# How do astronomer measures the size of any celestial objects?

What techniques and tools are available to the astronomers to measure the size of any celestial objects such as, stars or perhaps black holes that doesn't emit light nor reflects starlight?

If a celestial body is larger than the resolving power of a telescope, its size can be measured directly. This is the case for most galaxies, molecular clouds in the Milky Way and nearby galaxies, and even for a few nearby stars. EDIT: See discussion by Rob Jeffries on how these measurements are carried out for stars using interferometry.

For more distant stars, we can rely on our understanding of stellar evolution, which tells us pretty accurately the radius, once we know its spectrum (EDIT: or just a assume blackbody radiation and use the formula given by Rob). If the star is a member of a binary system whose orbit we observe roughy edge-on, we can measure how the luminosity declines as one star occults the other, and calculate the radius. This can also be used to measure the sizes of exoplanet. And for stars, the same technique is even possible using our own Moon as occulter. See a description here.

Black holes (BH) that don't emit light, cannot be measured (at least not until we are able to detect gravitational waves), but often BHs are surrounded by a disk of accreting gas, which is heated to million of degrees by friction as it spirals down the drain. Measuring the temperature of this gas tells us the mass of the BH, which is directly proportional to its radius ($R_{mathrm{BH}} simeq 3 M/M_odot$ km).

A nice technique for measuring the size of the quite small region of gas clouds around a supermassive BHs, even though they are billions of lightyears away, is called reverberation mapping. Here, some of the light emitted from the BH's accretion disk travels directly in our direction, while some of if travels in other directions, illuminating the clouds around it. When we measure the light from those clouds, the signal looks like the directly observed signal, but with a delay $t$ corresponding to extra length of the path that the light has taken. Since we know the speed of light $c$, we can calculate the extra distance as $d = ct$, i.e. the size of the system.

The main tool to measure the diameter of a star is interferometry combined with a parallax-based distance measurement - a brief review by Kervella (2008) might be useful. The principles behind interferometry are described here.

Interferometry involves measuring the light from a star using two (or more) telescopes that are separated by some distance. Together, the signals from these telescopes can be combined to give an angular resolution that can be (in the best circumstances) equivalent to a telescope with a diameter equal to the telescope separation. These measurements give the angular size of the star, which must then be multiplied by their distances to get a physical diameter.

One of the most successful experiments is the Chara array, which has yielded diameters for many nearby stars. Precisions can be as good as a few percent, but more usually 10% and of order 100 (predominantly nearby) stars have had their radii measured in this way.

A second main direct technique is to use eclipsing binary systems. The measured light curve can be used in an almost model-independent way to estimate the radii of the two stars involved. Of course most eclipsing binaries are close pairs with short orbital periods and with orbital inclinations that allow us to see the eclipse. They are therefore highly prized objects. Radii can be measured with precisions of 1%. A reasonably complete catalogue of the $sim 100$ known eclipsing binaries with precise radii can be found here.

Another technique is lunar occultation. The passage of a star behind the limb of the moon results in a changing diffraction pattern that can be used to estimate the angular size of the star. Again a distance is required to convert this into an actual diameter.

More distant stars are inaccessible - their angular diameters are simply too small. At the moment only indirect estimates of their radii are possible. For example, if we were to assume that a star radiates as a blackbody, then its luminosity ($L$), radius ($R$) and temperature ($T$) are related by Stefan's law. $$L = 4pi R^2 sigma T^4,$$ where $sigma$ is the Stefan constant. If the star has a measured flux at the Earth and we know how far away it is, then $L$ can be estimated. If we take a spectrum and estimate its temperature, then the equation above can be rearranged to give the radius in terms of the measured luminosity and temperature. Real stars are more complicated than blackbodies, but the principle is the same.

Neither of the above techniques can work for black holes, and the sizes (event horizon or Schwarzschild radius) of black holes have not yet been directly measured. The physics of a black hole is relatively simple(!) and so there is a direct relationship between their Schwarzschild radii and their masses (modified somewhat by rotation). Basically it is 3 km multiplied by the mass in solar units. Therefore a measurement of the black hole mass gives its "radius". The masses of black holes are measured by looking at the motions of stars and gas around them and applying our knowledge of how gravity works.

