How Big is the Milky Way?

How big is our galaxy, anyway?

And more than that–how do we know?

Consider that we can’t really take a photo like this of our galaxy. We’re inside it, and space travel has not advanced to the point where we can leave it just yet. There’s no way we can get a camera out to take a picture from this perspective.

Most things in the universe–like stars, planets, and even other galaxies–can be measured using their angular diameters. That is, we use trigonometry to find their actual sizes based on how large they appear to us in the sky.

But that doesn’t work for an object that we’re inside of.

In order measure the size of our own galaxy, early astronomers had to get a bit creative–with variable stars.

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The Solar Eclipse

solar eclipse.jpg

A solar eclipse is the most amazing astronomical sight you’ll ever see.

Not only is it the only time you’ll ever be able to clearly see the “new moon” phase of the moon, it’s the only time you’ll ever see the sun’s corona. And it’s the only time that, under very specific circumstances, you can actually look directly at the sun for a few moments.

But before you get too excited about that, let me tell you what’s happening in the sky—and give you a few important safety warnings!

(This is just the first of a few posts that will talk about solar eclipses; they’re all worthy of a read. Even if you don’t read all of mine, make absolutely certain you’re caught up on safety warnings before you view a solar eclipse!)

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The Celestial Sphere

When you look up at the night sky on a clear, dark night, it seems as if the stars are glittering like bright thumbtacks on a great canvas above you. (You can get a similar effect–with less light pollution–from a planetarium like the one above!)

In reality, space is not like a canvas, and stars are not like thumbtacks. It would be more accurate to describe us Earthlings as floating in a vast, cosmic ocean.

Astronomers know this. But still, it’s helpful to map the sky in exactly the way it appears to us: as a sphere around the Earth. And so we use a model called the celestial sphere.

Telescopes operate solely based on the celestial sphere: the mechanism that aims the telescope doesn’t need to know anything about how far away an object actually is in the cosmic sea, just where it is in the sky.

That makes the celestial sphere a useful reference tool. Researchers need to communicate with telescope operators and say, “Let’s look over there now.”

And so, everything is mapped on a spherical model that pretends the night sky is a finite globe, inside which the Earth hovers like a bubble.

So…what exactly is the celestial sphere?

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