ASTR 1210 (O'Connell) Optional Reading

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THE MOON, ECLIPSES, AND STONEHENGE

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.

A. ECLIPSES (DARK SHADOWS)

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.

• There are two types of eclipses: lunar and solar. They are produced by shadows cast by the Earth and the Moon, respectively.
  • A lunar eclipse occurs when the shadow of the Earth strikes the Moon

  • A solar eclipse occurs when the shadow of the Moon strikes the Earth

• The geometry of the two kinds of eclipses is illustrated in the following diagrams. (Click for enlargements):
  
  Lunar Eclipse Geometry

  
  

  
  Solar Eclipse Geometry

  
  

  Click on the icon for a video [650 KB] of the Moon's shadow crossing the Earth during an eclipse.

• Referring to these illustrations and the diagram on Study Guide 5 concerning the phases of the Moon, we see that:
  • A lunar eclipse can only occur near Full Moon, and

  • A solar eclipse can only occur near New Moon

• The core of the shadow, where solar light is totally blocked, is called the umbra. The umbra is shown as the dark cone in the diagrams above. It is surrounded by a larger region (lighter gray above) with partial blocking, called the penumbra. Eclipses can therefore be either "total" or "partial", depending on which part of the shadow is involved.

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

Solar Eclipses

• As viewed from the Earth, the Sun and the Moon have nearly the same angular size, about 0.5 degree.
  This similarity of apparent size is merely a coincidence, a product of the present-day size of the Moon's orbit (which is continually changing slowly). The Moon and Sun have, of course, vastly different intrinsic sizes.

  This coincidence of sizes permits the occurrence of total solar eclipses on the Earth, in which the Sun's light is completely blocked by the Moon. In this circumstance, the umbra of the Moon's shadow touches the surface of the Earth, as shown in the Solar Eclipse Geometry diagram above. The Moon just barely blocks all of the bright surface of the Sun as seen from locations inside the umbra, which covers only a small part of the Earth's surface at any time. As the Moon moves in its orbit, the umbra sweeps across the Earth.
  For viewers situated in the penumbra, only part of the Sun's surface will be blocked by the Moon, producing a partial solar eclipse. Observable effects can be detected without optical aid for eclipse fractions larger than about 5%. "Annular" eclipses are produced when the Moon appears to lie entirely within the Sun's surface; this can occur when the Moon is near its apogee (farthest from Earth in its orbit).
  Partial eclipses are much less dramatic than are total eclipses because even a small fraction of the Sun's surface produces large amounts of light; however, they last longer, are more frequent, and can be seen from a larger part of the Earth's surface.
  
• At the right is a time lapse video of a total solar eclipse. The Moon crosses in front of the Sun from right to left. Just as totality is beginning, the exposure time of the camera was increased and shows the "diamond ring" effect produced by the last small uncovered parts of the solar surface.
  During a total solar eclipse, when the tremendously bright surface of the Sun is just blocked by the Moon, we can observe the very faint structures surrounding the Sun's surface (called the "chromosphere" and "corona") that are normally hidden in the glare.
  The corona is the irregular white halo surrounding the darkened Sun in the central frame of the picture at the top of this page. You can also see the inner corona in the video (a thin annular glow during totality). Another image, showing the corona's more detailed structure, is here.
  Although special telescope designs can reject much of the light of the solar surface, total solar eclipses still offer the best views of the inner solar atmosphere for ground-based telescopes, so they are very important events scientifically.

• Total solar eclipses last at most 7 minutes at a given location and are visible only in a narrow strip on the Earth's surface (see the Solar Eclipse Geometry diagram). Relatively few people experience total solar eclipses.

• If the Moon were more distant from Earth, such that the Sun appeared much larger than the Moon, there would never be total solar eclipses. And if the Moon were nearer, so that it appeared much larger than the Sun, total solar eclipses would last longer and be more frequent, though they would be be less interesting aesthetically and scientifically.

Lunar Eclipses

• Lunar eclipses can also be total or partial, depending on the exact path of the Moon through the Earth's shadow cones (see this chart of the Moon's track during a typical total eclipse).
  
• During a total lunar eclipse, the Moon will become very dim. But residual sunlight passing through dust layers in Earth's atmosphere often tints the Moon bloody red, as in the picture at the right; click on thumbnail for an enlarged view of an eclipse.

• By contrast with a total solar eclipse, a total lunar eclipse can last up to 1.5 hours and is visible from about 1/2 of the Earth's surface. Most of Earth's inhabitants (with astronomical interests) have therefore seen 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).

B. 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:

• Almost perfect alignment of the Sun, the Moon and the Earth is needed for an eclipse.