## How do astronomer measures the size of any celestial objects? - Astronomy

I am starting out in amateur astronomy, but would like to get more out of the exercise of star-gazing than just - well, star-gazing. I would like to observe (not just read) how far the celestial objects are to us, how fast they are moving, and what gases they are composed of. To this end, a telescope may be a good beginning point, but certainly not the final stop - it would be great to have access to a spectrometer and an UV ray-detecting telescope. Are these hopelessly outside the price range of a conventional amateur?

Hi. It seems that you want to see how easy it is to do science with amateur astronomy equipment, and how much would it cost. The answer is that yes, amateur astronomers can do some significant science, not only redo old experiments but also contribute to the field. To do any UV astronomy is pretty impractical, but there are things you can do with other equipment, for different price ranges.

1) A very simple spectroscope: you can get pretty cheaply (a couple hundred $or less) a small prism spectroscope, but what you can see is mainly limited by the size of your telescope. In fact, that is true for almost anything. But with this spectrometer (you could even make it yourself) on a moderately sized telescope (say 8-12" diameter) you can see spectral lines in some of the brighter stars. You cannot do too much that is really scientific, since all you see are the lines on a piece of paper. For details, look in the back of a Sky & Telescope magazine. 2) A CCD. For about$500 you can get a very decent CCD camera (an electronic camera) for use on almost any telescope. Things people do are: search for supernovae, search for asteroids/comets, or monitor variable stars. All three areas have had very significant contributions made by amateur astronomers, and they continue to do so. You can find more information on the Sky & Telescope site (www.skyandtelescope.com), or search for the AAVSO (American Association of Variable Star Observers). A CCD will also allow you to do other things like look at Cepheid variables, track the motion of the moons of Jupiter/Saturn, and similar things that have been done but are nonetheless interesting.

3) A telescope. It is really essential to everything you do. The bigger, the better, but that isn't the most important thing. For info on telescope selection you can also look at the S&T site, but one of the more important things are good sky conditions and a motorized mount (essential for CCD work).

Another option is radio astronomy. This really requires some technical background because it is much more do-it-yourself than optical astronomy, but there is a lot of room for science. Look for either the Society of Amateur Radio Astronomers (SARA) or the amateur radio astronomy mailing list (ara). You can do things like monitor the rotation of the galaxy, listen to noise storms on Jupiter, monitor solar flares, observe meteors, or even search for extra-terrestrial intelligence. You can find info about all of this on the web.

This page was last updated July 18, 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.

## Elementary School Science

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This lesson contains affiliate links to products I have used and personally recommend. At no cost to you, I make a commission for purchases made through the links or advertisements. These commissions help to pay for the costs of the site and enable it to remain free for anyone who wants to use it.

The students will learn about astronomy.

The students will be able to define the terms: astronomy, astronomer, and telescope.

The students will be able to name famous astronomers and explain their contributions to astronomy.

The students will be able to describe the five fields of astronomy.

Questions that encompasses the objective:

Think about the Solar System. How did we learn that there are planets, stars, and other objects in space?

Prepare the Learner: Activating Prior Knowledge.

How will students prior knowledge be activated?

Warm up by asking students:

What do you know about astronomy?

Can you name any famous astronomers?

Common Core State Standards:

Materials and Free Resources to Download for this Lesson: ​

Page 9: History of Astronomy

Pages 10-12: Famous Astronomers

Pages 13-14: Fields of Astronomy

&ldquoA Day in the Life of&hellip&rdquo activity materials:

&ldquoFamous Astronomers&rdquo &ndash Here are some ideas

Arno Penzias and Robert Wilson

Giovanni Domenico Cassini

**The teacher can either provide fact sheets for the students or allow the students to do research on their own to gather information about their assigned astronomer**

###### Space Lesson Plans: How Big Is The Solar System?

Input:
What is the most important content in this lesson?
To reach this lesson&rsquos objective, students need to understand:

The field of astronomy and its importance in science.

The definitions to the terms: astronomy, astronomer, and telescope.

The names of famous astronomers and their contributions to astronomy.

The five fields of astronomy.

How will the learning of this content be facilitated?

For the first few minutes, the students will fill out the &ldquoWhat I Know About Astronomers&rdquo worksheet.

