“Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” –Douglas Adams
Well, maybe “peanuts” isn’t going to do. When you look out at the night sky, all sorts of objects are yours to discover, from our closest neighbors in the Solar System to the billions of stars in the Milky Way to the faint, extended and fuzzy galaxies stretching millions and billions of light years across the cosmos.
But all the flowery language in the world isn’t going to help you if you actually wanted to figure out just how far away these things are. Lucky for you, it’s science to the rescue!
The closest object to us is actually the easiest one to start with: the Moon!
We already know how big the Earth is: about 12,700 kilometers (or 7,900 miles) in diameter. In fact, that’s something we’ve known for over 2,000 years! Just by knowing that, and by assuming that the Moon is a lot closer to us than the Sun is (as you might’ve guessed from what you know about Solar Eclipses), you can figure out the size and distance to the Moon!
Because every so often — including tomorrow night — you’ll get a partial lunar eclipse!
When this happens, you can see part of the Moon blocked out by Earth’s shadow! And because you know how big the Earth is, if you assume that the Moon is very, very close to Earth compared to how far away the Sun is, then you know that Earth’s shadow on the Moon is roughly the same size as the Earth!
So if you can figure out, from viewing the Moon during a partial eclipse, what the relative sizes of the Moon and the Earth’s shadow are, you can figure out the size of the Moon. This is a lot easier if you know how to stitch your images together, as Rod Pommier did in this amazing composite, below.
You can figure out that the Moon is about 27% the diameter of the Earth, more or less. But you can also measure that the Moon takes up about half-a-degree in the sky! And as long as you know a little bit of geometry, you can figure out — from the Moon’s angular diameter and its physical diameter that you just measured — exactly how far away it has to be!
This is remarkable, because it’s a measurement you can make with absolutely no equipment or training. For objects in our Solar System, we can make as accurate measurements of their size as we want, because we can actually go there, and in a great many cases, we actually have!
But we’ve never been beyond our Solar System. And yet, when it comes to the stars, there are a great many that we can measure exactly how far away they are!
This is Sirius, the brightest star in the night sky and one of the closest. Before the invention of the telescope, the only way to estimate the distance to the stars was to assume that they’re very much like our Sun, intrinsically, and then measure their brightness relative to the Sun, and infer how far away they are from that.
If you do that for Sirius, you get an answer of about half a light year, which isn’t terrible, but is off by around a factor of 20. Thankfully, we can do better.
The reason you can see in 3D is because you have two eyes — two inputs — at two different positions in space! If you alternate winking one eye and then the other, the location of nearby objects will appear to shift more dramatically relative to distant, background objects. This is because the angle your left eye makes connecting to the object is different from the angle your right eye makes. And the closer that object is, the more severe the angular difference is!
This effect is known as parallax. For the stars, our eyes are insufficient; they’re too close together. But over the span of six months, we get a much longer baseline!
By seeing how the position of the nearby stars shifts — ever so slightly — against the background of much more distant stars, we can determine the distances with amazing accuracy! The Hipparcos satellite and later, the Tycho-2 catalogue, were able to measure hundreds of thousands (and then more than two million) of the nearest stars extraordinarily well, and so we know a great deal about the positions of not only objects within our Solar System, but a great many of the stars that lie beyond.
But what about the distant galaxies? The vast majority of stars in our own Milky Way are far too distant to measure a parallax for; how, then, could we possibly hope to measure the distance to this faint, fuzzy galaxies that lie well beyond the extent of our galaxy?
The key is that we have to find a way to connect what we know about stars that we can measure parallax for with stars that exist in these distant galaxies!
And this key was provided all the way back, more than 100 years ago, by Henrietta Leavitt.
Some stars are intrinsically variable in their brightness! Over well-measured periods of time, their brightness oscillates between maxima and minima. What Leavitt did was catalogue over 2,000 variable stars, as was her job, and she noticed something remarkable about the brightest of these objects: there was a strong relationship between how bright an object was, intrinsically, and how quick its period of oscillation was.
So if you can measure how quickly a star of this type — a classical cepheid variable — undergoes this oscillation, you know how intrinsically bright it is!
And if you measure how bright it appears to be, then you can figure out how far away it must actually be!
This is exactly the method that was used to first determine the distances to the galaxies, by Edwin Hubble in 1923, and it’s still used today! This is the first “rung” on the cosmic distance ladder, and by measuring other correlations between properties of known galaxies and applying them to more distant observed ones, we can extend our reach to the farthest galaxies seen in the visible Universe.
And that’s how we figure out the distances to all the objects in the night sky, from those within our Solar System to the stars, nearby galaxies, and beyond!
And if you liked this story, just you wait… this is just a tiny, tiny bit of the story that’s going into the book I’m writing! I know it’ll still be a couple years before it’s done, but I can’t wait to share it with you!