The term solstice, which derives from the Latin solstitium: sol- (sun) + -stitium (a stoppage), describes the similar extremes in the Sun's varying declination. Neither the Sun nor the Moon stands still, obviously what stops, momentarily, is the change in declination. The word tropic, as in Tropic of Capricorn, comes from ancient Greek meaning "to turn", referring to how ascending (or descending) motion turns to descending (or ascending) motion at the solstice. [2]

As Earth rotates on its axis, the stars in the night sky appear to follow circular paths around the celestial poles. (This daily cycle of apparent movement is called diurnal motion.) All the stars seem fixed on a celestial sphere surrounding the observer. In the same way that positions on Earth are measured using latitude and longitude, the apparent places of stars on this sphere are measured in right ascension (equivalent to longitude) and declination (equivalent to latitude). If viewed from a latitude of 50° N on Earth, any star with a declination of +50° would pass directly overhead (reaching the zenith at upper culmination) once every sidereal day (23 hours, 56 minutes, 4 seconds), whether visible at night or obscured in daylight.

Unlike the stars, the Sun and Moon do not have a fixed declination. Since Earth's rotational axis is tilted about 23.5° with respect to a line perpendicular to its orbital plane (the ecliptic), the Sun's declination ranges from +23.5° on the June solstice to −23.5° on the December solstice, as Earth orbits the Sun once every tropical year. Therefore in June, in the Northern Hemisphere, the midday Sun is higher in the sky, and daytime then is longer than in December. In the Southern Hemisphere, the situation is reversed. This obliquity causes Earth's seasons.

The Moon's declination also changes, completing a cycle once every lunar nodal period: 27.212 days. Thus, lunar declination ranges from a positive value to a negative one in just under two weeks, and back. Consequently in under a month, the Moon's altitude at upper culmination (when it contacts the observer's meridian) can shift from higher in the sky to lower above the horizon, and back.

The Moon differs from most natural satellites around other planets in that it remains near the ecliptic (the plane of Earth's orbit around the Sun) instead of Earth's equatorial plane. The Moon's maximum and minimum declination vary because the plane of the Moon's orbit around Earth is inclined about 5.14° with respect to the ecliptic plane, and the spatial direction of the Moon's orbital inclination gradually changes over an 18.6-year cycle, alternately adding to or subtracting from the 23.5° tilt of Earth's axis.

Therefore, the maximum declination of the Moon varies roughly from (23.5° − 5° =) 18.5° to (23.5° + 5° =) 28.5°. At the minor lunar standstill, the Moon will change its declination during the nodal period from +18.5° to −18.5°, for a total range of 37°. Then 9.3 years later, during the major lunar standstill, the Moon will change its declination during the nodal period from +28.5° to −28.5°, which totals 57° in range. This range is enough to bring the Moon's altitude at culmination from high in the sky to low above the horizon in just two weeks (half an orbit).

Strictly speaking, the lunar standstill is a moving position in space relative to the direction of Earth's axis and to the rotation of the Moon's orbital nodes (lunar nodal precession) once every 18.6 years. The standstill position does not persist over the two weeks that the Moon takes to move from its maximum (positive) declination to its minimum (negative) declination, and it most likely will not exactly coincide with either extreme.

However, because the 18.6-year cycle of standstills is so much longer than the Moon's orbital period (about 27.3 days), the change in the declination range over periods as short as half an orbit is very small. The period of the lunar nodes precessing in space is slightly shorter than the lunar standstill interval due to Earth's axial precession, altering Earth's axial tilt over a very long period relative to the direction of lunar nodal precession. Simply, the standstill cycle results from the combination of the two inclinations.

The azimuth (horizontal direction) of moonrise and moonset varies according to the Moon's nodal period of 27.212 days, while the azimuth variation during each nodal period varies with the lunar standstill period (18.613 years).

For a latitude of 55° north or 55° south on Earth, the following table shows moonrise and moonset azimuths for the Moon's narrowest and widest arc paths across the sky. The azimuths are given in degrees from true north and apply when the horizon is unobstructed. Figures for a time midway between major and minor standstill are also given.

The arc path of the full Moon generally reaches its widest in midwinter and its narrowest in midsummer. The arc path of the new Moon generally reaches its widest in midsummer and its narrowest in midwinter. The arc path of the first quarter moon generally reaches its widest in midspring and its narrowest in midautumn. The arc path of the last quarter moon generally reaches its widest in midautumn and its narrowest in midspring.

Azimuth of full Moon on horizon
(as viewed from 55° north)
narrowest arc widest arc
epoch moonrise moonset moonrise moonset
minor standstill 124° 236° 56° 304°
midway 135° 225° 45° 315°
major standstill 148° 212° 32° 328°
Azimuth of full Moon on horizon
(as viewed from 55° south)
widest arc narrowest arc
epoch moonrise moonset moonrise moonset
minor standstill 124° 236° 56° 304°
midway 135° 225° 45° 315°
major standstill 148° 212° 32° 328°

For observers at the middle latitudes (not too near the Equator or either pole), the Moon is highest in the sky in each period of 24 hours when it reaches the observer's meridian. During the month, these culmination altitudes vary so as to produce a greatest value and a least value. The following table shows these altitudes at different times in the lunar nodal period for an observer at 55° north or 55° south. The greatest and least culminations occur about two weeks apart.

