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Considering how massive our universe is, we know the distances to cosmic objects surprisingly well. What tools and clues do scientists use to measure distances that are so enormous they sound like made-up numbers?

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Considering how massive our universe is, we know the distances to cosmic objects surprisingly well. Like, we know the Moon's distance from Earth so precisely, we can tell it increases about 3.8 centimeters each year.

And we can also measure distances that are so enormous they sound like made-up numbers. To do that, astronomers use what's called the cosmic distance ladder. Each rung on this metaphorical ladder is a different way of measuring astronomical distances.

And from the bottom to the top, it goes all the way from the Moon to the most remote galaxies we've ever found. People have been measuring distances within the solar system for millennia. For example, the Ancient Greek astronomer Hipparchus used what's now high school trigonometry to estimate the distance to the Moon during a solar eclipse.

He was able to do that because eclipses look different from different places on Earth, for the same reason that, if you close one eye and hold out your thumb, it'll cover one spot on the wall, but if you switch eyes, it will cover a different spot. This effect is called parallax, and during a solar eclipse, the Moon acts as your thumb, while different cities are like different spots on the wall. So, by measuring what fraction of the Sun was visible in one city when it was a total eclipse in another, Hipparchus was able to do a little math and calculate the distance from the Earth to the Moon.

And he came pretty close to the right answer, too! We know that because today we can use light to get a much more direct measurement. We just bounce a laser beam off the Moon and time how long it takes to return.

We already know how fast light travels, so with these two pieces of information, we can solve for distance. This technique works incredibly well within the solar system. And it doesn't just tell us the distances to objects like the Moon and nearby planets.

It also shows us that many older measurements were extremely accurate, and that good old-fashioned trigonometry can be a solid way to calculate cosmic distances. Which is good to know, especially when it comes to objects we can't bounce light off… like the Sun, which is too bright, or objects outside the solar system, which are too far. In fact, trigonometry is the best way to measure distances to most nearby stars.

For those, astronomers still use the same trick the Greeks pioneered: parallax. As Earth moves from one side of the Sun to the other, nearby stars seem to move a tiny bit compared to more distant ones. Again, it's like when you hold your thumb up and close one eye.

As your perspective shifts from one eye to the other, closer objects seem to move, while distant ones seem to stay in the same place. So based on how much something seems to move, you can tell how far away it is. Parallax works great for stars within about 3,000 light-years from Earth, or several quadrillion kilometers.

But after that, the shifts get too small to measure. So for more distant objects, we need to climb up another rung on the ladder. Scientists first took that step in the early 1900s, when the astronomer Henrietta Swan.

Leavitt discovered a special type of star called a Cepheid variable. These stars are “variable” because their brightness fluctuates over time. And what makes them special is that the rate of those fluctuations is directly related to how bright they are on average.

That means that no matter how dim they seem from Earth, you can figure out how bright they truly are based on how quickly they vary. And, conveniently, brightness is tied to distance in a predictable way. It's like if you saw one candle right in front of you and another at the end of a hallway—the one closer to you would appear much brighter.

As a result, Cepheid variable stars became the first known standard candles. A standard candle is any object whose absolute, or intrinsic, brightness is consistent, meaning you can use its apparent brightness from Earth to figure out its distance. In fact, just 15 years after Leavitt's discovery, Edwin Hubble used Cepheids to measure distances to objects well outside our galaxy—showing, for the first time, the Milky Way was not the entire universe!

These days, we have lots of kinds of standard candles, which is helpful, because Cepheids aren't the brightest, and you can't always find one where you need one. For instance, in some cases, astronomers can use supernovas as standard candles. These exploding stars are so bright, we can use them to calculate the distances of galaxies deep in the universe.

And sometimes, even galaxies themselves can be standard candles: The faster a spiral galaxy rotates, the brighter it is, so we can use that fact to figure out the relative distances between galaxies. There are plenty of other standard candles, too, but the cosmic distance ladder isn't one big candelabrum. For extremely distant objects—or ones that don't contain a standard candle—astronomers have to get really creative to measure their distances.

They can make educated guesses by looking at how much the light has stretched out between the source and Earth or how light bends around a massive object. In short, there's no one-size-fits-all solution, and the cosmic distance ladder is still getting new rungs. What's incredible is that, even though astronomers are dealing with numbers that seem absolutely, ludicrously large, they can be pretty confident in their measurements, because each step of the cosmic distance ladder overlaps at least a little with the one before it.

So each rung of the ladder double-checks its neighbor. And that means we can map out distances to nearly anything from the Moon to the edge of the universe. Thanks for watching this episode of SciShow Space!

And if you want to find out more about the discovery of the first standard candle and the brilliant astronomer behind it, you can learn all about it in our Great Minds episode on Henrietta Leavitt right after this. {♫Outro♫}.