Characteristics and Origins of the Solar System

Lecture 11

September 24, 2001

The Moon

The Moon is an important object because we have been able to make detailed observations of it, and have also made measurements on it surface and obtained samples to study in Earth laboratories. For this reason, we have learned a great deal about the Moon, and have been able to obtain information which has greatly assisted us in figuring out the other objects in the solar system.

Keep an eye on this object as it moves through the evening sky during the next week.

Let’s start with a list of its basic properties.

1. The sidereal period of its orbit around the Earth is 27.5 days. The period of the phases, i.e. the time between successive full moons, is 29.5 days. Be sure you understand why these two periods are different.

2. The semimajor axis of the Moon’s orbit is 384,000 kilometers. This is a distance equal to 60 Earth radii.

3. The eccentricity of the Moon’s orbit is 0.055, so there is a 10 % difference between the distance of closest approach and greatest distance.

4. The diameter of the Moon is 3476 kilometers, versus 12756 for the Earth. The Moon is just over one quarter the diameter of the Earth. >>>> transparency with relative sizes of solar system objects.

5. The mass of the Moon is only 1.23 percent that of the Earth, so it is a much less substantial object.

6. Its density is 3.34 grams per cubic centimeter, as compared with 5.52 for Earth. The mean density of the Moon is close to that of rock, where that of the Earth is intermediate between rock and metal. This simple numerical fact about the Moon indicates that it does not have a massive metallic core like the Earth.

Another attribute of interest about the Moon is that it has no atmosphere.

Surface Feature of the Moon

Even observations with a small telescope reveal a lot about the surface geography of the Moon. Recognition of these terms will help a lot in appreciating the enormous discoveries of the Apollo program. Take a look at page 179 for reference.

• Maria (plural of Mare) are smooth, dark areas visible to the naked eye from Earth.
• Terrae (plural of Terra) are the bright areas that we know are of higher elevation. They are sometimes called the lunar highlands for this reason.
• Craters are big holes in the ground. These are all over the Moon, and there are five of them that are bigger than 200 kilometers in diameter. We now know that they were caused by impacts from huge meteors and asteroids (more later). One of the arguments that helped people come to this conclusion is the fact that some of them have rays coming out of them, which correspond to the ejecta blankets of matter which was thrown up into space and then settled back to the Moon.
• Mountains The Moon has mountain ranges, the most prominent of which is the lunar Apennines. In contrast to mountains on Earth, these are the edges of huge craters called impact basins.

Next time we will look at pictures of these features and discuss what was learned in the Apollo program of the 1960’s. Also look at your textbook for pictures and descriptions of these features.