Altitude at culmination
(as viewed from 55° north)
epoch greatest least
minor standstill 53.5° 16.5°
midway 58.5° 11.5°
major standstill 63.5° 6.5°
Altitude at culmination
(as viewed from 55° south)
epoch greatest least
minor standstill 53.5° 16.5°
midway 58.5° 11.5°
major standstill 63.5° 6.5°

The following table shows some occasions of a lunar standstill. The times given are for when the Moon's node passed the equinox—the Moon's greatest declination occurs within a few months of these times, depending on its detailed orbit. [3] [4] However, the phenomenon is observable for a year or so on either side of these dates. [1]

Times of lunar standstill
major standstill minor standstill
May 1988 February 1997
June 2006 October 2015
April 2025 March 2034 [5]
September 2043 [5] March 2053 [5]

A more detailed explanation is best considered in terms of the paths of the Sun and Moon on the celestial sphere, as shown in the first diagram. This shows the abstract sphere surrounding the Earth at the center. The Earth is oriented so that its axis is vertical.

The Sun is, by definition, always seen on the ecliptic (the Sun's apparent path across the sky) while Earth is tilted at an angle of e = 23.5° to the plane of that path and completes one orbit around the Sun in 365.25636 days, slightly longer than one year due to precession altering the direction of Earth's inclination.

The Moon's orbit around Earth (shown dotted) is inclined at an angle of i = 5.14° relative to the ecliptic. The Moon completes one orbit around the Earth in 27.32166 days. The two points at which Moon crosses the ecliptic are known as its orbital nodes, shown as "N1" and "N2" (ascending node and descending node, respectively), and the line connecting them is known as the line of nodes. Due to precession of the Moon's orbital inclination, these crossing points, the nodes and the positions of eclipses, gradually shift around the ecliptic in a period of 18.59992 years.

Looking at the diagram, note that when the Moon's line of nodes (N1 & N2) rotates a little more than shown, and aligns with Earth's equator, (from front to back, N1, Earth, and N2, seem to be the same dot), the Moon's orbit will reach its steepest angle with the Earth's equator, and in 9.3 years (from front to back, N2, Earth, N1 seem to be the same dot) it will be the shallowest: the 5.14° declination (tilt) of the Moon's orbit either adds to (major standstill) or subtracts from (minor standstill) the inclination of earth's rotation axis (23.439°).

The effect of this on the declination of the Moon is shown in the second diagram. During the course of the nodal period, as the Moon orbits the Earth, its declination swings from –m° to +m°, where m is a number in the range (ei) ≤ m ≤ (e + i). At a minor standstill (e.g., in 2015), its declination during the month varies from –(ei) = –18.5° to +(ei) = 18.5°. During a major standstill (e.g., in 2005-2006), the declination of the Moon varied during each month from about –(e + i) = –28.5° to +(e + i) = 28.5°.

However, an additional subtlety further complicates the picture. The Sun's gravitational attraction on the Moon pulls it toward the plane of the ecliptic, causing a slight wobble of about 9 arcmin within a 6-month period. In 2006, the effect of this was that, although the 18.6-year maximum occurred in June, the maximum declination of the Moon was not in June but in September, as shown in the third diagram.

Because the Moon is relatively close to the Earth, lunar parallax alters declination up to 0.95° when observed from Earth's surface versus geocentric declination, the declination of the Moon from the center of the Earth. Geocentric declination may be up to about 0.95° different from the observed declination. The amount of this parallax varies with latitude, hence the observed maximum of each standstill cycle varies according to position of observation.

Atmospheric refraction – the bending of the light from the Moon as it passes through the Earth's atmosphere – alters the observed declination of the Moon, more so at low elevation, where the atmosphere is thicker (deeper).

Not all the maxima are observable from all places in the world – the Moon may be below the horizon at a particular observing site during the maximum, and by the time it rises, it may have a lower declination than an observable maximum at some other date.

