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© Copyright 2001, Jim Loy
The Moon (on the left is a NASA photo of the crater
Copernicus) is the fifth largest moon in the Solar System. Most moons are much
smaller than their planets. The Moon, however, is relatively large, about 1/4
the diameter of the Earth, making the Earth-Moon system a double planet. Tiny
Pluto and its moon Charon are also a double planet; Charon is about half the
size of Pluto.
The moon looks pretty bright when it is full. But it is really made of very dark material. The seas (darker areas, which are actually lava planes) are about the brightness and color of asphalt. The albedo (percent of light reflected) of the moon varies from about 5% to about 18%. The earth is much much brighter.
The far side of the moon (which we never see) is sometimes called the dark side of the moon. That side gets plenty of sunlight, about as much as the near side. But, it would be slightly darker there, as it never gets light reflecting off the earth. So, I guess it makes some sense to call it the dark side.
Map of the Moon
Above is a map of the moon, modified slightly from a map (a mosaic of photographs) from Jet Propulsion Laboratory. I have labeled some of the prominent features, drawn the two vertical lines, and lightened the map. This map gives a good view of the dark maria (seas), which are actually lava planes, mostly on the near side. Each sea is a huge impact crater or series of overlapping impact craters. Mare Tranquilitatis is where men first walked on the moon. The crater Tycho is fairly prominent because of it's bright rays of ejecta, radiating in all directions.
As you can see, the near side of the moon looks much different from the far side. There is a surprising lack of seas on the far side. Apparently, most of the lava on the near side came from one or two asteroid impacts, probably when Mare Imbrium was created, as that crater is about the right age.
The Moon's Rotation
On the left we see a NASA photo of the moon. We always see the
same side of the moon. So, it would seem that the moon does not rotate (spin on
its axis), right? A surprising number of people are very insistent that the
moon does not rotate.
A person standing in one spot on the moon would see the
earth in the same part of the sky (directly overhead, for the person in the
diagram on the right), unmoving. But, he/she would see the sun appear to move
around the sky in about 29 earth days. The person would correctly deduce that
the moon has a solar day of about 29 earth days. This person would also see
that the stars appear to move around the sky in about 27 earth days. Again, the
person would correctly deduce that the moon spins on its axis once in about 27
earth days.
The moon definitely rotates. It just doesn't look like it, from where we stand.
The moon rotates at a constant rate. But, it's distance from earth, and speed in its orbit, varies quite a bit. This means that we can see a little ways around the edge of the moon. And the rotational axis of the moon is tilted somewhat; so we can see a little beyond the Moon's north and south poles. So, over a period of time, we can see about 57% of the moon, from earth.
In the movie, Crocodile Dundee II, Paul Hogan travels back to Australia. In one scene, they show the moon. And it looks just like the moon looks in America. It should be upside down (South on top).
The rotation of the moon is, to some extent, a matter of definition, as it is not rotating with respect to us observers here on earth. But the above definition (with the moon rotating) is more consistent with other rotations in astronomy and geometry. An earth-centered universe is a myth which was disproved long ago.
Which Direction Does The Moon Move?
The moon rises in the east, and sets in the west. It moves from east to west, right? If you think about it, you should realize that much of the moon's apparent motion is due to the spin of the earth. The moon appears to go around the earth about once a day. And, you probably know that the moon actually takes about a month to go around the earth. That's where we get the word "month."
Why don't you perform a simple observation. Look at the moon, some night, and see where it is with respect to the stars (or with respect to the sun). You don't need to know the stars very well, but you probably want to ignore the planets (Venus, Jupiter, and Mars, mainly, as their motion complicates this observation). Night after night, the moon moves against the background of stars. See if you can tell which way the moon is moving, against the background of stars.
Another simple observation would be to keep track of when the moon rises (roughly) or sets from night to night. Does it rise later each night, or earlier, or what?
You will see that the moon moves west to east, fairly rapidly. See the following diagram:
Which way does the earth spin?
