# Is there any thing special with an axial tilt of roughly 25°?

Half of the planets in the solar system have an axial tilt of $$25.5pm 3^circ$$: earth (23.5°), mars (25.3°), saturn (26.7°) and neptune (28.3°). If random collisions in the early solar system caused the axial tilt of the planets, it seems like there should be a lot more variation in the planetary tilts. Is there some mechanism that favors tilts around 25°, or is this just a statistical fluke?

Mars (the planet with an axial tilt closest to that of the Earth) has an axial tilt that varies greatly over time. For instance, 4 million years ago, the mean obliquity was $$∼35 ± 10°$$. From the same article:

Earth on the other hand has had its rotational features stabilized due to the effect of our large Moon.

So it is only by chance if the axial tilt of Mars is currently similar to that of Earth.

## Earth's axial tilt and the shadow from the Sun

I have watched the Sun's shadow make its trek from the solstice moments for many years. I have it marked and like to observe it as Earth's tilt reaches it's maximum point, from Summer to Winter and back. I used a heliodon to built a large structure (80 feet long/12 feet tall=roofline of my house) that gives me a long shadow line. It is not perfect, but reliable for my purposes. I have been observing this shadow-progression for the past 7 years and have the solstice shadows marked for record.

Is there someplace/website I can visit to see a real-time recording of the Sun's shadow this year, as compared to years previous? Thanks.

## Contents

Giacomo Maraldi determined in 1704 that the southern cap is not centered on the rotational pole of Mars. [3] During the opposition of 1719, Maraldi observed both polar caps and temporal variability in their extent.

William Herschel was the first to deduce the low density of the Martian atmosphere in his 1784 paper entitled On the remarkable appearances at the polar regions on the planet Mars, the inclination of its axis, the position of its poles, and its spheroidal figure with a few hints relating to its real diameter and atmosphere. When Mars appeared to pass close by two faint stars with no effect on their brightness, Herschel correctly concluded that this meant that there was little atmosphere around Mars to interfere with their light. [3]

Honore Flaugergues's 1809 discovery of "yellow clouds" on the surface of Mars is the first known observation of Martian dust storms. [4] Flaugergues also observed in 1813 significant polar-ice waning during Martian springtime. His speculation that this meant that Mars was warmer than Earth proved inaccurate.

There are two dating systems now in use for Martian geological time. One is based on crater density and has three ages: Noachian, Hesperian, and Amazonian. The other is a mineralogical timeline, also having three ages: Phyllocian, Theikian, and Siderikian.

Recent observations and modeling are producing information not only about the present climate and atmospheric conditions on Mars but also about its past. The Noachian-era Martian atmosphere had long been theorized to be carbon dioxide–rich. Recent spectral observations of deposits of clay minerals on Mars and modeling of clay mineral formation conditions [5] have found that there is little to no carbonate present in clay of that era. Clay formation in a carbon dioxide–rich environment is always accompanied by carbonate formation, although the carbonate may later be dissolved by volcanic acidity. [6]

The discovery of water-formed minerals on Mars including hematite and jarosite, by the Opportunity rover and goethite by the Spirit rover, has led to the conclusion that climatic conditions in the distant past allowed for free-flowing water on Mars. The morphology of some crater impacts on Mars indicate that the ground was wet at the time of impact. [7] Geomorphic observations of both landscape erosion rates [8] and Martian valley networks [9] also strongly imply warmer, wetter conditions on Noachian-era Mars (earlier than about four billion years ago). However, chemical analysis of Martian meteorite samples suggests that the ambient near-surface temperature of Mars has most likely been below 0 °C (32 °F) for the last four billion years. [10]

Some scientists maintain that the great mass of the Tharsis volcanoes has had a major influence on Mars' climate. Erupting volcanoes give off great amounts of gas, mainly water vapor and CO2. Enough gas may have been released by volcanoes to have made the earlier Martian atmosphere thicker than Earth's. The volcanoes could also have emitted enough H2O to cover the whole Martian surface to a depth of 120 m (390 ft). Carbon dioxide is a greenhouse gas that raises a planet's temperature: it traps heat by absorbing infrared radiation. Thus, Tharsis volcanoes, by giving off CO2, could have made Mars more Earth-like in the past. Mars may have once had a much thicker and warmer atmosphere, and oceans or lakes may have been present. [11] It has, however, proven extremely difficult to construct convincing global climate models for Mars which produce temperatures above 0 °C (32 °F) at any point in its history, [12] although this may simply reflect problems in accurately calibrating such models.