Events in Sydney, Australia Date/time RA Dec Az. Elev Lunar phase
Closest viewing of "true maximum" on Friday, 15 September during civil twilight Thursday, 14 September 19:53 04:42:57.32 +29:29:22.9 27° 46% waning
Highest visible maximum during civil twilight Tuesday, 4 April 07:49 06:03:11.66 +29:30:34.5 350° 26° 38% waxing
Highest visible maximum during darkness Tuesday, 4 April 09:10 06:05:22.02 +29:27:29.4 332° 21° 39% waxing
Lowest visible minimum during civil twilight Wednesday, 22 March 19:45 18:10:57.40 −28:37:33.2 41° 83° 50% waning
Lowest visible minimum during darkness Wednesday, 22 March 18:36 18:09:01.55 −28:36:29.7 80° 71° 50% waning
Events in London, England Date/time RA Dec Az. Elev Lunar phase
Highest visible maximum during civil twilight Friday, 15 September 05:30 06:07:12.72 +28:19:52.6 150° 64° 42% waning
Highest visible maximum during darkness Tuesday, 7 March 19:43 05:52:33.05 +28:18:26.9 207° 64° 60% waxing
Lowest visible minimum during civil twilight Friday, 29 September 17:44 17:49:08.71 −29:31:34.4 186° 43% waxing
Lowest visible minimum during darkness Saturday, 2 September 20:50 18:15:08.74 −29:25:44.0 198° 70% waxing

Note that all dates and times in this section, and in the table, are in UTC, all celestial positions are in topocentric apparent coordinates, including the effects of parallax and refraction, and the lunar phase is shown as the fraction of the Moon's disc which is illuminated.

In 2006, the minimum lunar declination, as seen from the centre of the Earth, was at 16:54 UTC on 22 March, when the Moon reached an apparent declination of −28:43:23.3. The next two best contenders were 20:33 on 29 September, at a declination of −28:42:38.3 and 13:12 on 2 September at declination −28:42:16.0.

The maximum lunar declination, as seen from the centre of the Earth, was at 01:26 on 15 September, when the declination reached +28:43:21.6. The next highest was at 07:36 on 4 April, when it reached +28:42:53.9

However, these dates and times do not represent the maxima and minima for observers on the Earth's surface.

For example, after taking refraction and parallax into account, the observed maximum on 15 September in Sydney, Australia was several hours earlier, and then occurred in daylight. The table shows the major standstills that were actually visible (i.e. not in full daylight, and with the Moon above the horizon) from both London, UK, and Sydney, Australia.

For other places on the Earth's surface, positions of the Moon can be calculated using the JPL ephemeris calculator. During a major lunar standstill, the moon was on the 29th parallel because eclipses of odd numbered saros occurred near March Equinox and even numbered saros occurring near September Equinox. During a minor lunar standstill, the moon was on the 18th parallel because eclipses of even numbered saros occurred near March Equinox and odd numbered saros occurred near September Equinox.


Galileo's Work Today

I am a member of the Spring 1995 History 333 ("Galileo in Context") class at Rice University. Our assignment was to study Galileo's work in a particular field and then recreate his work as closely as possible. The Astronomy Group built Galilean telescopes and used them to study the same heavenly bodies that Galileo did 375 years ago.

Our group experienced many of the exciting discoveries and frustrating problems that Galileo himself may have encountered. We used modern materials to make replicas of Galileo's telescopes (click here for more information on our Galilean telescopes). One fundamental difference between our telescopes and Galileo's is the fact that ours had only about 9x magnification while Galileo used telescopes with magnifications of up to 20 to study the moon. Our attempt to rigorously recreate Galileo's work was also thwarted by the uncooporative Houston spring weather. The skies were clear for only about five nights during the semester, making it impossible to observe all the phases of the moon as Galileo did.

During my first attempt at observing the moon, I encountered problems, both expected and unforeseen. I was initially frustrated in trying to find the moon due to the small field of view the telescope. The best technique, I found, was to search until a bright light (the light of the moon) was visible on the interior surface of the tube. By moving the telescope so that this light moved down the length of the tube towards my eye, I was able to bring the moon into view. After securing the mounting so that the telescope was fixed on the moon, I was ready to begin viewing.

Before my work on this project, I had had very little experience observing the heavens with a telescope. Therefore, on my first night of observing, I felt much of the same amazement and excitement that Galileo must have felt when he first saw the detail of the moon that is invisible to the naked eye.

This is my drawing of the moon as seen through a Galilean telescope at 11pm on April 26, 1995.

The 9x magnification of our telescope brought out numerous small shadows in the lower left face of the moon, along with a distinctly rough interface between the dark and light sides of the moon. The telescope also made it possible to see the variations in the darkness of the shadows, which gave the moon the distinct three-dimensional look. Galileo's conclusions about the imperfect surface of the moon may have been revolutionary, but they are not surprising once one has looked at the moon through a telescope. Before Galileo made his 20x telescope, the strongest telescopes people had been able to make had only about 3x or 4x magnification. Presumably these telescopes were not strong enough to bring out the details of the moon as Galileo's did It was this high magnification that allowed Galileo to make his discoveries before others. Galileo's effort to examine the moon through all its phases also played a big role in enabling him to discover and interpret the changing shadows.