The teacher will begin the class by handing out the &ldquoGuess the Astronomer&rdquo worksheet. On the worksheet is a box with the names of the three famous astronomers: Galileo Galilei, Nicolaus Copernicus, and Isaac Newton. There will be nine questions (three for each of the astronomers). The teacher should allow the students about 10 minutes to read through each question and write down their guess of which astronomer refers to each question. The teacher should tell the students that each astronomer will have three statements on the worksheet. At the end of 10 minutes, the teacher should review the worksheet with the students.

Galileo [G] Newton [N] Copernicus [C]

Believed the Earth was not the center of the Earth, but did not believe that the moon caused tides. (G)

First scientist to prove that the Solar System revolved around the Earth. (G)

Invented and improved the telescope. (G)

Defined the 3 laws of motion and universal gravitation. (N)

Invented the reflecting telescope in 1668. (N)

Explained that gravity helps the planet orbit around the Sun. (N)

Stated that the Sun is in the center and that the planets orbit around it (theory called &ldquoHeliocentrism&rdquo) (C)

Not only was he an astronomer, but a physician, scholar, economist, translator, mathematician, artist, and diplomat. (C)

Many people believe he started modern astronomy. (C)

After the worksheet is reviewed, the teacher will begin presenting the information on the Astronomers. If it is possible, project each page of the teacher&rsquos copy of the &ldquoAll About the Solar System&rdquo Science Journal worksheet onto the board using a projector or put into a PowerPoint document and project. The teacher&rsquos copy of the journal has certain words/phrases that are bolded red and highlighted. It is important the teacher explain to the students those words/phrases are to be highlighted in their (students) journal. For this lesson, the teacher should review these pages:

Page 9: History of Astronomy

**This page reviews what astronomy is and its history**

Pages 10-12: Famous Astronomers

**These pages will review the three famous astronomers from the beginning of class: Galileo Galilei, Nicolaus Copernicus, and Isaac Newton**

Pages 13-14: Fields of Astronomy

**This page explains the different fields of astronomy: Observational Theoretical Solar Planetary Stellar**

After the information has been presented, the students will break into pairs. On the board, the teacher should write the names of famous astronomers (See activity section). Each pair will pick an astronomer. Together, the students will create a poster board depicting what life was/is like for that astronomer. (Refer to materials section as to how information/research should be obtained). Allow the students about 15-20 minutes to complete the poster board.

**Each student/pair can either present their &ldquoPlanet&rdquo or a gallery walk can be set up. For the gallery walk, each pair will place their planet poster on desks throughout the room. The students will walk around and look at their peers&rsquo poster**

After the presentation or gallery walk is over, the students should reconvene and the teacher should discuss the activities. The teacher should review the vocabulary words with the students.

The students will then fill out the &ldquoWhat I Learned About Astronomers&rdquo worksheet.

They will then share with the class, or with a partner, some things they wrote down in their &ldquoWhat I Learned About Astronomers&rdquo page.

**The students should keep both of the "What I Know" and "What I Learned" sheets in a folder to put them all together into a book when the unit is finished.​**

## How do astronomers determine masses of planets and other celestial objects? It's not hard to scale their volume, but it's not enough to find mass out. Do they define prevailing materials and substances and speculate on the density to calculate mass using volume or what?

Usually masses are determined by observing the orbits of things around them, in the case of planets it's usually the planets' moons that are used. A lot of stars are in binary systems, and the binary orbits were used to understand the relationship between brightness and mass, which can be used to determine the mass of solo stars. For more distant things like galaxies, we can see how much the light of even farther objects gets deflected by gravity, and use that to calculate the mass.

And what do you do with planets that have no moons like Mercury or Venus?

For clarity, the planet-moons part would only apply in the Solar System. For exoplanets, it's usually done with the same technique as binary stars (using the star-planet orbit). There's some cases where the planet mass is estimated from theoretical models (but those are very uncertain) and through microlensing (which involves the bending of light by massive objects).

/u/iorgfeflkd answered your question well, but I wanted to add that for objects outside our solar system, it's usually easier to directly measure the mass than the radius/volume. For almost every binary star system, the stars are just points of light to us, so directly measuring the radius is impossible - but watching their movement and change in velocity can be done, which allows us to work out the mass.

Typically we measure the mass of stars indirectly though, because we have a really good handle on stellar evolution, and because stars are fairly simple, so it's possible to give the mass of a star based on unresolved stuff like its colour & brightness.