• The Earth and Sun lie (by definition) in the ecliptic plane and therefore the Moon must also be in that plane for an eclipse to occur. (Hence, the origin of the word "ecliptic.")
  If the Moon's orbit lay exactly in the plane of the ecliptic, there would be a total eclipse of the Sun and a total eclipse of the Moon at each new and full Moon, respectively, each month. Eclipses would be routine celestial phenomena, and the psychological impact of solar eclipses would greatly diminish.

• But the Moon's orbit is tilted 5 degrees out of the ecliptic plane and only crosses the plane at two points, called nodes. The Moon moves through the two nodes once each month.
  5 degrees sounds small, but remember that it is 10 times the angular diameter of the Moon.

• Therefore, the Moon must be near one of its nodes and that node must lie almost directly on the line running from the Sun through the Earth for eclipses to occur.
  • Viewed on the celestial sphere from the Earth, the node is where the Moon's celestial path crosses the ecliptic. See the diagram below (click for enlargement). Only if the Sun and Moon are both near the node at the same time can a solar eclipse occur. If the Sun and Moon are both close to the node but the alignment is not perfect, a partial eclipse will occur, as in the figure.
    

• The direction to a node on the celestial sphere (i.e. with respect to the stars) changes only slowly. Good alignments between the "line of nodes" and the Sun-Earth line therefore only occur at approximately 6-month intervals. [Equivalently, once the Sun has passed one node along its ecliptic path, it will not approach the other for 6 months.]

• The perspective drawing below shows the orientation of the Moon's orbit in 3-D space. Eclipses occur only near those times when the line of nodes points near the Sun (called "eclipse seasons" and marked "favorable for eclipse" in the drawing). On average, there are 2 solar and 2 lunar eclipses each year.
  

  
  

• Because of gravitational interactions with the Sun, the line of nodes of the Moon's orbit moves with time, taking 18.6 years to make a complete rotation around the Earth. This produces an 18-year-long sequence of eclipses called the saros cycle.

• During the cycle, solar and lunar eclipses occur roughly once every 6 months. But the dates of the eclipses change and don't repeat for about 19 years.
  The dates and paths on the Earth's surface for solar eclipses from 2000 to 2020 are shown here.

• The change in the line of nodes also produces changes in the extreme northerly or southerly positions of the Moon as seen from Earth by +/- 5 degrees. This affects, for instance, the duration of a moonlit night, so that the cycle can be recognized without necessarily observing eclipses. The saros cycle was known to many ancient astronomers.

C. STONEHENGE

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.

• Solsticial Alignments: A line from the monument center to the "Heelstone", which lies on the centerline of the Avenue, points to sunrise at the summer solstice (the northernmost sunrise of the year and the longest day of the year). The reverse points to sunset at the winter solstice. The Heelstone is a large, isolated stone lying outside the circular structures between two parallel banks. [Click on the thumbnail at right for a chart of the alignments.]
  Note that such "solsticial" orientations are not simply East-West (which is much more common in ancient buildings). The heelstone is north-east of the center of Stonehenge. A sketch of the Sun's path as it rises over the heelstone on the summer solstice as seen from the center is shown below:

  

• Lunar Alignments: The so-called "Station Stones" are four stones lying just inside the bank (labeled "SS" in the plan drawing). Lines drawn through Station Stones 92 and 93 or 91 and 94 align with the N/S maxima of the Moon's rise or set during the 18.6-year nodal revolution cycle.
  Diodorus, a Greek historian during the 1st century BC, refers to a "19 year" cycle associated with Stonehenge and the Moon---almost certainly the lunar nodal cycle described above.
  British astronomer Fred Hoyle suggested that the circle of 56 "Aubrey Holes" could have been used as an analog computer to track the motion of the node of the lunar orbit (56 years, or 3 saros cycles, is required to bring solar eclipses back to approximately the same locations on Earth's surface), though this idea has found little support.

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.

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Web links:

Espenak's Eclipse Home Page: everything you could possibly want to know about eclipses.
Video of NASA expedition to 29 March 2006 total eclipse
Total Solar Eclipse of 21 August 2017, which crossed the USA
Introduction to Archaeoastronomy (UMd)
Introduction to Stonehenge.Some of the illustrations above are taken from this site.
Stonehenge Factsheet from the Royal Astronomical Society
Stonehenge (Wikipedia)
Murder at Stonehenge (segment in the "Secrets of the Dead" PBS series).
3-D Model of Stonehenge
Recent excavations at the Stonehenge complex (Jan 2007)

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Last modified June 2021 by rwo

Some eclipse images copyright © Fred Espenak. Diagrams of eclipse geometry copyright © Brooks-Cole Publishing Co. Stonehenge images from various sources. Text copyright © 1998-2021 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.