ASTR 1210 (O'Connell) Study Guide 13


Apollo 17 landing site in Taurus-Littrow Valley

After the Sun, the Moon is the most important extraterrestrial object. Seen in a small telescope, the Moon's surface is strange and remarkable --- completely un-Earthlike and suggesting how poor our intuition about the character of other planets is likely to be. It features fantastic topography, with towering mountain peaks, thousands of craters, and deep valleys which have never been subject to weathering. Because its surface contains a fossilized history of the early solar system, the Moon is a goldmine of critical astrophysical information.

The Moon is also an accessible extraterrestrial body --- not easily accessible, but we were able to reach it in the 1960's. During the Apollo program, twelve humans made six landings on the Moon and returned invaluable rock and soil samples and reams of other data.

In Lunar Motions and Their Consequences we described several of the important phenomena associated with the Moon that are easily visible without telescopes in the sky. In this Guide, we consider the physical nature of the Moon itself and what we have learned about its history. We also describe another phenomenon -- the tides -- familiar to humans since prehistoric times, whose association with the Moon was not explained until the work of Issac Newton in the 17th century.

A. General

Full Moon Composite

B. Main Terrain Types

The two main lunar terrain types are highlighted in the image of the full Moon at right. Click for more images and information.

The maria ("seas") make up the conspicuous dark pattern we see as the "Man in the Moon." They were so named by early telescopic astronomers because they looked like large bodies of water (but in actuality they are completely dry). Maria are mainly confined to the near-side of the Moon. The far-side consists almost entirely of highlands regions. Click here for a photomosaic of the lunar far-side.

C. Impact Topography

The Moon's surface testifies to the fact that the surface topography of most rocky planets is shaped largely by brutal impacts of asteroids, planetesimals, and comets. Although they should have realized this earlier, astronomers have only widely accepted the importance of impacts for the last 60 years.

On the Moon, and most other solar system bodies with hard surfaces, the impact history is preserved in the form of extensive cratering. But impacts are responsible for most of the other major lunar surface features as well, including the mountains and maria.

The surface density of craters (i.e. number per square km) can be used to crudely age-date different regions on planetary surfaces:

LOLA Map E Limb

Topographic Map of East Limb of Moon (Lunar Orbiter Laser Altimiter)
The "near-side" is at the left; the "far-side" on the right. Click for the full image.

D. Topographic Features

Click for illustrations

E. The Apollo Missions

NASA's Apollo program was the US response to the USSR's breakthrough achievements in launching the first artificial satellite ("Sputnik," October 4, 1957) and the first human into space (Yuri Gagarin, 1961). In May 1961, President Kennedy committed the US to send Americans to the surface of the Moon within 9 years.

Apollo was a triumph of human courage, skill, and organization. The lunar landings were of great scientific value in learning about lunar geology and surface history and represented tremendous steps forward in human exploration.

After 1976, there were no further spacecraft studies (human or robotic) of the Moon until the 1990's. A new NASA program intended to send humans back into deep space revived US scientific investigations of the Moon, and other nations (including Japan, India, and China) have recently begun lunar exploration. This site provides a complete summary of spacecraft sent to the Moon.

Apollo 15 landing site and rover

F. Geology of the Moon

The Apollo program returned rock samples and a wealth of other information about the geology of the Moon.

G. Interior

Origin of Moon

H. Origin

I. History

J. Tides and Their Orbital Effects

Apollo 11 Lunar Module returns to the Command Module
after the first human landing on the Moon (July 1969)

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Last modified December 2020 by rwo

Cratering rate drawing by Barbara Cohen (Univ. of Tennessee) . Lunar formation drawing by Toby Smith (Univ. of Washington). Tide diagram © 2006 by Encyclopedia Britannica. Text copyright © 1998-2020 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.