As I continued my observations, a number of other problems with our telescope and mounting became obvious. At least one of these problems was one with which Galileo probably had to deal. The small field diameter of our telescopes (my telescope had a calculated field diameter of 964.8 arcseconds, or 16 arcminutes) made it impossible to see the whole face of the moon at once. In the picture above, the concentric circles indicate the field of view visible at one time. The outer circle shows the total view possible with the telescope, while the inner circle roughly indicates the maximum field I was able to get in focus. The small field of view made it necessary to move the telescope around in order to see the whole moon when observing and drawing it. Many of Galileo's drawings of the moon indicate that he had to deal with the same problem. His pictures seem to have been drawn more as representations of the moon than as accurate replicas of what one sees through a telescope. For example, in the picture below, the size of the circular crater on the terminator (perhaps Albategnius) is greatly exaggerated. It is speculated that Galileo represented it this way in order to emphasize the effect of shadowing in creating a three-dimensional image.

From Galileo's Sidereus Nuncius (p 44)

Keeping the image withing the field of view also became a problem. In spite of our efforts to make the mounting as stable as possible, we could not entirely eliminate the wobble from the telescope. Any slight wind or shakiness of the viewer's hand, coupled with the tiny field of view, would move the telescope enough for the object under study to move out of the field. This effect made observing somewhat more difficult, although it had a more detrimental impact on observations of the more distant (and therefore smaller) planets than on observations of the moon.

Full moon observed and drawn March 16, 1995 by Nicole Peterson.

One final problem that future groups will find easily correctable is the height of the eyepiece of our telescope. Once I got the telescope angled at the moon, I discovered that the eyepiece was too high for my eye to reach while I was sitting, yet too low to see into while kneeling. The half-crouch position that I finally had to settle for became uncomfortable and made the observing much more difficult. A suggestion for the future is to design each person's mounting so that the eyepiece sits where it is most comfortable for that person in order to eliminate this annoying problem.


8.5 Cosmic Influences on the Evolution of Earth

In discussing Earth’s geology earlier in this chapter, we dealt only with the effects of internal forces, expressed through the processes of plate tectonics and volcanism. On the Moon, in contrast, we see primarily craters , produced by the impacts of interplanetary debris such as asteroids and comets. Why don’t we see more evidence on Earth of the kinds of impact craters that are so prominent on the Moon and other worlds?

Where Are the Craters on Earth?

It is not possible that Earth escaped being struck by the interplanetary debris that has pockmarked the Moon. From a cosmic perspective, the Moon is almost next door. Our atmosphere does make small pieces of cosmic debris burn up (which we see as meteors—commonly called shooting stars). But, the layers of our air provide no shield against the large impacts that form craters several kilometers in diameter and are common on the Moon.

In the course of its history, Earth must therefore have been impacted as heavily as the Moon. The difference is that, on Earth, these craters are destroyed by our active geology before they can accumulate. As plate tectonics constantly renews our crust, evidence of past cratering events is slowly erased. Only in the past few decades have geologists succeeded in identifying the eroded remnants of many impact craters (Figure 8.19). Even more recent is our realization that, over the history of Earth, these impacts have had an important influence on the evolution of life.

Recent Impacts

The collision of interplanetary debris with Earth is not a hypothetical idea. Evidence of relatively recent impacts can be found on our planet’s surface. One well-studied historic collision took place on June 30, 1908, near the Tunguska River in Siberia. In this desolate region, there was a remarkable explosion in the atmosphere about 8 kilometers above the surface. The shock wave flattened more than a thousand square kilometers of forest (Figure 8.20). Herds of reindeer and other animals were killed, and a man at a trading post 80 kilometers from the blast was thrown from his chair and knocked unconscious. The blast wave spread around the world, as recorded by instruments designed to measure changes in atmospheric pressure.

Despite this violence, no craters were formed by the Tunguska explosion. Shattered by atmospheric pressure, the stony projectile with a mass of approximately 10,000 tons disintegrated above our planet’s surface to create a blast equivalent to a 5-megaton nuclear bomb. Had it been smaller or more fragile, the impacting body would have dissipated its energy at high altitude and probably attracted no attention. Today, such high-altitude atmospheric explosions are monitored regularly by military surveillance systems.

If it had been larger or made of stronger material (such as metal), the Tunguska projectile would have penetrated all the way to the surface of Earth and exploded to form a crater. Instead, only the heat and shock of the atmospheric explosion reached the surface, but the devastation it left behind in Siberia bore witness to the power of such impacts. Imagine if the same rocky impactor had exploded over New York City in 1908 history books might today record it as one of the most deadly events in human history.

Tens of thousands of people witnessed directly the explosion of a smaller (20-meter) projectile over the Russian city of Chelyabinsk on an early winter morning in 2013. It exploded at a height of 21 kilometers in a burst of light brighter than the Sun, and the shockwave of the 0.5-megaton explosion broke tens of thousands of windows and sent hundreds of people to the hospital. Rock fragments (meteorites) were easily collected by people in the area after the blast because they landed on fresh snow.

Link to Learning

Dr. David Morrison, one of the original authors of this textbook, provides a nontechnical talk about the Chelyabinsk explosion, and impacts in general.

