Reading the Moon: Step 2

The Moon has a story to tell. And much of its story is like an open book, born of a violent past, worn on the lunar surface as ancient round pockmarks and streaks of fresh, bright material. It's a story that scientists started piecing together in 1609 when Galileo Galilei first turned a telescope on the Moon.

Galileo and You

Galileo Galilei Galileo Galilei did not invent the telescope, but he was the first to realize its importance for astronomy, and he used his great engineering skills to craft improved lenses for the crude devices, which were at first called spyglasses or eyeglasses (a colleague of Galileo's later dubbed them telescopes). When Galileo turned his own handmade device on the Moon in 1609, he turned conventional thinking on its head. Amazed at what he saw, Galileo wrote that the Moon "is like the face of Earth itself," marked with mountains and valleys. The discovery surprised the heck out of his contemporaries, too, most of whom still believed Aristotle's idea that the heavens were made of unearthly matter -- ether or quintessence -- that was immutable and incorruptible, meaning it couldn't have jagged edges. With a small telescope, or even a pair of binoculars, you can share Galileo's thrill as the Moon's mountains and valleys jump out in stark relief. See our Telescope Buying Guide

The book evolved slowly. In the 1800s, people claimed to see settlements on the Moon. And it wasn't until the middle 20th Century that scientists agreed on what caused the Moon's craters.

Then beginning in the 1960s, Apollo astronauts added many chapters to the book by bringing back rocks and photographs from the surface.

You can easily read the Moon's story with a touch of guidance, so we asked Andrew Chaikin, Apollo historian, to share some insights and simple tips for anyone who might want glance up at the sky, with or without a telescope:

Even if you don't have a telescope or a pair of binoculars, you can go out and look at the Moon and make a discovery. There are two kinds of rock that you can see from Earth, even with the naked eye.

When you look up at the Moon you see a bright disk and dark splotches. The bright areas are the ancient crust leftover from the Moon's formation, and the dark areas are relatively newer rock that formed from lava that erupted from the interior. Those are the so-called seas.

They are made out of lava rock like we find in Hawaii. Except that in Hawaii, if you pick up a rock, it's likely to be maybe 100 years, or maybe it's a year old, or maybe it formed last week. But on the Moon, it's likely to be 3.5 billion years old.

With binoculars, you'll start to see some of the surface features. In particular, you can see some of the Moon's largest craters. The craters tell you that the Moon was the victim of some cosmic target practice. You can see that the Moon was a pretty violent place when the planets were forming. Each one of those craters is the result of an impact that was so violent it dwarfs any atomic weapons we have on Earth.

The next question one might ask is why doesn't the Earth show the same kind of scars. Well, it does have a few, like Meteor Crater in Arizona. But most of the impact craters on the Earth were erased by the forces of wind and rain and the shifting continents, including mountain-building events and volcanism.

But the Moon is pretty much untouched for the last 3 billion years, so it's kind of like a museum for that violent era.

If you have a good telescope, you can start to look the details of craters. Some of the big craters have walls that look terraced like a staircase, and this is because right after the crater was blasted out of the Moon's crust, the rock that formed the walls of the crater was fractured by the impact and slumped down, or collapsed, to form a series of stair steps.

Many of these large craters have flat floors covered with fairly dark rock that look somewhat like seas or ponds. This is actually impact melt, which is material that had been melted by the giant impact and then rained back down into the crater and cooled.

If you look in the middle of many of these large craters, you'll see some very large mountains sticking up. It's hard to tell how big these things are when you're looking through a telescope, but you can rest assured that these mountains are thousands of feet high. These are called central peaks, and they formed when the crust rebounded after the impact.

There are craters all over the Moon in various stages of preservation. Many are much more beaten up, and have craters on top of craters.

But the youngest craters have what look like rays of bright material coming from them. These rays are made of ejecta that were blasted out of the crust when the crater formed and sprayed across the landscape. The best example is the crater Tycho, which is in the southern highlands. It's one of the best-preserved large impact craters on the Moon. You can see its ray pattern across much of the Moon's near side with binoculars, or sometimes even with the naked eye if you've got good eyes.

Finally, the dark lava seas are themselves filling in giant impact craters that are called impact basins. These basins can be hundreds of miles (kilometers) across. Mare Imbrium is one such giant impact basin. You can still see its round shape.

The basins are places where absolutely gigantic impacts took place. If they had been much bigger, they could have broken the Moon apart.

Through a telescope or binoculars, you'll also see that the Moon has a grayish-tan color. It's a very subtle shade. And it is exactly the color that the astronauts described when they went there. So you can actually get a little sense of what they saw by looking at the Moon yourself.

Moon Guide: Step 1 | Step 2 | Step 3 | Moon Map | Top 10 Cool Facts