The Celestial Sphere

celestial-sphereThe 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?

I mean, this model—the “celestial sphere”—even tries to claim that all the stars sit on the plane of the sphere like thumbtacks on a ceiling. And that the planets in the solar system follow 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. Even if it pretty much epic fails at explaining why.

The why, naturally, is for later. Don’t worry, I’ll dive into it headfirst soon enough.

imagesHey, lookie! I managed to find the exact same image of the celestial sphere that’s in my textbook!

The diagram above should speak for itself. I’m only going to explain a few things.

First of all, the arrows indicating direction of motion along the equator do not refer to Earth’s rotation—they point west, so like all items on this diagram, they refer to the way the sky moves above us.

The westward rotation of the night sky is a result of the eastward rotation of Earth. Just wrap one hand around your fist, rotate your fist inside like a ball-and-socket joint, and you’ll see what I mean.

Here’s another thing. It’s not easy to tell because of how tiny this image is, but Earth’s poles line up with the north and south celestial poles. The labels of north, south, east, and west on the flat plane indicate the cardinal directions relative to the observer.

(Yes, in case it’s too tiny to see, there’s a silhouette of a guy standing on the face of the Earth.)

You might have heard of the zenith, the point in the sky directly above your head. This is not a definable location. No matter where you stand on the globe, the zenith is going to be directly overhead. Not in any constellation or at any star.

Likewise, the nadir is the point on the celestial sphere directly beneath your feet and opposite the zenith. Unless the Earth one day turns transparent, it is impossible to see the nadir. It’s not a spot on the ground, it’s the spot in the sky that would be under you if you dug a hole straight down to the other side of the planet.

Let me point out something else—the tilt of the Earth in this image is completely arbitrary. Generally, Earth is tilted about 23.5 degrees from what is generally considered “up” in the plane of the solar system. In this diagram, it’s tilted way farther than that.

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


Let me give you some perspective to help make sense out of this diagram. Remember the celestial sphere above, the one from my textbook? The one that’s sitting weirdly on its side? You are that silhouette person, and the horizon drawn there is your horizon.

You are standing at about 32 degrees latitude, by my estimate. You’re looking north, and you’re ignoring the fact that…ahem…the north star is actually pretty darn dim, even though it’s sort of the brightest 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 make 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, is not visible.


To recap, here are all three together. There are no arrows in the “looking east” diagram, but the stars are heading upward from the horizon the observer is facing.


We’ve still got a little more room in this post, so I’m going to cap our discussion of the celestial sphere with angular distance. Because the celestial sphere is an inaccurate representation of the universe and thus can be any size, as long as it surrounds the Earth, metric distances are just about impossible to work with.

After all, it doesn’t matter how much surface area you give the celestial sphere—so why should we measure the distances between stars that way?

Angular distance solves this problem by measuring distances in degrees, arc minutes, and arc seconds. Note that arc minutes and arc seconds are units of distance, not time. They are unrelated to standard seconds and minutes.

PointABGoodness, it was just short of impossible to find a suitable diagram for this one. But basically, you measure the angle formed between two stars and your eye, with your eye as the vertex.

Do this the same way you would with a protractor or compass in math class—except, obviously, neither tool is big enough to accurately line up with stars in the sky.

So astronomers use a special sort of protractor. A sextant!


It’s not really as complicated to use as it looks, I promise.

Anyway, because degrees are a bit too large to work with accurately, astronomers divide them up into arc seconds and arc minutes. An arc minute is 1/60th of a degree, and an arc second is 1/60th of a minute. Okay, so maybe they’re related to standard seconds and minutes just as far as the conversions go.

imagesThe same technique can be used to measure angular diameter—but this measurement is only useful on larger objects like the moon.

There are equations to help work this all out, but I already swamped you with math in my last post. I promise, I won’t ask you to calculate a single number here. 😉

Anyway…those are the basics of the celestial sphere. Next up tomorrow in chemistry is the fun stuff—elements and compounds. And next up in astronomy will be precession—one of my favorite topics!

See you around, and thanks for reading!

About Emma

I'm a college student, astronomer, writer, Democrat, global warming activist, hardcore trekkie, daring adventuress, and dreamer. I write for different reasons. Sometimes because I want to share my process of discovery with others. Sometimes because I think a story is too incredible to keep it to myself. Sometimes just because I want to be heard. Regardless, I hope you enjoy it!
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