ASTR 1210 (O'Connell) Supplement


The Moon is the most conspicuous of the denizens of the night sky, and for several nights each month completely dominates the sky, so it always was of major interest to ancient astronomers. Its motion from day-to-day against the star background is also faster (about 13 degrees per day) than those of the Sun or planets and can be followed easily without the need for complicated instruments.

Because it orbits the Earth rather than the Sun and is very nearby by planetary standards, the Moon is associated with a number of unique phenomena, some of which are quite dramatic.

The lunar phases are the most obvious of the important cyclic phenomena associated with the Moon. Others are eclipses (two types), changing positions of north/south maximum distances from the celestial equator, and polar precession, all of which we discuss here, and the tides, which are covered in Study Guide 13.

All ancient cultures must have been aware of the phases and eclipses. (Precession is too subtle to detect without careful observations, and tides are only obvious along coastlines.) Although the Greeks were the first to arrive at the explanation of them (around 500 BC), others left traces of their fascination with them.

A. Lunar Phases

Unlike the planets and stars, the Moon is perceptibly extended on the sky to the unaided eye. Throughout a given month, the Moon exhibits drastic changes in apparent shape, from crescent to round and back. The shapes are called phases of the Moon. Overall lunar brightness changes in proportion to the bright area. .

Ancient societies had many colorful supernatural explanations for the lunar phases (e.g. regular consumption and regurgitation of the Moon by a giant celestial sow). But only one of them arrived at the right answer: the Greeks, who understood the phases as early as 500 BC.

Lunar Phases
Our modern understanding of the Moon is as follows: Technical Note:


B. Polar Precession

Precession is a motion of the Earth, not the Moon, but it is partly induced by the Moon's motion in its orbit. It is a cyclical, long-period wobble in the orientation of the Earth's polar axis projected on the celestial sphere. This is shown schematically in the animation at the right.

C. Eclipses (Dark Shadows)

Eclipses are shadow effects in which either the Sun or the Moon appears to "go out" for a time. Both can be beautiful and dramatic events for properly situated observers on Earth. In particular, total solar eclipses have tremendous psychological impact because the Sun seems to disappear in the middle of an ordinary day with no guarantee of return. The sudden extinguishing of the Sun can be terrifying for people who believe the Sun is a living god if they could not predict it in advance.

The picture at the top of the page shows a series of photographs taken before, during, and after a total solar eclipse.

Two solar eclipses. (Left) Total eclipse in March 1970 from Virginia.
(Right) Annular eclipse in May 2012 from Texas (J. Thumberger).

Solar Eclipses

Lunar Eclipses

Multiple exposure of a 2008 lunar eclipse over the Southern Cascade Mountains.
The exposure time was increased during the faintest phases. (Sean Bagshaw)

D. Eclipse Prediction

The basic geometry of eclipses is simple, but predicting their occurrence and type (total, partial, annular) depends on understanding the complex nature of the lunar orbit:

Web links:
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Last modified December 2020 by rwo

Some eclipse images copyright © Fred Espenak. Lunar phase image copyright © by Antonio Cidadao. Precession and lunar phase diagrams copyright © by Nick Strobel. Precession animation by Scott R. Anderson. Diagrams of eclipse geometry copyright © Brooks-Cole Publishing Co. 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.