The best-known recent crater on Earth was formed about 50,000 years ago in Arizona. The projectile in this case was a lump of iron about 40 meters in diameter. Now called Meteor Crater and a major tourist attraction on the way to the Grand Canyon, the crater is about a mile across and has all the features associated with similar-size lunar impact craters (Figure 8.21). Meteor Crater is one of the few impact features on Earth that remains relatively intact some older craters are so eroded that only a trained eye can distinguish them. Nevertheless, more than 150 have been identified. (See the list of suggested online sites at the end of this chapter if you want to find out more about these other impact scars.)

Mass Extinction

The impact that produced Meteor Crater would have been dramatic indeed to any humans who witnessed it (from a safe distance) since the energy release was equivalent to a 10-megaton nuclear bomb. But such explosions are devastating only in their local areas they have no global consequences. Much larger (and rarer) impacts, however, can disturb the ecological balance of the entire planet and thus influence the course of evolution.

The best-documented large impact took place 65 million years ago, at the end of what is now called the Cretaceous period of geological history. This time in the history of life on Earth was marked by a mass extinction , in which more than half of the species on our planet died out. There are a dozen or more mass extinctions in the geological record, but this particular event (nicknamed the “great dying”) has always intrigued paleontologists because it marks the end of the dinosaur age. For tens of millions of years these great creatures had flourished and dominated. Then, they suddenly disappeared (along with many other species), and thereafter mammals began the development and diversification that ultimately led to all of us.

The object that collided with Earth at the end of the Cretaceous period struck a shallow sea in what is now the Yucatán peninsula of Mexico. Its mass must have been more than a trillion tons, determined from study of a worldwide layer of sediment deposited from the dust cloud that enveloped the planet after its impact. First identified in 1979, this sediment layer is rich in the rare metal iridium and other elements that are relatively abundant in asteroids and comets, but exceedingly rare in Earth’s crust. Even though it was diluted by the material that the explosion excavated from the surface of Earth, this cosmic component can still be identified. In addition, this layer of sediment contains many minerals characteristic of the temperatures and pressures of a gigantic explosion.

The impact that led to the extinction of dinosaurs released energy equivalent to 5 billion Hiroshima-size nuclear bombs and excavated a crater 200 kilometers across and deep enough to penetrate through Earth’s crust. This large crater, named Chicxulub for a small town near its center, has subsequently been buried in sediment, but its outlines can still be identified (Figure 8.22). The explosion that created the Chicxulub crater lifted about 100 trillion tons of dust into the atmosphere. We can determine this amount by measuring the thickness of the sediment layer that formed when this dust settled to the surface.

Such a quantity of airborne material would have blocked sunlight completely, plunging Earth into a period of cold and darkness that lasted several months. Many plants dependent on sunlight would have died, leaving plant-eating animals without a food supply. Other worldwide effects included large-scale fires (started by the hot, flying debris from the explosion) that destroyed much of the planet’s forests and grasslands, and a long period in which rainwater around the globe was acidic. It was these environmental effects, rather than the explosion itself, that were responsible for the mass extinction, including the demise of the dinosaurs.

Impacts and the Evolution of Life

It is becoming clear that many—perhaps most—mass extinctions in Earth’s long history resulted from a variety of other causes, but in the case of the dinosaur killer, the cosmic impact certainly played a critical role and may have been the “final straw” in a series of climactic disturbances that resulted in the “great dying.”

A catastrophe for one group of living things, however, may create opportunities for another group. Following each mass extinction, there is a sudden evolutionary burst as new species develop to fill the ecological niches opened by the event. Sixty-five million years ago, our ancestors, the mammals, began to thrive when so many other species died out. We are the lucky beneficiaries of this process.

Impacts by comets and asteroids represent the only mechanisms we know of that could cause truly global catastrophes and seriously influence the evolution of life all over the planet. As paleontologist Stephen Jay Gould of Harvard noted, such a perspective changes fundamentally our view of biological evolution. The central issues for the survival of a species must now include more than just its success in competing with other species and adapting to slowly changing environments, as envisioned by Darwin’s idea of natural selection. Also required is an ability to survive random global catastrophes due to impacts.

Still earlier in its history, Earth was subject to even larger impacts from the leftover debris of planet formation. We know that the Moon was struck repeatedly by objects larger than 100 kilometers in diameter—1000 times more massive than the object that wiped out most terrestrial life 65 million years ago. Earth must have experienced similar large impacts during its first 700 million years of existence. Some of them were probably violent enough to strip the planet of most its atmosphere and to boil away its oceans. Such events would sterilize the planet, destroying any life that had begun. Life may have formed and been wiped out several times before our own microbial ancestors took hold sometime about 4 billion years ago.

The fact that the oldest surviving microbes on Earth are thermophiles (adapted to very high temperatures) can also be explained by such large impacts. An impact that was just a bit too small to sterilize the planet would still have destroyed anything that lived in what we consider “normal” environments, and only the creatures adapted to high temperatures would survive. Thus, the oldest surviving terrestrial lifeforms are probably the remnants of a sort of evolutionary bottleneck caused by repeated large impacts early in the planet’s history.