If the binary system is an eclipsing binary system, that is the stars pass in front of one another as they orbit, then it is possible to get a measurement of their radii relative to the size of their orbit (specifically the semi-major axis). To measure the size of the orbit you need a number of radial velocity (velocity along observer's line of sight) measurements, but fairly routine for eclipsing binaries. Eclipsing binaries are one of the rare examples where it is possible to measure both the mass and radius of the stars involved.

Although we do have a fairly good idea of how a star evolves, there are still lots of uncertainties present in the models used for stellar evolution. For example, there are uncertainties associated with how convection is modelled, there are examples where the models cannot reproduce the radii and temperature of stars in observed binary systems, at least for low mass stars (say <2 solar masses). There is currently a lot of research going into trying to improve these models because for single stars, the models are often the only way to obtain a mass, something that is important if you are trying to understand the environment around a planet-hosting star.

## Astronomers measure new distances to nearby stars

Astronomers from the U.S. Naval Observatory (USNO), in collaboration with others from the REsearch Consortium On Nearby Stars (RECONS), have determined new distances to a group of faint young stars located within 25 parsecs (pc) of the sun. These measurements, based on parallax observations obtained over periods ranging from nine to twelve years, include new measures of the star known as TRAPPIST-1, which has been recently identified as having a system of up to seven Earth-sized planets orbiting around it.

The paper describing the measurements, whose lead author is Dr. Jennifer Bartlett of the USNO, has been published in the Astronomical Journal.

Measuring the distances to nearby stars is accomplished by a technique called "trigonometric parallax," in which the tiny apparent annual shift of a star's position is related to the diameter of the Earth's orbit around the sun. By measuring the tiny angle produced by this motion, astronomers can use trigonometry to determine the distance to the star.

The distances to these stars are measured in parsecs, short for "parallax second." One parsec is the distance at which a star would show an annual parallax shift of one second of arc on the plane of the sky it is equivalent to 3.26 light-years.

Utilizing the 0.9-meter telescope at the Cerro Tololo Inter-American

Observatory (CTIO) in Chile, Dr. Bartlett and her colleagues measured the parallaxes of 32 stellar systems, most of which are very cool faint red dwarf stars. Of these systems, 17 have never had previous parallax measurements, and out of those, 14 have been found to lie less than 25 pc from the Earth. One of these new, nearby stars, 2MASS 2351-2537AB, also shows evidence of actually being two new nearby stars, i.e., a binary.

In addition to these newly-measured star systems, the astronomers also obtained new parallax measurements for 15 other, previously-known nearby stars. Among these is the star known as "TRAPPIST-1," which has been recently shown to host a system of at least seven planets. Using over 12 years of observations made with the 0.9-meter telescope at CTIO, Dr. Bartlett presents a new parallax of 79.29 +/- 0.96 mas, yielding a distance of 12.61 pc, about 4 percent more distant than previous measurements.

The team also measured the brightness of each of these star systems in three regions of the optical and near infrared electromagnetic spectrum and analyzed how the brightness of each system varied over the course of their parallax observations. Although the team caught TRAPPIST-1 flaring during July 2009, they found the overall variability of this star to be low. In other words, its brightness varied enough to be detectable but not enough to be considered significant.

For many of these star systems, the team obtained optical spectra using the 1.5 m CTIO telescope. This study is the first to identify LP 991-84 as a M 4.5 V type star, confirming its cool, dim, red nature. As a graduate student, Dr. Bartlett discovered that this star is within 10 pc of the sun. This study measured its parallax more precisely to be 115.90 +/- 1.33 mas, or 8.63 pc.

From their assessment of the positions, motions, variability, and spectra of these 32 star systems, the astronomers concluded that 13 of them are young, probably less than 120 million years old. TRAPPIST-1 and 2MASS 2351-2537AB, however, do not appear to be particularly youthful.

On the other hand, LP 991-84 may be 1 billion years old, or more.

"I am amazed at what we can find in our backyard—well, if your backyard is 25 pc deep," Dr. Bartlett said. "We are still looking and identifying stars within the sun's immediate vicinity, its neighborhood so to speak. What will we find next?"

