How do scientists
measure millions
of light-years?

The full narration of the video.

The Andromeda Galaxy is about two and a half million light-years away. No one has ever been there, and no ruler could ever stretch that far. So where does that number come from? From one measuring trick handed to the next. Like a ladder, each rung resting on the one below. You can try the first rung right now. Hold out your thumb. Close your left eye, then your right. Your thumb seems to jump against the background.

The closer something is, the more it jumps. The farther away, the less. Measure the jump, and you can work out the distance. Earth circles the Sun. Look at a star, then look again six months later. Now your two eyes are about three hundred million kilometers apart. With this trick, a satellite has measured stars within a few thousand light-years very precisely. But any farther, and the stars barely jump at all.

The second rung uses lamps. If you know how bright a lamp really is, then how dim it looks tells you how far away it is. In nineteen twelve, astronomer Henrietta Leavitt found something: a kind of star that brightens and dims. The slower it changes, the brighter it truly is. Let's call it a blinking star. Count how many days it takes to blink, and you know how bright it really is.

First, measure nearby blinking stars with the thumb trick. Now this lamp is calibrated. In nineteen twenty-three, Edwin Hubble found a blinking star in Andromeda. The math put it far outside our own Milky Way. Andromeda was another galaxy. But farther out, blinking stars are too faint to see. The third rung needs a brighter lamp: an exploding star, called a supernova.

One kind of supernova shines about the same every time. Roughly five billion times brighter than the Sun. Bright enough to outshine its whole galaxy. Find a galaxy with both blinking stars and one of these supernovas. Use the blinking stars to calibrate the supernova. Then you can measure beyond a billion light-years. For the farthest places, there's one more trick: look at the color of the light. The universe is growing. The longer light travels, the more it gets stretched, and the redder it looks.

In nineteen twenty-nine, Hubble found something: the farther a galaxy is, the redder its light. Measure how red, and you can estimate how far. So these numbers aren't measured in one go. They're built up, rung by rung. If a lower rung is a little off, every rung above it is off too. Andromeda's two and a half million light-years has an error of about five percent. And two ways of measuring how fast the universe grows still don't agree today. Scientists are still working on it.

Next time you hold out your thumb and switch eyes, remember: the first rung of the ladder that measures the universe starts with that simple move.