Evidence of a geologically recent, extreme ice age on Mars was published in 2016. Just 370,000 years ago, the planet would have appeared more white than red. [13]

Mars' temperature and circulation vary every Martian year (as expected for any planet with an atmosphere and axial tilt). Mars lacks oceans, a source of much interannual variation on Earth. [ clarification needed ] Mars Orbiter Camera data beginning in March 1999 and covering 2.5 Martian years [14] show that Martian weather tends to be more repeatable and hence more predictable than that of Earth. If an event occurs at a particular time of year in one year, the available data (sparse as it is) indicates that it is fairly likely to repeat the next year at nearly the same location, give or take a week.

On September 29, 2008, the Phoenix lander detected snow falling from clouds 4.5 kilometres (2.8 mi) above its landing site near Heimdal Crater. The precipitation vaporised before reaching the ground, a phenomenon called virga. [15]

Martian dust storms can kick up fine particles in the atmosphere around which clouds can form. These clouds can form very high up, up to 100 km (62 mi) above the planet. [16] The first images of Mars sent by Mariner 4 showed visible clouds in Mars' upper atmosphere. The clouds are very faint and can only be seen reflecting sunlight against the darkness of the night sky. In that respect, they look similar to mesospheric clouds, also known as noctilucent clouds, on Earth, which occur about 80 km (50 mi) above our planet.

Measurements of Martian temperature predate the Space Age. However, early instrumentation and techniques of radio astronomy produced crude, differing results. [17] [18] Early flyby probes (Mariner 4) and later orbiters used radio occultation to perform aeronomy. With chemical composition already deduced from spectroscopy, temperature and pressure could then be derived. Nevertheless, flyby occultations can only measure properties along two transects, at their trajectories' entries and exits from Mars' disk as seen from Earth. This results in weather "snapshots" at a particular area, at a particular time. Orbiters then increase the number of radio transects. Later missions, starting with the dual Mariner 6 and 7 flybys, plus the Soviet Mars 2 and 3, carried infrared detectors to measure radiant energy. Mariner 9 was the first to place an infrared radiometer and spectrometer in Mars orbit in 1971, along with its other instruments and radio transmitter. Viking 1 and 2 followed, with not merely Infrared Thermal Mappers (IRTM). [19] The missions could also corroborate these remote sensing datasets with not only their in situ lander metrology booms, [20] but with higher-altitude temperature and pressure sensors for their descent. [21]

Differing in situ values have been reported for the average temperature on Mars, [22] with a common value being −63 °C (210 K −81 °F). [23] [24] Surface temperatures may reach a high of about 20 °C (293 K 68 °F) at noon, at the equator, and a low of about −153 °C (120 K −243 °F) at the poles. [25] Actual temperature measurements at the Viking landers' site range from −17.2 °C (256.0 K 1.0 °F) to −107 °C (166 K −161 °F). The warmest soil temperature estimated by the Viking Orbiter was 27 °C (300 K 81 °F). [26] The Spirit rover recorded a maximum daytime air temperature in the shade of 35 °C (308 K 95 °F), and regularly recorded temperatures well above 0 °C (273 K 32 °F), except in winter. [27]

It has been reported that "On the basis of the nighttime air temperature data, every northern spring and early northern summer yet observed were identical to within the level of experimental error (to within ±1 °C)" but that the "daytime data, however, suggests a somewhat different story, with temperatures varying from year-to-year by up to 6 °C in this season. [28] This day-night discrepancy is unexpected and not understood". In southern spring and summer, variance is dominated by dust storms which increase the value of the night low temperature and decrease the daytime peak temperature. [29] This results in a small (20 °C) decrease in average surface temperature, and a moderate (30 °C) increase in upper atmosphere temperature. [30]