## What is Extragalactic Astronomy? (with pictures)

The dawn of extragalactic astronomy was in 1917, when American astronomer Heber Curtis observed a stellar nova within M31, the formal name for what was then called the Great Andromeda Nebula. At the time, spiral nebulae such as Andromeda were thought to lie within our own galaxy, with a size only several times larger than that of our solar system and a distance less than 50,000 light-years. They thought the Milky Way Galaxy represented the entire universe.

After observing the nova in M31, Curtis searched the photographic record, noticing 11 additional novae in the region. If M31 was just a stellar nebulae, why were there so many novae within it, and why were these characteristically fainter than other novae? Reasoning from the observation that these novae were about 10 magnitudes fainter than novae known to occur in our own galaxy, Curtis declared that the Great Andromeda Nebula was in fact an "island universe," distinct from the Milky Way and located 500,000 light-years away. Astronomers did not accept his hypothesis at first, and a scientific debate began.

In 1920, Harlow Shapley, another American astronomer, challenged Curtis to a Great Debate on important astronomical issues of the time, including whether spiral nebulae like Andromeda were really outside our own galaxy. Many fellow astronomers followed the debate, but the final results were inconclusive. It was not until 1925, when Edwin Hubble (after whom the Hubble Space Telescope is named) published observations from the 100-inch Hooker telescope, then the largest in the world, that he had discovered Cepheid variable stars in the Andromeda nebulae and used them to measure its distance, found to be an enormous 2.5 million light-years. The era of extragalactic astronomy had begun, and the Andromeda Nebula was renamed the Andromeda Galaxy.

For the past 80 years, extragalactic astronomy has been an active area of research. By measuring the relative speed of galaxies using their optical signature, it was found that all galaxies are moving away from each other and the entire universe is expanding. In 1998 observations of Type Ia supernova even suggested that the expansion is accelerating. Cosmologists now think it is likely that the universe will end in a "Heat Death" where accelerating expansion causes all matter to disperse and freeze.

An important episode in extragalactic astronomy is the discovery and investigation of quasars, QUasi-stellAR radio sources. These bright point sources were known to be very luminous and very remote, among the most distant objects known, with some as far away as 13 billion light-years. Although quasars were first observed in the 1950s, it wasn't until the 1970s that a scientific consensus began to emerge on the nature of quasars: they were active galactic nuclei, consisting of supermassive black holes sucking in several solar masses worth of material per century and releasing tremendous amounts of radiation in the process. Formal models have been built to describe this, and one of the greatest mysteries in extragalactic astronomy was solved.

Today, millions of galaxies have been photographed and classified by scientists, sometimes even using the help of the public (as in GalaxyZoo). Galaxies are either spiral or elliptical. It is estimated that there exist about a hundred billion galaxies in the observable universe. Interestingly, this is about the same as the number of neurons in a human brain.

Michael is a longtime contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Michael is a longtime contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

## Frequently bought together

### Review

". has infectious enthusiasm that makes people want to buy a telescope

### From the Back Cover

The night sky is alive with many wonders??distant planets, vast star clusters, glowing nebulae, and expansive galaxies, all waiting to be explored. Let respected astronomy writer Philip Harrington introduce you to the universe in Star Watch, a complete beginner?s guide to locating, observing, and understanding these celestial objects. You?ll start by identifying the surface features of the Moon, the banded cloud tops of Jupiter, the stunning rings of Saturn, and other members of our solar system. Then you?ll venture out beyond our solar system, where you?ll learn tips and tricks for finding outstanding deep-sky objects from stars to galaxies, including the entire Messier catalog??a primary goal of every serious beginner.

Star Watch features a detailed physical description of each target, including size, distance, and structure, as well as concise directions for locating the objects, handy finder charts, hints on the best times to view each object, and descriptions of what you?ll really see through a small telescope or binoculars and with the naked eye.

Star Watch will transport you to the farthest depths of space??and return you as a well-traveled, experienced stargazer.

## An astronomer's guide to stargazing from your backyard

With the proper equipment, you can enjoy the beauty of the night sky.

This is a challenging time for families. Schools across the US are struggling to provide a meaningful online experience. The coronavirus pandemic has cut off or restricted many entertainment options. As an astronomer, I believe a great way for families to fill the void and have a meaningful science experience in the time of COVID-19 is to turn their attention to the stars they can see right outside their homes.

The night sky is, and always has been, safe and free.

Here are five ways you can get started.