Impacts in Our Future?

The impacts by asteroids and comets that have had such a major influence on life are not necessarily a thing of the past. In the full scope of planetary history, 65 million years ago was just yesterday. Earth actually orbits the Sun within a sort of cosmic shooting gallery, and although major impacts are rare, they are by no means over. Humanity could suffer the same fate as the dinosaurs, or lose a city to the much more frequent impacts like the one over Tunguska, unless we figure out a way to predict the next big impact and to protect our planet. The fact that our solar system is home to some very large planets in outer orbits may be beneficial to us the gravitational fields of those planets can be very effective at pulling in cosmic debris and shielding us from larger, more frequent impacts.

Beginning in the 1990s, a few astronomers began to analyze the cosmic impact hazard and to persuade the government to support a search for potentially hazardous asteroids. Several small but sophisticated wide-field telescopes are now used for this search, which is called the NASA Spaceguard Survey. Already we know that there are currently no asteroids on a collision course with Earth that are as big (10–15 kilometers) as the one that killed the dinosaurs. The Spaceguard Survey now concentrates on finding smaller potential impactors. By 2015, the search had netted more than 15,000 near-Earth-asteroids, including most of those larger than 1 kilometer. None of those discovered so far poses any danger to us. Of course, we cannot make a similar statement about the asteroids that have not yet been discovered, but these will be found and evaluated one by one for their potential hazard. These asteroid surveys are one of the few really life-and-death projects carried out by astronomers, with a potential to help to save our planet from future major impacts.

Link to Learning

The Torino Impact Hazard Scale is a method for categorizing the impact hazard associated with near-Earth objects such as asteroids and comets. It is a communication tool for astronomers and the public to assess the seriousness of collision predictions by combining probability statistics and known kinetic damage potentials into a single threat value.

Purdue University’s “Impact: Earth” calculator lets you input the characteristics of an approaching asteroid to determine the effect of its impact on our planet.


Could the Moon sit on the Earth? - Astronomy

A few friends and I are currently in debate about space. They say that there is no sound in space and that it is because there is no air in space. For instance if someone were talking to you, you couldn't hear what they were saying. I found it hard to believe either of those claims. I argued that there has to be air out there and that even if there was no air, there would still be sound because things like radio waves and light waves travel through space. Could you please clear us up on this argument.

Answer by Dave: I'm afraid that your friends are right. In empty space, there is no air, and what we call "sound" is actually vibrations in the air. Now, like you've said, there are indeed light waves and radio waves in space, but these waves are not sound, but light. Light does not need air to travel, but then you don't hear it you see it, or it is interpreted by your radio set and then translated into sound.

Astronauts in space do talk to each other. In the spacecraft, there is plenty of air, so they just talk normally. When they are spacewalking, they talk by means of radios in their helmets. The radio waves, again, have no problem in space, but they're not sound. They're radio, which has to be converted into sound by the astronauts' headsets.

But can't there be vibrations in matter that isn't air? And if there are gases in space, then why can't sounds move through them?

Answer by Lynn: You're right that there are gases in space, and it's true that these gasses can propagate sound waves just like Earth's air allows sound to travel. The difference is that interstellar gas clouds are much less dense than the Earth's atmosphere. (They have fewer atoms per cubic foot.) So if a sound wave was traveling through a big gas cloud in space and we were out there listening, only a few atoms per second would impact our eardrum, and we wouldn't be able to hear the sound because our ears aren't sensitive enough. Maybe if we had an amazingly large and sensitive microphone we could detect these sounds, but to our human ear it would be silent.

There can also be vibrations in matter that's not gaseous: for example, the solid Earth or even the Sun (see the related link below). But although sound can travel through Earth, it can't travel from Earth to Mars because there's essentially no matter (gases, liquids, solids) in between the two planets for it to travel through.

So it's not strictly true that no sound vibrations can travel through space at all, but it is true that humans would not be able to hear any sounds in space.

But in movies when they show a large space ship exploding and another spaceship nearby they often play a large exploding sound. I'm wondering in large explosions (maybe not as small as a spaceship exploding, but say in a supernova) could a person hear the sound because possibly the explosion releases gases in which the acoustic energy is transported through the vacuum between the explosion and some observer in a spaceship (or possibly on earth) if the supernova or spaceship explosion was relatively nearby?

Answer by Lynn: I know in movies a lot of times they play sounds when things explode, but I don't know of any cases where this would actually be realistic. Because space is a vacuum, gases released into space expand very quickly, and as they expand their density decreases.