Before and after the Viking missions, newer, more advanced Martian temperatures were determined from Earth via microwave spectroscopy. As the microwave beam, of under 1 arcminute, is larger than the disk of the planet, the results are global averages. [31] Later, the Mars Global Surveyor's Thermal Emission Spectrometer and to a lesser extent 2001 Mars Odyssey's THEMIS could not merely reproduce infrared measurements but intercompare lander, rover, and Earth microwave data. The Mars Reconnaissance Orbiter's Mars Climate Sounder can similarly derive atmospheric profiles. The datasets "suggest generally colder atmospheric temperatures and lower dust loading in recent decades on Mars than during the Viking Mission," [32] although Viking data had previously been revised downward. [33] The TES data indicates "Much colder (10–20 K) global atmospheric temperatures were observed during the 1997 versus 1977 perihelion periods" and "that the global aphelion atmosphere of Mars is colder, less dusty, and cloudier than indicated by the established Viking climatology," again, taking into account the Wilson and Richardson revisions to Viking data. [34]

A later comparison, while admitting "it is the microwave record of air temperatures which is the most representative," attempted to merge the discontinuous spacecraft record. No measurable trend in global average temperature between Viking IRTM and MGS TES was visible. "Viking and MGS air temperatures are essentially indistinguishable for this period, suggesting that the Viking and MGS eras are characterized by essentially the same climatic state." It found "a strong dichotomy" between the northern and southern hemispheres, a "very asymmetric paradigm for the Martian annual cycle: a northern spring and summer which is relatively cool, not very dusty, and relatively rich in water vapor and ice clouds and a southern summer rather similar to that observed by Viking with warmer air temperatures, less water vapor and water ice, and higher levels of atmospheric dust." [28]

The Mars Reconnaissance Orbiter MCS (Mars Climate Sounder) instrument was, upon arrival, able to operate jointly with MGS for a brief period the less-capable Mars Odyssey THEMIS and Mars Express SPICAM datasets may also be used to span a single, well-calibrated record. While MCS and TES temperatures are generally consistent, [35] investigators report possible cooling below the analytical precision. "After accounting for this modeled cooling, MCS MY 28 temperatures are an average of 0.9 (daytime) and 1.7 K (night-time) cooler than TES MY 24 measurements." [36]

It has been suggested that Mars had a much thicker, warmer atmosphere early in its history. [37] Much of this early atmosphere would have consisted of carbon dioxide. Such an atmosphere would have raised the temperature, at least in some places, to above the freezing point of water. [38] With the higher temperature running water could have carved out the many channels and outflow valleys that are common on the planet. It also may have gathered together to form lakes and maybe an ocean. [39] Some researchers have suggested that the atmosphere of Mars may have been many times as thick as the Earth's however research published in September 2015 advanced the idea that perhaps the early Martian atmosphere was not as thick as previously thought. [40]

Currently, the atmosphere is very thin. For many years, it was assumed that as with the Earth, most of the early carbon dioxide was locked up in minerals, called carbonates. However, despite the use of many orbiting instruments that looked for carbonates, very few carbonate deposits have been found. [40] [41] Today, it is thought that much of the carbon dioxide in the Martian air was removed by the solar wind. Researchers have discovered a two-step process that sends the gas into space. [42] Ultraviolet light from the Sun could strike a carbon dioxide molecule, breaking it into carbon monoxide and oxygen. A second photon of ultraviolet light could subsequently break the carbon monoxide into oxygen and carbon which would get enough energy to escape the planet. In this process the light isotope of carbon ( 12 C) would be most likely to leave the atmosphere. Hence, the carbon dioxide left in the atmosphere would be enriched with the heavy isotope ( 13 C). [43] This higher level of the heavy isotope is what was found by the Curiosity rover on Mars. [44] [45]

## Answers and Replies

I have a hypothetical question. If there was a relatively Earth-like planet out there that somehow experienced a rather catastrophic meteor impact large enough to alter its axial tilt close to 0 degrees, would such an event render the planet essentially a desert world? I.e. there are no more seasons, no more poles, oceans essentially dried up (or would they be dried up?)

Obviously the way the question is worded betrays my ignorance, so forgive me. Thanks for your time.

There would still be poles and it seems to me that there would still be 3 different "seasons" and 1 of them would occur twice in its' year. As it orbited its' Sun, its' "seasons" would change in this order:

1)a season where the north pole would be in full sunlight(potentially no ice cap, not desert, probably a very rainy season) but the south pole would be in complete darkness(potentially frozen for the majority of that season).

