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The Earth and the Moon
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In the previous section the alignment of the Sun, Earth and Full Moon
in the Earth’s orbital plane was shown to result in a lunar eclipse,
a phenomenon during which the Earth blocks all direct sunlight to the
Moon. With direct sunlight blocked the only sunlight that reaches the
Moon is light that is “bent” or refracted around the Earth’s atmosphere
giving the Moon a dull, reddish appearance. In this section the
discussion turns to the solar eclipse, a phenomenon during which a
similar alignment of celestial bodies occurs with the Sun, the
New Moon and the Earth aligned in the Earth’s orbital plane as
shown in the figure. Recall that the New Moon is defined as that
lunar phase during which the “far side” of the Moon is illuminated
while the Earth-facing side lies in darkness.
| Celestial Alignment During Total Solar Eclipse |
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Globe image courtesy of
GraphicMaps.com |
| Total Solar Eclipse |
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Eclipse image courtesy of
Wikipedia |
As with lunar eclipses, solar eclipses can occur only when the
Moon’s inclined orbit around the Earth passes through the plane
of the Earth’s orbit around the Sun at one of two orbital nodes.
If the Moon is in its New Moon phase as it passes through a node the Sun, Moon and Earth will align causing the Moon
to block direct sunlight to part of the Earth’s surface. Notice in the
figure above that the Moon’s darkly shadowed umbra and its partially
shadowed penumbra reach the surface of the Earth.
Someone positioned
within the Moon’s umbra during this alignment will witness a total
solar eclipse during which the Moon’s apparent size completely blocks
the Sun’s rays and only the Sun’s corona is visible.
The figure on the left was taken during a total solar eclipse as
seen from France in 1999. For someone positioned within the Moon’s
penumbra the celestial alignment is witnessed as a partial solar
eclipse during which the Moon only partially blocks the Sun rays.
The image on the right was taken during a partial solar eclipse as
seen from Florida in 2001.
The Moon's Angular Size
Notice in the discussion above that emphasis was placed on the role
of the Moon’s “apparent” size in a solar eclipse. The apparent or
angular size of a celestial body is the size or diameter that the
body appears to approximate in the sky expressed in breadth of
degrees. The angular or apparent size of a celestial object is
related to its true size or diameter and its distance from the
Earth by the following equation:
Something very interesting occurs when the Sun’s and Moon’s
respective true diameters and distances from Earth are plugged
into this equation. The Sun’s true diameter is approximately
1.39 million km and the Moon’s true diameter is 3,476 km. The
Sun’s average distance from the Earth is 149.6 million km, and
the Moon’s distance from the Earth varies between from 356,400 km
at perigee (closest approach to Earth) to 406,700 km at
apogee (farthest distance from Earth) during its orbit around
the Earth. Based on these numbers the Sun’s angular or apparent
size is approximately 0.53 degree. At its closest approach to
the Earth the Moon’s angular or apparent size is 0.56 degree, while
at its farthest point the Moon’s angular size is 0.49 degree. This
means that the apparent Moon’s size in the sky is approximately the
same as the Sun’s! As a result the Moon is able to block the Sun’s rays
very effectively when the three celestial bodies are aligned during
a solar eclipse.
Annular Eclipses
But these numbers reveal another more interesting facet to the solar
eclipse phenomenon. Notice that when the Moon is at apogee or its
farthest distance from the Earth the Moon’s angular size of 0.49 degree
is actually smaller than the Sun’s angular of 0.53 degree. When the Sun,
Moon and Earth align for a solar eclipse with the Moon near its farthest
point from the Earth the result is called an annular solar eclipse
as illustrated in the figure below.
| Celestial Alignment During Annular Eclipse |
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| Annular Eclipse |
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Eclipse image courtesy of
Wikipedia |
When the Moon is at a sufficiently great distance from the Earth the
Moon’s shadow includes a third region called the antumbra. The
Moon’s antumbra begins where the umbra ends, is bordered on both sides
by the penumbra and is sometimes called the “negative shadow”. The
antumbra is created because the Moon’s angular size of 0.49 degree
is not large enough to obscure the entire disc of the Sun at 0.52
degree.As a result sunlight from a ring or annular region around
the Moon reaches those parts of the Earth lying within the Moon’s
antumbra and an annular eclipse is observed. The photo on the right
is an example from NASA’s archives. An annular eclipse,
therefore, requires a very special alignment of the Sun, Moon and
Earth with the New Moon passing through an orbital node while near
apogee or at some sufficiently large distance from the Earth.
The final section summarizes key facts about the phases of the Moon
and the celestial motion of the Earth, Moon and Sun. In addition a
a number of interesting online resources are suggested for further
study of the Moon.
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Basics of Celestial Motion. Copyright 2006
S. E. Scruggs
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