So say you were in a spaceship in the middle of a big space battle and a nearby ship exploded. The exploding ship would release gases and technically sound could travel along with them. However, since space is a vacuum, these gases will spread out very rapidly and the density will drop off very fast with distance from the explosion. (If you think about it, the amount of air in the ship is probably not very large compared to the volume of space between two ships.) So by the time the explosion reached your ship nearby, any sounds carried by the gas would still be too faint to hear. It seems more likely to me that what you would hear would be the shrapnel from the explosion banging into the hull of your ship. As you point out, it depends on distance. If the your ship was directly next to the exploding ship, you would be more likely to hear something, but it would also be bad news for your ship and crew!

It's pretty much the same for a supernova. The gases from a supernova explosion expand rapidly, and the density will drop off fast. I'm not sure how close you would have to be to hear a supernova, because I'm not sure where you would have to be to get densities close to Earth atmospheric values, and you might need a computer simulation to tell exactly. But to get some idea of how the density of gas would drop off as you expand the material of a star, I did a really simple calculation. If you took a star 50 times the mass of the sun and distributed its mass over a sphere of space with a radius equal to the planet Mercury's orbital distance, the density would already be 10 times less than atmospheric density at sea level on Earth. Mercury is pretty close to the sun, and you wouldn't be able to hear sounds even at that distance! In reality, not all the star's mass is ejected into space, and the gas that is expelled has shock waves, which are compressed. But the basic idea is that you would have to be extremely close to get densities high enough to hear anything. So we won't ever hear a supernova explosion on Earth, for example. It's a little sad, but space really is silent.

Page last updated on June 22, 2015.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.


What Is a Supermoon?

There are two supermoons in 2021—and the next one is Wednesday morning, May 26, 2021 at 7:14 a.m. EDT . The Moon will appear full both Tuesday and Wednesday night. What is a supermoon? We agree that it’s a catchy word and anything that encourages us to explore the night sky is positive—but let’s also get our facts straight.

What Is a Supermoon?

Generally speaking, a supermoon is a full moon that appears larger than a typical full moon due to it being closer to Earth.

However, there’s a bit more to it than that! In fact, there are a couple definitions of “supermoon” out there. Let’s go through the two most popular ones, which we’ll refer to as the “broad” definition and the “strict” definition:

  • Broad Definition: A supermoon is a new or full moon that occurs when the Moon is near perigee (the point in the Moon’s orbit where it is closest to Earth). By this definition, there can be several supermoons in a year. This term “supermoon” was coined by astrologer Richard Nolle in 1979 and specifies that the Moon must be within 90% of perigee.
  • Strict Definition: A supermoon is the single closest new moon and full moon of the year. By this definition, there can be only two supermoons each year (a full moon supermoon and new moon supermoon).

In popular usage, most folks go by the broad definition, since it’s much more exciting to be able to talk about multiple supermoons instead of just one or two. Plus, “supermoon” tends to refer only to full moons, rather than both full and new moons. (This makes sense, given that new moons are essentially invisible from Earth.)

Another measure that’s used to determine if a full moon is a supermoon is its physical distance from Earth. The exact distance cutoff varies, but we generally adhere to the idea that a full moon occurring at a distance closer than 224,000 miles (360,000 km) is considered a supermoon.

How Many Supermoons Are There in 2021?

Different publications use slightly different thresholds for deciding which full Moons qualify as supermoons, but for 2021 the Almanac and NASA agree that April and May are supermoons—90% of perigee, its closest approach to Earth.

Supermoons in 2021

A Supermoon Lunar Eclipse

May’s full Moon is particularly notable for two reasons:

  1. It’s the closest supermoon of the year, sitting at a distance of 222,116.6 miles from Earth—though only slightly closer (about 100 miles) than April’s supermoon.
  2. It coincides with a total lunar eclipse in some areas, which means that it will take on a reddish hue during the eclipse’s maximum. In other words, it will be a “blood moon.”

These two full Moons are virtually tied, with the full Moon on May 26, 2021, slightly closer to the Earth than the full Moon on April 26, 2021, but only by about 98 miles (157 kilometers), or about 0.04% of the distance from the Earth to the Moon at perigee.

The eclipse will only be visible in some parts of the world, unfortunately. If you’re located in western North America, however, you’re in luck! Read more about this year’s eclipses here.

Why Do Supermoons Occur?

It all comes down to the fact that the Moon’s orbit around Earth is not a perfect circle—in fact, it’s an elliptical (oval) shape.

Because of this, the Moon’s distance from Earth changes as it travels around our planet. Additionally, Earth doesn’t sit directly in the middle of this elliptical orbit, so there are points in the Moon’s orbit where it is closest and farthest from Earth. These points are called perigee and apogee, respectively.

  • Perigee is the point in the Moon’s orbit where it is closest to Earth.
  • Apogee is the point in the Moon’s orbit where it is farthest from Earth.

The Moon makes one full orbit around Earth in about 29.53 days, which means that it reaches its perigee and apogee points about once a month. When this occurs at the same time as a full moon, it’s called a perigee syzygy—or, more commonly, a supermoon!