2)a season where it receives an even amount of sunlight(mainly) from north to south pole and it would have a normal day and night(potentially a fertile planet during this season, potentially flooding from ice melt from the southern ice)

3)a season where the south pole will be in full sunlight(potentially no ice cap, not desert, probably a very rainy season), north pole would be in complete darkness(potentially frozen for the majority of the season)

4)a season where it receives an even amount of sunlight(mainly) from north to south pole and it would have a normal day and night(potentially a fertile planet during this season, potentially flooding from ice melt from the northern ice)

That's an approximation and without being specific about certain other variables.

It's hard to say exactly because there are atmospheric variables that would have a lot to do with average temperature, temperature ranges which would effect how much ice could form(if any) and how long it would last through the other seasons, how much heat and cold weather patterns could regulate, presence of green house gases, among other things.

## What's so special about Earth's axial tilt of 21° 1/2 degrees?

I understand that Earth's tilt of 21° 1/2 influences a lot the dynamic of seasons around the whole globe. But. How different would it if it was a smaller angle? What about a wider angle?

The greater the angle, the greater the change in seasons.

A smaller angle would mean a smaller change in seasonal light variation. It would have a huge impact on the biology of our planet. For example, many animals have a birthing season (spring) to give their offspring the chance to be as old as possible (And therefore hardier) before encountering their first winter. As the angle decreases, so would the necessity for that.

Imagine too all the plants that are evergreen and those that are not. There are special adaptations that have to be made for this, which places constraints on the structures of your leaves, your trunk, your branches, even the chemistry of yourself etc. (For example many trees have to be prepared to withstand much heavier loads during winter months because of snow on branches.) Less seasonal variation means being able to optimise yourself for whatever the average condition is.

As for our axial tilt being "special" .. it's no more special than one of 23, or 25, or 19, or 17. One thing you could say is that is has a significant axial tilt IE one that results in noticeable seasonal variations, which means our ecosphere has had to adapt to be able to withstand regular change. But many axial tilts would result in significant seasonal variations, so it's hard to regard that as "special".

Edit: There's a nice simple diagram on this page, bottom right:

The greater the angle, the greater the change in seasons.

We're looking at the Earth from the "side," and we're going to pretend we're in the northern hemisphere for this example. Since the earth is [effectively] a sphere, no matter how much it's rotated one way or another, it always get about the same amount of sun. During the Spring and Fall, when the tilt plane is circumferential to the orbit, both the north and south hemispheres get about the same amount of sun, but when you move into the summer, the northern hemisphere is tilted towards the sun. At the equator (shown in orange) this doesn't really change anything, because the the day/night ratio stays about the same. However, as you go north, you can see that there is more sunlight falling on that part of the planet. This effect continues the further you go, and when you reach the poles, they are effectively always day or night. Seen here.

This is really where, I believe, the answer to the second part of your question lies, if you were to increase the tilt further, more and more of the earth would be in either light or darkness for 6 months at a time. If you were to decrease the tilt, the opposite happens.

This is part of the reason why the poles are always colder. The sunlight cross section here is split into 10 equal parts. You can see that the portions that are 'responsible' for lighting the poles have more ground to cover than those at the equator, this spread the light out, leaving less per unit area of land, ergo, colder.

It's late and my brain is starting to fail me. I think I got everything in there, but if someone wants to piggyback off my comment to make any corrections or to explain the finer points, please do.

EDIT: As others have been so kind to point out, current tilt of the earth lies at 23.5 degrees. However:

"Today, the Earth's axis is tilted 23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees."

## 3 Answers 3

If the person is wandering across the broiling wastes of the sunward side, then a gyrocompass (since there's probably no magnetic pole anymore) is needed to determine a north-south line, and a simple stick-and-shadow sundial can be used with that north-south line to gauge both latitude and longitude. No need for a reliable clock, since the sun is always in a constant, predictable position.

If the person is wandering across the frozen wastes of the night side, then the traveler will need the gyrocompass and a small ephemeris with key equatorial star positions to use in place of the sun, and a sextant to use in place of the stick. A reliable calendar (not clock) is needed here, since the star positions do not change significantly each hour.