Well, the stars, sun, moon, and planets all seem to be moving east to west. So
the earth is spinning in the opposite direction, west to east. Hey, the moon is
also moving west to east, coincidence? Well, the earth is moving around the sun
from west to east. In fact all nine of the planets are moving around the sun
from west to east. And, all of the planets, except Uranus (which is lying on
its side) and Venus (which is spinning slowly backward), are rotating from west
to east. And most of the moons, of the various planets, are orbiting their
planets from west to east. And most of the asteroids are orbiting the sun from
west to east.
It's not a coincidence. The Solar System started out as a cloud of dust and gas, and it was rotating from west to east. That's why most of its parts are still rotating and orbiting from west to east.
The Distance To The Moon
Above is a picture of the earth-moon system, roughly to scale. When I was a child, I memorized that the average (mean) distance to the moon was 238,857 mi. Now it's 238,856 which is probably more accurate. The moon's orbit is elliptical, and the distance varies quite a bit (it varies from 225,745 mi. to 251,967 mi.).
Now they actually know the distance to the moon to
within a few inches. Apollo 11 astronauts left an array of corner mirrors on
the moon. A corner mirror is three mirrors arranged like a corner of a box, all
at right angles to each other. Such a mirror reflects light directly back
toward the source of the light. Scientists shine a laser at the moon, and
measure the time it takes to reflect back. They can measure this time to within
a tiny fraction of a second. So they deduce the distance to the moon (at any
given time) to within a few inches.
The speed of light in a vacuum is a constant. But it is not known very accurately, not anywhere as accurately as the distance to the moon is known. What this means is:
So essentially, they measure the distance to the moon in fractions of a light-second, not in inches or meters.
These scientists really measure the distance from their telescope to the Sea of Tranquility. And, if they want to know the distance between the earth and the moon, they have to adjust for the distances which these places are from the centers of the earth and the moon. And these are not known with tremendous accuracy, either.
All the great accuracy of their measurements is not lost, however. Mainly, these scientists are studying the way in which the distance changes over time. This helps them predict the orbit of the moon with great accuracy.
The moon is slowly getting farther from the earth. This is a result of the earth losing energy because of the tides. The moon gains energy. And the earth-moon system does not gain or lose any energy. I have recently found out that the orbit of the moon probably oscillates from near to far and back to near, over and over. The distance is currently increasing about 3.74 cm/year.
Center of gravity of the Earth/Moon system
The Moon goes around the Earth, right? Actually, both the Earth and the moon go around the center of gravity of the Earth/Moon system. This center of gravity (see the fulcrum of the balance in the picture above) is actually inside the earth. It is closer to the surface of the Earth, than it is to the center of the Earth. But it is fairly accurate to say that the Moon goes around the Earth.
Phases of the Moon
The movement of the Moon in its orbit causes
the phases of the Moon. These are called "new moon" (when solar eclipses
occur), "first quarter" (the right half of the moon, as we look at it, is in
sunlight), "full moon," and "last quarter" (left half is in sunlight). The
phases in between are described as either crescent (less than half of the
visible side is in sunlight), or gibbous (more than half is in sunlight). In
the diagram on the left, just imagine the phases that the little guy on the
Earth (on the left) sees, as the moon orbits the Earth. I have drawn an
animation, above right.
Earth Shine
Last night I saw a great example of earth shine. Earth
shine, of course, has nothing to do with moonshine (the alcoholic beverage).
Earth shine is when you can see the dark portion of the moon, when it is a
faint gray instead of its normal pitch black. This is caused by the sunlight
shining on the earth and being reflected to the moon and lighting up the dark
portion. This light is reflected back to earth, where we see it.
Angular diameter
As seen from the Earth, the Moon appears to be almost exactly the same size as the Sun. This means that they have about the same angular diameter. This coincidence makes eclipses of the sun especially spectacular. The Moon is currently moving away from the Earth, and so it will appear to get smaller as the millenia go by.
Here are some data about the Moon: