ASTR 121 (O'Connell) Optional Reading
THE MOON, ECLIPSES, AND
STONEHENGE
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.
A. ECLIPSES (DARK SHADOWS)
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.
- 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 button 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 4
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.
- As viewed from the Earth, the Sun and the Moon have nearly the
same angular size, about 0.5 degree.
They do, of course, have vastly different intrinsic sizes, and
this similarity of apparent size is merely a coincidence
deriving from the present size of the Moon's orbit (which is
continually changing slowly).
- Total and Partial Solar Eclipses
This coincidence of sizes permits the occurrence of total solar
eclipses on the Earth, in which the Sun's light is totally blocked
by the Moon. In this circumstance, the umbra of the Moon's shadow
touches a small part of the surface of the Earth, as shown in the
Solar Eclipse Geometry diagram above. The Moon just barely blocks all
the bright surface of the Sun as seen from these locations. As the
Moon moves in its orbit, the umbra sweeps across the Earth's surface.
If the Moon were more distant from Earth, such that the Sun appeared
much larger than the Moon, there would never be total eclipses. And
if the Moon were nearer, so that it appeared much larger than the Sun,
eclipses would be less interesting aesthetically and
scientifically---though they would last longer and be more
frequent.
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, you 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.
- 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.
- By contrast, 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. (Residual sunlight
passing through dust layers in Earth's atmosphere often tints the Moon
bloody red during a lunar eclipse: click on thumbnail at right for a
better view.)
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.")
- 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 near enough 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, in the figure above, 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).
There are typically 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 18 years.
- 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 saros 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" 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.
- Solsticial Alignments: A line from the center to the
"Heelstone" points to sunrise at the summer solstice (the northernmost
sunrise 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 thumbnail
at right for a plan of the present-day Stonehenge.]
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 on drawing above. See the plan. 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 (1st century BC) refers to a "19 year" cycle associated
with Stonehenge and the Moon; almost certainly the lunar nodal
cycle.
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), through this idea has since been
discarded.
- Stonehenge is situated at a unique latitude: where the lunar and
solar sight lines just described form a
rectangle. It is possible
that the Stonehenge people chose this site for the monument because
of this fact.
- 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 (lifespan 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:
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.
Astronomical alignments: both solar and lunar.
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