If the person is wandering along the terminator, then they already know their longitude, and can use the sun-glow or a sextant-with-equatorial-star to calculate latitude.

A simple square-and-protractor to measure the angles would be handy, as most folks aren't very good at estimating angles by eye.

Of course, since the traveler is likely to have space travel (how else did they get there?), they could simply have a GPS satellite constellation in orbit, which makes the answer much easier.

If it's tidally locked to the sun, by definition there's no such thing as axial tilt. That can be disregarded.

The first thing is to not think of latitude and longitude in the way it's oriented on Earth. For simplicity, we'll assume the same numbering system as on Earth (360 degrees = full circle), and for timekeeping, we'll assume one orbit around the sun is 100 days (Earth standard).

The center of your navigational grid on the lit side is going to be the sun. Directly overhead marks the zero position, the hot pole. Instead of 90 degrees, it's going to be labelled 0. The terminator--ignoring atmospheric diffraction of light--is going to be 90. That's going to be your latitude. On the daylight side, easy to find you simply use a sextant to measure the height of the sun over the horizon, and better than the Earth, you don't have to wait until midday to do it you can do it at any time.

On the cold side, you rotate the grid. Now, your reference is a recognizable star or constellation closest to your planet's orbital axis, so it's the closest thing to unmoving you can find in the sky. Essentially, instead of finding a pole star for your planet's rotation as on Earth, you find it for your planet's revolution around the sun. Find one above and below the orbital plane, and you've got a way of determining stellar latitude by measuring the distance of one (or other features, as the sky is mapped) above the horizon.

Picture it this way: Imagine that on the Eurasian side of the Earth, latitude and longitude are exactly like they are now, but on the side with the Americas, it's turned sideways so that the "pole" is centered on the Galapagos islands. Now 90 degrees latitude, as centered on the Galapagos, would be the same great circle around the planet as 0/180 degrees longitude based on the Eurasian Grid. The Eurasian side is the dark side of the planet, the Galapagos side the light side.

Okay, so on the Eurasian night side, further measurement is relatively easy, once you've invented reasonably accurate timekeeping mechanism (ie, clocks). You've got astro-north, and astro-south. You know that stars near the astro-equator (which would be the plane of the planet's orbit) take a half-year (50 days) to go from horizon to horizon. At day 1, it rises above the horizon, is at its apex at day 25, and set on day 50.

If you have accurate timekeeping, you know that at a specified location at a given date a given star should be a certain number of degrees above the horizon. If you measure the height, you can measure the difference between where it is and where it should be on a given date as seen from your latitude, and that can be used to calculate the longitude.

Now, this only works if you have accurate timekeeping devices so you know your date/time compared to the reference point, but really, not a lot of difference from the problem of measuring longitude on Earth before the advent of accurate portable clocks.

On the day side, it's trickier. Solar latitude is trivial to determine, as mentioned. Solar longitude becomes harder without the stars. You're initial assumption (with the sun "bobbing" up and down, as seen from the planet) would make it easier, but that can't happen if the planet is tidally locked.

## Earth's 23 degree tilt thought due to collision that caused Moon .. but Mars has 25 degree tilt. What's the theory there?

The formation of the Moon wasn't a guaranteed thing just because there was a giant impact, it takes a specific set of initial conditions to end up forming a large moon. Proto-Earth either had to have been hit by a shallow, glancing blow in order for a large enough amount of the material ejected to end up in a high enough orbit to collapse into a single object rather than remain torn apart into a ring system, OR it had to have been hit so hard that both the proto-Earth and Theia (the impacting body) were almost totally vaporized and formed a rapidly spinning doughnut shaped cloud of hot material called a 'synestia', which the moon would have formed from later.

As for adjusting a planet's axial tilt, that takes nothing special, just an off-axis impactor. In fact any impact no matter how small adjusts the axial tilt of the Earth by a very tiny amount, usually so small that it is undetectable. It simply takes a giant impact to supply enough change in kinetic energy to change the degree of tilt of the axis of rotation.

We actually think that at least four of the planets in our solar system were hit with giant impacts, Earth, Mars, Venus, and Uranus, though the others almost certainly were hit also, they just don't show any obvious signs.

