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What is a Solar Eclipse?

/ Emma / 18 Comments
solar eclipse.jpg

This post has been updated and republished from Sept 2017.

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 it’s not just an astronomical event. It’s an experience. You can see the moon’s shadow rushing toward you. Nature falls silent. It’s night during the day.

And there’s one coming up on April 8, 2024.

So, let me tell you a bit about what’s happening in the sky—and give you a few important safety warnings!

(If you’re wondering what happened to our unit on active galaxies, not to worry–I’m still publishing the next post this week!)

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How Big and Bright are Galaxies?

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The brightest galaxy in our night sky is the Andromeda Galaxy, our nearest intergalactic neighbor.

Hey…notice we’re talking about intergalactic neighbors now? 😀

In most of the posts on this blog, we’ve talked about interstellar neighbors, interstellar space, etc. But now, we’ve graduated to the intergalactic frontier, and we’re not turning back–at least not for a little while!

Anyway. The Andromeda Galaxy is the brightest galaxy in our night sky. But as you may remember from my eons-old article on apparent visual magnitude…that doesn’t tell us much about how bright it actually is.

It is the closest galaxy to us, the denizens of the Milky Way, so that tells us one thing: if it appears brighter than other galaxies, it probably is.

But…let’s say we put all galaxies in the universe on an even playing field, magically all exactly the same distance from Earth.

How big and bright would they appear then?

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How Far Away are Galaxies?

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Well, I’ll give you a spoiler: they’re ridiculously far away.

Let’s consider for a moment what a light-year actually means. It sounds like a unit of time, but it’s actually the distance that light travels in one Earth year.

Think of it this way: if your name is Bob, and you can travel a certain distance in one year, that distance could be called a Bob-year.

I know it’s strange to think of light traveling at a certain speed. When you flip a light switch, the room immediately brightens. When you shine a flashlight, its beam immediately falls across the nearest surface.

But that just goes to show how insanely fast light travels. If it takes 2 million years for light to get from one object to another…imagine how far apart those objects are?

Well, that’s the case for our home galaxy, the Milky Way, and our nearest galactic neighbor, the Andromeda Galaxy.

But…wait a second. How do we know that?

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How Big is the Milky Way?

/ Emma / 7 Comments

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 Celestial Sphere

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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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