ASTR 121 (O'Connell) Study Guide
5: 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, and the
tides (discussed in the section on the Earth). There is good
evidence that the most remarkable megalithic monument, Stonehenge,
incorporated knowledge of lunar cycles.
A. ECLIPSES (DARK SHADOWS)
Eclipses are dramatic events during which 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 disappears 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.
- 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
- 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 are, of
course, of 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).
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
the bright surface of the Sun as seen from these locations.
If the Moon were more distant, 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.
- During a total solar eclipse, when the tremendously bright surface
of the Sun is just blocked by the Moon, astronomers can observe the very
faint structures surrounding the Sun's surface (called the
"chromosphere" and "corona") which are usually 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. Another
image, showing its more detailed structure, is here.
- 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. You can also see the inner corona (which produces a thin
annular glow around the eclipsed Sun).
- 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 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 at one of its nodes AND that node
must lie directly on the line running from the Sun through the Earth
for eclipses to occur. See the diagram below (click for enlargement):
- Good alignments between the "line of nodes" and the Sun-Earth
line occur at approximately 6-month intervals. Eclipses occur
only near these times (called "eclipse seasons" and marked "favorable for
eclipse" in the drawing above). 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 took place 3500-1500 BC in several major phases. A
massive effort, involving transport of 5 ton stones up to 240 miles.
Image above shows Stonehenge as it might have appeared in the period
2000-1550 BC.
- 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 the isolated stone at lower right in the drawing above.
[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).
- 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.
- 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.
Homework:
Reading: FMW: Sec. 3.4 and 3.7
Optional: Stonehenge is only briefly discussed in the text. For more
information, see Gerald Hawkins, Stonehenge Decoded (1966) or
John North, Stonehenge: A New Interpretation of Prehistoric Man and
the Cosmos (1996).
Web links:
Last modified
January 2004 by rwo
Eclipse images copyright © Fred Espenak.
Diagrams of eclipse geometry copyright © Wadsworth Publishing Co.
Stonehenge images from various sources. Text copyright ©
1998-2003 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.
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