Mars probably suffered from a smaller, but still giant, impact comparable to Earth's, however the angle was either not shallow enough or the energy was too low, and only a small amount of material ejected from the impact remained in orbit to collapse and form three tiny moons. These were Deimos, Phobos, and the biggest of the three, an unnamed but theorized innermost moon that only had a short lifespan before tidal interactions with Mars draw it downwards until it broke up into a ring system, which decayed over time. The evidence of this impact includes the moons themselves, Mars' axial tilt, and finally Mars' peculiar terrain, which neatly splits the surface of Mars into two broad areas the southern hemisphere highlands, heavily cratered and very old, and the northern hemisphere lowlands, which are much lower in elevation and incredibly smooth by comparison. It's possible that Mars' giant impact happened late in its formation, erasing much of the cratered northern hemisphere's crust and replacing it with a smooth plain of cooled lava, which was shielded from many future impacts by the formation of an ocean of water which could have persisted for as much as a billion years, washing away big craters and preventing small ones from forming entirely.

Venus is a bit more difficult to pick apart, as its entire surface is fairly flat and new, due to Venus apparently remaining geologically active for far longer than Mars did and eventually covering all the old crust with huge lava flows. What tells us Venus may have been hit with a giant impact is the fact that it spins incredibly slowly, at around human walking speed, and in fact spins backwards compared to the other planets (or is upside-down, if you follow the right hand rule for determining planet axial tilt, which would give Venus a tilt of 177.4 degrees). What we believe caused this is a giant impact which struck Venus while moving in the opposite direction it had already been spinning previously there is no preference for what inclination or direction an impactor approaching a planet will approach by, so this is entirely plausible. Rather than striking Venus in more or less the same direction it had been spinning, speeding it up and changing the axis of rotation, the impact came at such an angle that it almost stopped Venus entirely, arresting most of its rotational momentum. This impact could have formed a moon also, however like Mars' theorized third moon, Venus' moon would have been doomed by tidal forces to be drawn ever closer and eventually be shredded into a ring, and by now there is nothing left. This is because since Venus only spins very very slowly, its moon would have been orbiting Venus at a rate faster than the surface rotated underneath it. This causes tidal drag, which slowly removed energy from an object's orbit, bringing it closer and increasing the strength of the effect. By contrast, immediately after the formation of the Earth and the Moon the Earth would have had a day length of less than ten hours, and the Moon would have had an orbital period of around 20 hours, and so the surface would be rotating faster than the Moon and inducing a tidal boost, the opposite of tidal drag. Even today this effect occurs, though it is much weaker as the Moon orbits about ten times farther away and Earth spins much slower now, but still the Moon's orbit does increase in height by about 3 cm per year.

Finally, Uranus seems to have had the largest giant impact event of all, energetic enough to change the axis of rotation of a planet with a mass more than a dozen times that of Earth by over 90 degrees, and eject enough material into orbit to form all of the moons that orbit the planet. We think that the impactor that struck Uranus was at least a super-Earth massed object, which would have made it the fifth largest object in the solar system were it still around today, and in fact would have been bigger than every smaller object including the four terrestrial planets and all moons and asteroids, combined.

## Calendar-Worthy Events on a no-axial spin distant future Earth

The Question Let's try this again. Again. One concise question: "What significant and regular calendar-worthy events would either nomadic or stationary humans perceive on a distant future Earth that (long ago) stopped spinning about its axis (but is otherwise identical to our current Earth)?"

I cannot stress this enough but the following is not asking a different question, only providing examples of helpful answers: I am specifically interested in understanding how astronomy events would be changed because of Earth's lack of axial-spin (moon, stars, other planets, etc) and how significant and regular weather patterns would form on the planet (seasons, season length, temperature, precipitation, storms, etc).