### Naked eye

You can see a lot with just your eyes. But the night sky is a strange landscape for most people. Just as you would when traveling somewhere unfamiliar, you’ll need a map. Sky and Telescope – an astronomy news publication – has a guide to get started, with printable sky maps for any month. The objects of the night sky migrate through a complete cycle over the course of a year.

As the Earth moves around the sun, different stars and constellations come into view, so you can enjoy new sights all through the year. You may find it more convenient to have a planisphere, or sky wheel, a rotating plastic disk that shows the night sky for any date and time. They can be bought for US$10-$20 online.

Spotting planets are trickier since they move among the stars, but there are interactive maps online that show them in the night sky for any time at any location. Your view of the night sky depends on your latitude, so it varies with your specific location. Even easier, there are smartphone apps that take all the work – and maybe also the fun – out of navigating the night sky.

Hold your phone up and the apps identify stars and overlay the constellation shapes. Some respond to voice commands and add detailed information on celestial objects or show the International Space Station as it whizzes overhead.

### Binoculars

You probably have a pair of binoculars somewhere in your house. If you don’t or you want an upgrade, new binoculars cost anywhere from $35 to over$500, and the lower end of the range is just fine for stargazing.

Perhaps you use them at concerts or for bird-watching. Well, they’re also perfect for stargazing. There are two numbers on binoculars. They represent the magnification and the lens diameter, so 7 x 50 will magnify an image by a factor of 7 using 50-millimeter lenses. At a dark location, your naked eye will see about 3000 stars. With binoculars, this number goes up to 100,000.

The moon is spectacular through binoculars. It’s fun for kids to track a cycle of the moon phases over a month, and then do an activity that shows why the moon has phases.

### Small telescopes

If you want to make a bigger commitment to exploring the night sky, consider getting a small telescope. Peering through a telescope opens up a world of star clusters, galaxies, and nebulae. You can see Saturn’s rings and the moons of Jupiter.

It’s a good idea to read a guide before you take the plunge. Small telescopes range from a hundred to several thousand dollars, but you can get a good starter one for as little as $200. These basic telescopes usually have a viewfinder attached to help locate objects, but you’ll need sky maps to get the most out of them. If you can shell out at least$400, you can acquire what are called “GoTo” telescopes that have motors and are computer-controlled, where the telescope does the work of finding the deep sky objects. You just have to type the name in or choose from a list. Now you’ll be ready to learn tricks of the trade, like using a red LED flashlight to preserve your night vision and looking slightly to the side, which lets you see deeper because the cells near the edge of your retina are more sensitive to low light levels.

### Online resources

The internet is a great resource for backyard astronomy. You might want to start with a Crash Course in naked-eye astronomy. In addition to reviews of apps and binoculars and telescopes, there are tutorials on how to use your new telescope.

You’ll also want to check out Sky and Telescope’s weekly “Sky at a Glance.” BBC’s Sky at Night magazine has a longer monthly summary of what you can observe.

And if you just want to be inspired by the visual splendor of a dark night sky, there are a number of time-lapse videos you can enjoy.

### Dark skies

Hopefully, you’re excited about backyard astronomy, but what you can actually see will depend on where you live.

For tens of thousands of years, the night sky was a familiar friend to our ancestors, and they used it to navigate, tell time, and project their myths into the stars and constellations. But the glories of the night sky have been steadily eroded by industrial activity and artificial lights.

You can see recent measures of night sky brightness in a zoomable map of the U.S., where it’s clearly harder to find a dark sky in the eastern half of the country.

To measure how sky brightness affects what you can see, amateur astronomers use something called the Bortle scale, where 9 is an inner-city and 1 is a pristine wilderness. Light pollution is the effect of artificial lighting on the night sky, and you can also see how it affects the familiar Big Dipper and Orion constellations.

National parks are great places to enjoy the sky because they try to protect it from artificial lights, and in normal times many national parks offer astronomy programs. Many are also beginning to resume those programs with COVID-19 restrictions in place.

If you want to contribute to the effort to raise awareness of light pollution by monitoring the sky brightness where you live, the U.S. national observatories run a project called “Globe at Night.”

Anyone can collect data and help a research project by doing citizen science, which is when non-scientists gather data and contribute to a collective research effort. You can become a citizen scientist and submit your own observations from a computer or smartphone.

This article was originally published on The Conversation by Chris Impey at the University of Arizona. Read the original article here.