  • Syzygy” is the astronomical term for when three or more celestial bodies (such as the Sun, the Moon, and Earth) line up. When the Sun, Earth, and Moon form a syzygy, we experience a full or new moon, depending on whether the Moon is between the Sun and Earth or Earth is between the Sun and the Moon.

Where Did the Term “Supermoon” Come From?

Although it has been all over the news in recent years, “supermoon” is not an official astronomical term. In fact, it didn’t even exist until astrologer Richard Nolle coined it in 1979!

At the time, Nolle defined a supermoon as “a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit.” This definition is what most people go by today, though we tend to pay attention only to the full moon supermoons, since they’re a lot more interesting to look at!

Does a Supermoon Really Look Bigger?

Given that a supermoon full moon is closer to Earth than a normal full moon, it does appear larger—about 7% larger, technically speaking. This means that the difference between a full moon at perigee and a full moon at apogee can be up to 14%, which is significant.

Here’s the key fact, however: Unless you were somehow able to compare a normal full moon and a supermoon side by side in the sky, it’s nearly impossible to perceive a 7% difference in the Moon’s size.

Even if you could somehow place the year’s biggest possible Moon (the perigee full moon) next to the smallest one (an apogee full moon) in the sky, you’d just barely tell the difference. And that’s with the absolute extreme Moons!

The bottom line is that it’s difficult to truly perceive any difference at all in the Moon’s size from one month to the next, or one night to the next, so don’t get your hopes up about seeing a gigantic Moon out there.

The Moon Illusion

Okay, if you want to be guaranteed of seeing a huge- LOOKING Moon, it’s easy… Simply watch the Moon when it’s rising or setting!

A Moon down near the horizon will always look enormous, thanks to a well-known phenomenon called the Moon illusion, which makes our minds exaggerate the size of objects near the skyline.

Try it! If you want a truly massive supermoon, you can have it—any night!


Could the Moon sit on the Earth? - Astronomy

I will be going to Oslo this June. I understand that, north of some specific latitude, the sun does not set at all for one or more nights. What path does the sun follow during this time? I've heard it described as tracing a small halo over the northernmost point in the sky, never dipping below the horizon as it retrogrades back to its starting point in the circle. Is this even close?

The "critical" latitude is 66.5 degrees. But Oslo is only at 60 degrees and so you should not be seeing the "midnight sun" there.

The path of the Sun depends on the latitude of the place. At the latitude of 66.5 degrees north, the Sun will not set on June 21. On this day, the Sun rises at north, goes towards east reaching higher portions of the sky reaching a maximum elevation of about 47 degrees above the horizon at south, then go towards west and just touch the horizon (without setting) at north. Thus, the Sun never sets and goes in a circle in the sky. Now consider the extreme case of the north pole. There, the Sun will be tracing circles of roughly constant elevation for months!

Also, this year boasts a full moon on June 24. I'm intrigued by the thought of a full moon and the sun in the sky at the same time, and I was hoping to take a photograph of something I may never see again. My question: I assume the sun will be at its westernmost point in the sky when the moon rises in the east, and they will travel in opposite directions until the sun is in the east as the moon sets. How close in terms of degrees will they approach each other? That is, if the moon were directly overhead when the sun is due north touching the horizon, they would be 90 degrees apart, and I would need a pretty wide lens. Is this close to the truth?

You will almost never see the full moon and the Sun at the same time. The reason for this is that all the planets, Moon and the Sun lie in a plane in the sky called the ecliptic and this plane is tilted to the Earth's equator by about 23.5 degrees. On full moon day, the Moon and the Sun are roughly (not exactly) on opposite sides of Earth. Hence, if the Sun is at a declination of 23.5 degrees (which it will be close to summer) in the constellation of Gemini, then the Moon will be at a declination of -23.5 degrees in the constellation of Sagittarius.

Places on the Earth north of 66.5 degrees will never see the part of the ecliptic that is in Sagittarius (even though some parts of the constellation that are above the declination of -23.5 degrees may be seen depending on the latitude of the place). Hence, if you are at a latitude of say 80 degrees, the Sun will be above the sky all day during summer and the Moon will never rise during full moon.

However, the Moon's orbit is inclined to the ecliptic by about 5 degrees which is the reason why we do not see a solar eclipse during every new moon. Hence at latitudes close to 66.5 degrees, one might be able to see the Sun and the full moon for a very short time simultaneously if the geometry of the Moon is just right. However, the Sun and the full moon will be on opposite portions of the sky and so nobody will be able to photograph it unless there is an exceptional camera that can take a picture of the entire sky.

In Oslo, you will find a normal moonrise during full moon. The Sun will be up for a very long time and the full moon will rise shortly after sunset. Soon after moonrise, the Moon will set again and then the Sun will again rise. For the very same reason that you have the Sun for almost 24 hours, you will have the full moon in the sky for a very short time only.

This page was last updated on July 18, 2015.

About the Author

Jagadheep D. Pandian

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.