THE MOON: A Spaceflight Away - 2. The Moon Seen from Earth

Moon, Earth, and Sun, as viewed from above the solar system

Figure 1. Moon, Earth, and Sun, as viewed from above the solar system.

Phases of the Moon

Figure 2. Phases of the Moon as seen from Earth.

For thousands of years, human beings wondered about the Moon. Where did it come from? they asked. What was it made of? What was its surface like? Could life exist there? People wanted to know the answers because the Moon was such a big and bright object in the sky.


They were also curious because the Moon behaved so oddly. It could be seen both by day and by night. Unlike the stars, it moved across the sky. And, strangest of all, it seemed to changed shape! Over each four-week period, it would go from a full circle, to a thinner and thinner crescent, to nothing at all, and then back to a full circle again.


Why did the Moon behave in this way? Several hundred years ago, scientists began to discover the answers.


The Moon in Motion

Just as the Earth orbits the Sun, they found, so the Moon orbits the Earth. Racing at 2,300 miles (3,700 kilometers) per hour, the Moon takes 27 1/3 days to go all the way around our planet.


But as the Moon circles the Earth, it doesn't give off light of its own. It shines by reflecting light from the Sun. That is why the Moon gradually seems to change shape in the sky. We see only the part of the Moon that is facing the Sun (see Figure 1).


The side of the Moon that faces the Sun is lit up. This is the Moon's daylight side. When the Earth is between the Sun and the Moon, we see all of the daylight side, and we say that the Moon is full (see Figure 2).


As the Moon swings around the Earth, we see less of the daylight side and more of the dark side. We say that the Moon is waning. After a while, all we can see is a thin crescent. Finally, that disappears, too.


When the Moon is between the Sun and the Earth, we can see none of the daylight side. The dark side is turned towards us, and we say that the Moon is new.


As the Moon continues its trip around the Earth, its daylight side and crescent shape come back into view. It grows and grows – people call this waxing – until it gets back to its starting point behind the Earth. Then we see the full Moon again. All the different shapes of the Moon are called its phases.


About twice each year, the Moon passes exactly between the Earth and the Sun. Then the Moon blocks off all or part of the bright disk of the Sun. This is called a solar eclipse.


The Moon looks a coppery-red during a lunar eclipse


Once in a while, when the Moon is on the side of the Earth away from the Sun, it travels through the Earth's shadow. Then, for about an hour, very little sunlight gets through to the Moon. This event is called a lunar eclipse.


Can you think of anything else that the Moon does as it orbits the Earth? It spins, like a top, about its own axis. We know that it does this because the Moon always keeps the same side facing the Earth. In fact, the time it takes for the Moon to do one full spin must be exactly the same as the time it takes for the Moon to go once around the Earth.


You can do an experiment to see how this spin works. Put a chair in the middle of the room to represent the Earth. Now, pretending that you're the Moon, walk around the chair sideways so that you are always looking at the chair. By the time you have walked around the chair once, you will also have dome one complete turn. And, like the Moon, you will have kept your same side facing the chair.


Through the Telescope

People began to figure out the Moon's movements, its phases, and its eclipses a long time ago. But they were still puzzled by the Moon itself. What, for instance, was the Moon's surface like? they wanted to know. Was it covered, in part, by oceans, lakes, and rivers? Was it teeming with life? Or was it a strange place, unlike anything that had been seen before?


Human eyes alone couldn't provide the answers. But, beginning in 1609, scientists used telescopes to get close-up views of the Moon's surface. At first, the telescopes were small and simple. Later, they became much larger, and the views they gave became more detailed.


Telescopes showed that, on the side of the Moon seen from Earth, there are several great dark patches. Some early scientists thought that these were bodies of water. They named them maria – the Latin word for "seas."


In between the dark maria, telescopes showed much brighter regions. These were believed to be the Moon's continents, or terrae, that rose above the surrounding seas.


Most surprising of all, telescopes showed craters. These round pits with craggy, raised edges came in all sizes and peppered the Moon's surface. They looked like the remains of old volcanoes, and that's exactly what many early scientists thought they were.


Highlands, Lowlands, and Craters Galore

Today, we know that the Moon is a very dry world. There is not a drop of liquid water – let alone whole seas – on its surface. We still keep the old names for the moon's dark patches: Mare Tranquilitatis, the Sea of Tranquility; Mare Imbrium, the Sea of Rains; Oceanus Procellarum, the Ocean of Storms, and so on. But we now know that the maria are actually great dust-covered plains. They are flat, low-lying regions of land, not water.


The terrae, on the other hand, are highland regions. They contain all the Moon's great mountains and mountain ranges, and are much lighter in color than the flat maria. Many of the Moon's mountains rise 15,000 feet (about 4,200 meters) or more above the surrounding plains. The highest of all soar more than 25,000 feet (about 7,000 meters), nearly as high as the Earth's tallest peaks.


The terrae, or highlands, are also where most of the Moon's craters are found. Thousands and thousands of craters can be seen through a large telescope. Some are so small that they would fit inside your living room. Others are 100 miles (160 kilometers) or more across.


Some craters are very deep and ringed by tall mountains. Others are shallow. Mountain peaks may rise from the middle of their flat floors, and bright streaks may spread out for great distances in every direction from their edges. In fact, almost every kind of crater imaginable exists on the Moon.


Scientists used to argue a lot about how the Moon's craters had been formed. There were those who thought that these round, walled pits were the remains of dead volcanoes. Other scientists believed that the craters were scars made by rocks from space – meteorites and asteroids – that crashed into the Moon's surface a very long time ago.


Who was right? Today, scientists believe that most of the craters were formed by rocks, both big and small, that smashed into the Moon in its distant past.


Some craters are found on the smooth floors of the maria. Other strange features are found here, too. There are giant, winding canyons, called rilles, that may be hundreds of miles long, several miles wide, and more than a thousand feet deep. There are ridges, as large as 300 feet (about 85 meters) high and 20 miles (32 kilometers) wide, that wind their way across the maria plains. And there are strange domes - round bulges up to a few miles across and a few hundred feet high. Some of them have a small crater on top.


Even seen from Earth, the Moon seems to be a fascinating place. But there's so much that we can't find out using just a telescope. We can't tell what the Moon's dust and rocks are made of, and we can't do experiments to try to find out what's inside the Moon. Worst of all, we have to guess what is on the Moon's "far side" – the side that we can never see from Earth.


In the 1950s, scientists decided that to learn more about our closest neighbor in space, we would have to explore it with spacecraft. At first, the spacecraft would not carry people. Later, though, they would take astronauts on some of the most exciting missions of all time.