ASTR 1210 (O'Connell) Optional Reading


The lunar phases are only the first of the unusual but easily observed cyclic phenomena associated with the Moon. Others are eclipses, which we discuss here, polar precession (covered in Study Guide 5), and the tides. All ancient cultures must have been aware of these (except precession, which is too subtle to detect without careful observation). Although the Greeks were the first to arrive at the explanation of them, others left traces of their fascination with them. 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 types 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 in the middle of a normal day 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.

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

The solar atmosphere during total eclipses.
(Left) Coronal streamers during the February 1998 eclipse (A. Gada, processed by J. Lodriguss).
(Right) Chromosphere showing (red) prominences during August 1999 eclipse (L. Viatour).


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" ("giant stone") monuments surviving from prehistoric times, older than about 500 BC, in northern Europe. (Click on the thumbnail at right for information on megalithic sites in Great Britain and Ireland.) These consist mainly of standing stones, dirt mounds and ditches, and evidence of former wooden structures, now long decayed. Four examples are shown here.

Very little is known about the peoples who built these. They left no records of any kind. 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 some of them, including Stonehenge.

Construction at Stonehenge took place ca. 3100-1500 BC (over 1500 years!) in several major phases. This was a massive effort, involving, for instance, 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. Here are some more views of the modern Stonehenge.

The current-day structure consists of a series of concentric circular ditches, banks, and post-holes with a number of large standing stones clustered in the center and a few at the periphery. A long straight "Avenue," marked by banks, runs north-east from the main structure and ultimately connects with an ancient settlement complex several km away.

Astronomical alignments: there are both solar and lunar alignments built into Stonehenge.

Stonehenge is situated at a unique latitude: where the lunar and solar sight lines just described cross at right angles. It is possible that the Stonehenge people chose this site for the monument because of this fact and that this is the reason they invested so much effort (estimated at 1.5 million person-days) in building it.

Before solar and lunar orientations could be built into Stonehenge, its planners must have observed the sky for many cycles---in the case of the Moon, many times 19 years. And they needed a method to pass the information on from one generation to the next (the lifespan then was only ~30 yrs). No stone, paper, or other forms of records have been found.

The most obvious stone structures (the massive trilithons, see below) were constructed last but have no clear astronomical significance.

Web links:

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Last modified May 2018 by rwo

Some eclipse images copyright © Fred Espenak. Diagrams of eclipse geometry copyright © Brooks-Cole Publishing Co. Stonehenge images from various sources. Text copyright © 1998-2018 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.