Assumptions Please assume the following

• The lack of Earth's spin eliminated our equatorial bulge, flooding the oceans to North and South, creating a large North Ocean and a large South Ocean and one planet-spanning mega-continent along the equator.
• The lack of spin created 6 (current) months of "day" and 6 (current) months of "night". There will be two twilight periods in between: dawn and dusk. I do not know how long they will be (but would like to <-- mods, not asking a new question). This is NOT a tidally-locked scenario, so please don't direct me to those sources or talk about the planet's "day" side and "night" side.
• Assume humans have found ways to survive the temperature, storms, radiation, and agricultural difficulties. I'm not asking how they survive but what they will experience by surviving in this situation.
• Assume there are 3 human populations experiencing this situation: (1) those that stay at a fixed location somewhere along the equatorial mega-continent, (2) those that travel along the dawn twilight band, (3) those that travel along the dusk twilight band.

Relevance to others The guys says to add this. These answers will help anyone working in the same no-axial spin scenario I am or anyone working in a scenario that has disturbed the Earth's axial rotation speed or anyone working on a scenario that requires an understanding of the climate or astronomical effects of Earth's axial spin.

Conclusions so far Feel free to correct any of the below if you think otherwise.

## How did we decide that the North Pole was the top of the earth?

Generally speaking (disregarding the slight tilt of the earths axis and alteration between solstices), the North Pole is considered the top of the earth. Now, I understand that's where the strongest magnetic pull is located, but that doesn't automatically make it the top, for all we know, it could be the bottom, side, or some random spot in-between.

Models usually show our solar system rotating around the sides of the sun, but what if we actually spin around it's top and bottom like the hands go around a clock rather than the shadow around a sundial?

I guess since space is just space with no absolute top and bottom, the top can be whatever you want, so how did we come up with horizontal rotations with north being at the top?

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Probably because the people doing the most research and discovery of it lived in what was later designated as the top half of the earth. I imagine in an alternate reality where the tilt of the Earth and resulting climate was more favorable to human expansion in the Southern Hemisphere our view would be flipped. I’m oversimplifying human civilization but hopefully my point is clear.

I would love to hear an actual answer to this question from a more educated perspective.

The notion that north should always be up and east at the right was established by the Greek astronomer Ptolemy (90-168 AD). “Perhaps this was because the better-known places in his world were in the northern hemisphere, and on a flat map these were most convenient for study if they were in the upper right-hand corner,” according to historian Daniel Boorstin. The artificial convention of placing North at the top of a map began a few centuries ago when European navigators from the Northern hemisphere started using the North Star and the magnetic compass. Before that, the top of the map was to the East, which is where the word orientation comes from.

Everyone did astronomy. European map makers very recently deemed it the top. It should be the east

I could be wrong, but I remember in my Astronomy class learning about something called a procession whereas in our life time we won’t notice it, but like the earth actually is very gradually tilting and every 5,000 or 10,000 years the earth will be “upside down”. Again in our lives we won’t notice it but the earth will actually flip

You know how one "orient's" a map or themselves? Oriental means eastern, it's a leftover from when maps had east at the top. It's arbitrary, my cartography prof said north ended up on top due to there being more people (especially white people) in the northern hemisphere vs southern.

The most populous continent, Asia, is also in the northern hemisphere.

Not just people, 70% of all land on earth is in the northern hemisphere. The southern is mostly water.

Can I ask if people take cartography as part of a curriculum -- maybe surveying or scientific field work or nautical careers or as part of military training -- or if it's just one of those classes people want to take because they find the idea interesting?

Heh, “Especially white people”

Okay, so, for the second half:

Models usuall show our solar system rotating around the side of the sun

That's because that's how it actually is. Almost everything in the Solar System revolves in what's called the plane of the ecliptic. This includes all the planets revolving around the sun almost in exactly the same plane (like a spinning plate), as well as all the planets rotating along their own axis in roughly that same plane. Earth's axial tilt is roughly 23 degrees, so it's pretty much "up" (thank your local axial tilt for briging you seasons). The planets also rotate in the same direction (counter-clockwise if you're looking from "above" the sun). Of course someone's always gotta be special, so Uranus would like a word . . .

The reason that the sun, the planetary orbits, the planets, and the asteroids all go the same direction relates back to the formation of the solar system. It basically formed out of a whirlpool in a bunch of relatively uniform matter, because the stuff that made the sun and the planets was just a uniform oatmeal of elements. Imagine a whirlpool of water, but the water magically turns into the sun and planets . . . that's what happened. So all the momentum from the formation of the solar system is still there in the form of both orbits and planetary spinning. Again, very roughly.

