Worlds of David Darling > Children's
Encyclopedia of Science > The Moon > Chapter 2
THE MOON: A Spaceflight Away
a book in the Discovering Our Universe series by David Darling
2. The Moon 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.
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.
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
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
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
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.
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
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
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
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.