29:50 Modern Astronomy
Fall 1999
Lecture 34 ...November 17, 1999
The Moon- Part 1:

THE MOON

Let's start with the discussion of the first astronomical object, which is the Earth's Moon. The Moon is the closest astronomical object, so in many ways we have the most detailed information. It is also the only astronomical object on which human beings have set foot. Finally, the Moon is in many ways the Rosetta Stone for our understanding of the history and physical processes involved in the formation of the Solar System.

Just the facts, Ma`am: Let's start with the physical characteristics of the Moon. These are not particularly exciting, but are necessary for knowing exactly what we are dealing with. As is always true in the physical sciences, numbers are a key to understanding the nature of objects.

• Sidereal Period: 27.3 days (period of phases 29.5 days...Why are they different)
• semimajor axis of orbit: 384,000 kilometers
• eccentricity of orbit: 0.055 (explains why solar eclipses vary; a 10 percent different in distance of Moon.)
• inclination of orbit of plane of ecliptic: 5 degrees.
• Diameter: 3476 versus 12756 kilometers for Earth. (First unveiling of the famous picture).
• Mass: kg (0.0123 )
• Density: 3340 kg/m versus 5520 kg/m for the Earth.

Why are we interested in the density?: because it tells us what materials are made of. When we get to exotic areas of astronomy, it will even tell us what the laws of physics are. Let's see some examples:

• water: 1000 kg/m
• rock:
• air: 1.29
• brass: 8600
• steel: 7830
• lead: 11300
• gold: 19300

Thus the Moon has a mean density comparable to that of rock, whereas the Earth has a density intermediate between rock and metal. For an exercise, calculate the mean density of the Sun and see where it fits in!

The final attribute of interest about the Moon is the fact that it has no atmosphere. This despite the fact that it is at the same distance as the Earth, and about the same temperature. Why is that? It turns out you could figure it out from information presented already, but it would be hard. Later in the semester I will deal more with the existence and characteristics of atmospheres in the Solar System.

The surface features of the Moon. Because of the proximity of the Moon, we can tell a lot about it from telescopic observations, even with a small telescope.

Map of Moon from Canadian Astronomical Society. This is a fairly low resolution picture, but it is nicely drawn and shows some important features. You can see some smooth dark areas, and some lighter areas. These are detectable with the naked eye from the Earth as the features in the Man in the Moon. The scientific terms for these are the Maria (Latin plural of Mare for seas), and Terrae which is the Latin of lands.
We now know that the Terrae are higher than the Maria by a few hundred meters. The Sea of Tranquillity is where the first manned landing was in July 1969.

The feature which is most conspicuous in a telescopic observation, but not detectable with the naked eye are the famous lunar craters. These are big circular holes in the ground. Some of these are almost prominent enough to see from the ground with the naked eye; during the waning gibbous moon you can really see Copernicus if you know where to look and what to look for.

Craters can be extremely large; Clavius has a diameter of 220 kilometers (more than 100 miles, and its depth is several kilometers. There are five craters with diameters greater than 200 kilometers. These craters often have central mountains. As you probably know, craters were formed by impacts of giant meteors on the Moon's surface in the past. This much was figured out by the 1960's from telescopic observations from Earth. These observations were inadequate to tell how long ago this happened. The answer to this required the Apollo moon landings and was quite a surprise.

Some of the craters have bright patterns of rays coming out from them. The best examples are the crater Tycho, near the South Pole of the Moon, Copernicus, and Tycho. The technical term for these is ejecta blankets; when the meteor that struck the Moon at this point made its impact, the resulting explosion threw a large amount of debris above the Moon, which then fall back. These ray patterns are fun to see in a small telescope, and were one of the strong indicators that the craters were formed by impacts.

There are also mountain ranges on the Moon, indicated by the letters on this map, such as the lunar Apennines. An astute observer might note that these mountains seem to form partial arcs surrounding the Maria, and that they look like huge craters which have been partially filled in. We now know that this is the case. Thus there is a fundamental difference between mountains on the Earth, which are thrown up by collisions of tectonic plates, and those on the Moon, which are the remainder of rims of ancient megacraters called Impact Basins.

A final type of feature to be noted is called a Rille, and looks like a dry river or creek bed. In the pre-Apollo days there was considerable excitement about these. There was debate as to whether they had been formed by flowing water or lava. If the latter, it might indicate that conditions could have been suitable for life at an earlier point in the Moon's history.

Look at the pictures on p188 of your textbook, which give very nice views of the crater Copernicus and the Lunar Apennine mountains.

Now let's look at some pictures of these features on the Moon, so you have an idea of what I have been talking about. I also strongly encourage you to go up to the roof and look through the telescopes there.

Those who are interested can check out lunar atlases in the Physics Library.

In the 1960's came a crash program to land people on the Moon before the end of the decade. This was a dumb political decision, but it led to some exciting scientific results. One of the reasons it was dumb is that the late 1960's corresponded to the time of Solar Maximum when the Sun was at a high point of its 11 year cycle of sunspots, solar flares, etc. There were some reconnaisance missions, first of all the Ranger series of spacecraft which made crash landings, taking pictures up to the time of crash. There was then the Surveyor program, which had landers which returned pictures of the surface and made soil measurements. Next came the Lunar Orbiter pictures, which led to a planet-wide, high resolution survey of the Moon.
Lunar orbiter picture of (I think) the crater Copernicus (Chapter 16, frame 11659).

The finale was a set of six lunar landings between July 1969 and December 1972. We have not been back since, and there are no plans to have a manned expedition to the Moon. Nonetheless, these expeditions returned a lot of fascinating information, and were high drama. This Fall NASA will launch a geological surveying mission to the Moon to better understand its geological history.
Chapter 55 of Video Disk with Ranger Landing and Apollo 17, the last Apollo mission.

The most valuable scientific information came from analysis of the rock samples returned from the Moon to the Earth. I recommend seeing these sometime if you are at the Smithsonian Museum of Natural History on the Mall in Washington.

Next time I will deal with what we learned from analysis of these rocks, in particular from the age of formation.

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