ASTR 121 (O'Connell) Optional Reading


The lunar phases are only the first of the unusual but easily observed phenomena associated with the Moon. Others are eclipses, which we discuss now, polar precession (covered in Study Guide 5), and the tides (discussed later in the section on the Earth). There is good evidence that the most remarkable of the ancient megalithic monuments, Stonehenge, incorporated knowledge of lunar cycles.


During an eclipse either the Sun or the Moon appears to "go out." Both can be dramatic events, for properly situated observers on Earth. In particular, total solar eclipses have tremendous psychological impact because the Sun seems to disappear with no guarantee of return. The picture at the top of the page shows a series of photographs taken before, during, and after a total solar eclipse.


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:


Stonehenge, on the Salisbury plain in south-central England, is the best known of thousands of "megalithic" monuments surviving from prehistoric times in northern Europe. (Click on the thumbnail at right for information on megalithic sites in Great Britain and Ireland.) Very little is known about the people who built these. Though scholarly debate has raged over the purpose of such structures, there is good evidence that their builders incorporated astronomical knowledge of the Sun, Moon, and bright stars in many of them, including Stonehenge.

Construction at Stonehenge took place 3500-1500 BC (2000 years!) in several major phases. This was a massive effort, involving transport of 5 ton stones up to 240 miles. The image above shows Stonehenge as it might have appeared in the period 2000-1550 BC.

The current-day structure consists of a series of concentric circular ditches, banks, and post-holes with a number of large stones clustered in the center and a few at the periphery.

Astronomical alignments: both solar and lunar.

Web links:

Back to Study Guide 5 Guide Index

Last modified May 2010 by rwo

Eclipse images copyright © Fred Espenak. Diagrams of eclipse geometry copyright © Brooks-Cole Publishing Co. Stonehenge images from various sources. Text copyright © 1998-2010 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.