The Celestial Sphere


The celestial sphere is certainly a strange way to think about the night sky.

It makes sense to use globes to diagram the Earth. The Earth, after all, is a roughly spherical planet, and flat paper maps have a way of distorting distances.

The sky, though? Seriously? I mean, we all know the universe isn’t exactly a defined sphere that barely extends past Earth’s surface, right?

Funnily enough, the celestial sphere depicts all the stars as sitting on the plane of its surface like thumbtacks on a ceiling. Planets are described as following regular paths around this odd-looking sphere.

Pretty strange way to think about the night sky, right?

Well…I have to say, astronomers do have a point.

A scientific model doesn’t have to be visually accurate to perform a useful function, and the celestial sphere is a prime example. It makes the most sense if you think of it as an inside-out globe. Don’t imagine that you’re God, looking down on the sphere of space that encases Earth.

Instead, view the celestial sphere as if you’re a human standing on Earth, and you’re inside it.

Now you see how it’s useful?

The night sky really does look, from our vantage point, like a finite sphere. In fact, that’s how the ancient astronomers viewed it, before astronomy graduated from a philosophy to a science. Until methods were developed to actually measure what physically happened beyond the Earth’s surface, humans could do little more than speculate.

And those speculations resulted in—surprise! —the belief that the Earth was the center of a very small, spherical universe, and everything in the heavens moved in strange ways around our planet.

It’s funny how important the ancient astronomers viewed Earth, when it really is quite a backwater planet in the grand scheme of things in the universe.

So, the celestial sphere may be a far cry from accurate, but it actually does a good job of describing what we see in the sky above us. It’s not meant to explain why.

Here we see the Earth, surrounded by the celestial sphere. Now, how to break down this diagram?

Let’s start with the familiar. Notice that the north celestial pole is a projection of the physical north pole on the night sky; the same goes for the south celestial pole and Earth’s south pole. Additionally, the celestial equator is a projection of the Earth’s equator on the night sky.

What about the ecliptic, lines of right ascension and declination, and the equinox, then?

Right ascension and declination are actually quite simpler than they look. You’re familiar with latitude and longitude, right? Those are the gridlines that give you the GPS coordinates of any spot on the globe.

Right ascension and declination perform the same purpose as latitude and longitude lines, but on the celestial sphere. They form a grid across the sky that can describe the location (relative to Earth, of course) of any celestial object.

Declination is described in degrees just like latitude and might as well be a projection of latitude lines on the sky. But right ascension, the vertical lines on the sky that span from the north celestial pole to the south celestial pole, are different.

Why?

Notice the red arrow, indicating westward rotation. It’s referring to the celestial sphere. The Earth rotates eastward on its axis, making the sky above appear to rotate in the opposite direction around us.

See what that means? The “vertical” lines of longitude and right ascension move in opposite directions. As any one longitude line pans eastward over the globe, the right ascension lines above sweep across it in the other direction. That means that right ascension cannot be a projection of longitude lines.

Right ascension and declination are especially useful to astronomers because, unlike the altitude-azimuth coordinate system (which I’ll be covering in future posts), they don’t change depending on the observer’s location. They are locked to the celestial sphere.

Two points of interest we don’t see in this particular diagram are the zenith and the nadir. The zenith is the point in the sky directly above your head, no matter where you stand on the globe—not a fixed location. And the nadir is the opposite: the point on the sky directly beneath your feet. If the Earth were to one day turn transparent, you would be able to see it.

Got the celestial sphere down pretty well? Okay, let’s add some motion.

Imagine you are standing at about 32 degrees north latitude, looking north and ignoring the fact that…ahem…the north star is actually very faint, even though it’s sort of the brightest one in this diagram.

Think of your cardinal directions. If you’re looking north, then west is to your left and east is to your right. So the direction of the arrows makes sense.

If you stood on this spot and looked north all night, this diagram shows you what you would see. The stars of the dippers would rotate around the north star. Why? Because Earth rotates on its axis, and Polaris—the north star—just happens to be at the north pole.

This image is just slightly inaccurate…Polaris isn’t actually exactly at the north pole…but close enough. It’s not that far off.

So, what if you were looking east instead?

Basically, you’d see the stars rising up from the horizon. They would continue over your head and then disappear below the horizon in the west.

Looking south, you would see the stars move in an arc over the south celestial pole—which, from our example vantage point of 32 degrees north latitude, is not visible. Because the south celestial pole is below the horizon here, we see no circumpolar constellations—the constellations that remain above the horizon at all times.

Keep in mind that the celestial sphere is a model for how Earth’s inhabitants perceive the night sky, not a physical map of where stars are located in the universe. The stars appear as if they are thumbtacks in the inner surface of the celestial sphere, when physically, they might be very far apart. Their proximity in various constellations is an optical illusion.

With this in mind, you’d think the celestial sphere would be pretty useless for measuring the distance between the stars.

As it turns out…this isn’t quite true. This is where angular distance comes into play.

Angular distance does not describe the linear distance between stars in the sky. Of course, there are ways to measure that instead—ways that we’ll cover in future posts. But for now, we’re not talking about saying, “Star A is ___ meters from Star B.” Instead, we’ll use angles, with the center of the Earth forming the vertex of the angle.

Imagine a line extending from the center of the Earth to somewhere—anywhere—on the equator, fixed and immovable. Now imagine another line extending from the center of the Earth to anywhere else on the planet. These two lines connect at a vertex and form an angle.

That’s how you get latitude, in fact. For example, if you start with a point at the equator and choose another point directly northward that’s at 30 degrees north latitude, the angle formed with the center of the Earth will be 30°.

The same method can be used to measure the angular distance between stars.

You’ll probably notice that this diagram uses the observer’s eye as the vertex of the angle. This is okay—the stars are so far away from the Earth that changing the vertex within the Earth’s (relatively very tiny) diameter doesn’t make a difference in measurements.

Angular distance is measured in degrees, arc minutes, and arc seconds. Note that arc minutes and arc seconds don’t have anything to do with their namesake units of time. The only connection is that degrees are divided into 60 arc minutes and arc minutes are further divided into 60 arc seconds, just as hours are divided into 60 minutes and minutes are divided into 60 seconds.

Hold on a second, though. Humans don’t have some kind of innate calculator in their heads—how are we supposed to measure the angle between stars? I mean, it’s not like we can hold our old protractor from geometry class up to the night sky and make an accurate measurement.

That’s why astronomers use a special sort of protractor: a sextant!

Some sextants have more complex features than others, but they all serve the same purpose. The metal arc you see at the bottom of this sextant has measurements in degrees, and the observer looks through a sighting scope at the object in question. The sextant determines the angular distance between the object and the horizon.

In some of my beginner astronomy labs, I had the unenviable task of using a particularly rudimentary sextant. There was no sighting scope, just sort of vague sighting guides, and the measurement was obtained by dangling a string over the protractor at the bottom! Once we’d sighted our object, us students had to hold the string steady where it had fallen and record the tick mark it was on.

Modern, advanced sextants are a lot easier to use and a lot more precise, thankfully.

With some objects, particularly those in our own solar system, it’s also useful to measure angular diameter. We can do this for objects large enough and close enough to Earth to have an observable shape, such as the moon and planets. Stars just appear as a single point of light and are not usually measured this way.

Well, there you go—those are the basics of the celestial sphere. Next up, we’ll explore precession, one of my favorite topics!

Questions? Or just want to talk?

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