As to why the North pole is "up" and the South pole is "down", well . . . yeah, that's white European Imperialism. Because from a logical standpoint that's really the only arbitrary part of the whole thing.

## Earth's Seasons Change at Wednesday's Not-So-Equal Equinox

The seasons on Earth will officially change Wednesday, heralding their shifting nature with an astronomical feat: the autumnal equinox.

On Sept. 22, at 11:09 p.m. EDT (8:09p.m. PDT), the fall season will begin in the Northern Hemisphere while the Earth's Southern Hemisphere residents ring in their spring. This date — one of two each year — is called an equinox, from the Latin for "equal night," alluding to the fact that day and night are then of equal length worldwide.

But this is not necessarily so.

The not-so-equal equinox

The definition of the equinox as being a time of equal day and night is a convenient oversimplification.

For one thing, it treats night as simply the time the sun is beneath the horizon, and completely ignores twilight. If the sun were nothing more than a point of light in the sky and if Earth lacked an atmosphere, then at the time of an equinox the sun would indeed spend one half of its path above the horizon and one half below.

But in reality, atmospheric refraction raises the sun's disk by more than its own apparent diameter while it is rising or setting. Thus, when we see the sun as a reddish-orange ball just sitting on the horizon, we're looking at an optical illusion.

It is actually completely below the horizon. So from our point of view, the day on an equinox appears longer than it actually is. This illusion means that the appearance of equal day and night, from a skywatcher's view, will come several days later.

In addition to refraction hastening sunrise and delaying sunset, there is another factor that makes daylight longer than night at an equinox: Sunrise and sunset are defined as the times when the first or last speck of the sun's upper limb is visible above the horizon — not the center of the disk.

This is why, when you check your newspaper's almanac or weather page on Wednesday of this week to look up the times of local sunrise and sunset, you'll notice that the duration of daylight from sunrise to sunset still lasts a bit more than 12 hours — not exactly 12 as the term "equinox" suggests.

In New York City, for instance, sunrise is at 6:43 a.m. and sunset comes at 6:54 p.m. So the amount of daylight is not 12 hours, but rather 12 hours and 11 minutes.

Not until Sept. 26, will the days and nights truly equal (sunrise is at 6:47 a.m., sunset coming 12 hours later).

And at the North Pole, the sun currently is tracing out a 360-degree circle around the entire sky, appearing to skim just above the edge of the horizon.

At the moment of this year's autumnal equinox, it should theoretically disappear completely from view, and yet its disk will still be hovering just above the horizon. But it will take another 52 hours and 10 minutes later until the last speck of the sun's upper limb finally drops completely out of sight.

This strong refraction effect also causes the sun's disk to appear oval when it is near the horizon. The amount of refraction increases so rapidly as the sun approaches the horizon, that its lower limb is lifted more than the upper, distorting the sun's disk noticeably.

Equinoxes on other planets?

The word equinox is also used for either of the two points in the sky where the sun is located on the first day of spring and autumn. These points are the intersections of the ecliptic with the celestial equator, but they're not necessarily confined to Earth.

To determine when another planet experiences equinoxes, we need to know its axial tilt. The Earth's axis is tilted at a 23.44-degree angle.

The planet whose axial tilt is most similar to ours is Mars, whose axis is tilted at a 25.19-degree angle. The Martian autumnal equinox this year comes on Nov. 12, while the spring equinox occurs next year on Sept. 13.

Mercury has no significant axial tilt, so the sun (which appears about 2 1/2 times larger than here on Earth)always shines directly down on Mercury's equator.

In contrast, Uranus has an axial tilt of 97.77 degrees, giving it seasonal changes completely unlike those of the other major planets. Uranus rotates more like a tilted rolling ball.

At the time of a Uranian solstice, one pole continually faces the sun while the other pole faces away. Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness.

Near the time of the equinoxes, the sun faces the equator of Uranus giving a period of day-night cycles similar to those seen on most of the other planets.

Uranus reached its most recent equinox in 2007. The next will come in 2049.

Joe Rao serves as an instructor andguest lecturer at New York's Hayden Planetarium. He writes about astronomy forThe New York Times and other publications, and he is also an on-camerameteorologist for News 12 Westchester, New York.