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When it comes to exploring space, one of our biggest struggles is trying to see more. We build bigger telescopes to collect more light, we send them into space where they can intercept more wavelengths, and we're always trying to think of new ways to see what is currently invisible. But sometimes, seeing less is more.

For thousands of years, astronomers have studied events where one celestial object passes in front of another, blocking its light. They're called occultations, and as the more distant object, or part of it, disappears from view, astronomers can sometimes make incredible discoveries.

The word "occultation" comes from the Latin word meaning "to hide." If you've ever seen a solar or lunar eclipse, you have seen an occultation. An asteroid, planet, or moon passing in front of a distant star is also a type of occultation, and astronomers can take advantage of all of these events to study the object that's getting blocked or the one doing the blocking.

For instance, in 1977, astronomers planned to observe an occultation where Uranus would pass in front of a distant star. They were hoping to pin down some of the planet's properties that were still unknown, including its size. Basically, they knew how fast the star was moving across the sky, so by measuring the time between when the star disappeared behind Uranus and when it appeared on the other end, they would be able to calculate how wide the planet was.

So on the day of the occultation, astronomers got ready, pointed their telescopes at Uranus, and then, the star blinked out of sight just before they expected it to. And then, it came back, and it flickered on and off a few times before disappearing behind the planet. And when it came back out the other side, the star again dipped out of sight a few times before returning for good.

Ultimately, the astronomers realized they were looking at rings. Uranus's rings are so thin that they hadn't been visible from Earth, but this subtle crossing between two bodies was just what astronomers needed to discover them. And, in addition to figuring out the size of Uranus, astronomers were able to measure the size of its rings.

Scientists have also used occultations to study the Moon. Starting in the 1930s, British astronomers became interested in using what's called a grazing lunar occultation to study the Moon's geography. This type of occultation whenever the edge of the Moon just barely grazes a star, from our perspective, I mean. Like, it doesn't literally graze a literal star; if that happened, we would have big problems.

If you watch one of these events from just the right spot, you can often see the star disappear and reappear, like a light flashing on and off. What's happening is that the star is disappearing every time it passes behind a mountain on the Moon and reappearing in the valleys. It's like what happens when you see, like, a low moon when you're driving. First you see it, then it gets blocked by a house, and then it reappears in a gap to get blocked again when you pass some trees, and so on.

In that scenario, the appearance and disappearance of the Moon can tell you something about the landscape of Earth, and, similarly, over the next few decades, these astronomers used various occultations to map the landscape at the edge of the Moon. Keep in mind that this was in the 1930s, years before spacecrafts showed up, but even when the 1960s came along and orbiters began mapping the Moon in more sophisticated ways, areas around the poles were out of reach because they weren't lit well enough. So for several more decades, these grazing occultations provided some of the best data on the geography of these parts of the Moon.

Finally, similar types of occultations sometimes happen when a nearby asteroid passes in front of a star. When this happens, it casts a faint shadow on Earth that can be a few hundred kilometers across, and observers under its shadow will see the star blink out of view at slightly different times. So by recording exactly when the star gets blocked by the asteroid and reemerges, astronomers can figure out the diameter of the part of the asteroid that crossed their specific location.

For example, say people are observing an asteroid roughly the shape of a kidney bean or, like, the letter C. As the top and bottom of the asteroid cover the star, observers in those parts of the shadow will see the star blink out of view. But the middle of the asteroid will take an extra few moments to line up with the star and blot it out, so observers in that part of the shadow will see the star disappear slightly later. Each of these observers will also see the star emerge at slightly different times, depending on the asteroid's shape.

By having a network of astronomers all over the occultation zone, researchers can figure out the cross-section of the asteroid at each location. Then, they can piece that together to approximate the asteroid's shape. It can be hard to assemble a network of astronomers in exactly the spot an asteroid's shadow will pass over, but even amateur astronomers can help collect these observations and contribute. And overall, this technique can give impressively good measurements of an asteroid's size and shape, even when the asteroid itself is barely visible from Earth.

Today, astronomers have good tools for predicting occultations, and despite all the other tools we have for peering into the depths of space, occultations are still important for exploring distant worlds. Because sometimes it takes a little darkness to really see.

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