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The Earth and the Moon
This discussion of the phases of the Moon has (hopefully) dismissed any
misconceptions about how the relative positions of the Earth, Moon and Sun
affect how the Moon appears as well as the fundamental role that the lunar
phases play in lunar and solar eclipses.
- The Moon orbits the Earth in about 27.3 days and passes through eight distinct
lunar phases approximately every 29.5 days. The Moon also rotates about its own
axis of rotation in the same 29.5 days with the result that the same side of
the Moon is always facing the Earth, and the "far side" is not visible from
Earth.
- At each point in the Moon's orbit only half of its surface is illuminated
by the Sun, and the Moon's appearance during each lunar phase is a result of how
much of the Earth-facing side is illuminated. At the New Moon the Moon
lies between the Earth and the Sun and only the far side of the Moon is illuminated
while the Earth-facing side lies in darkness making the Moon invisible.
During the "waxing" phases an increasing portion of the Earth-facing side is visible
until the Full Moon when the Earth is between the Moon and the Sun and the entirety
of the Earth-facing side in illuminated. During the "waning" phases less and less
of the Earth-facing side is illuminated until the Moon returns to its New Moon
position.
- The Moon's orbit around the Earth is inclined 5 degrees with respect to the
Earth's orbital plane around the Sun, also known as the ecliptic plane. As a result
of this inclination the Moon passes through the ecliptic plane at two points called
the orbital nodes. A lunar eclipse, during which the Earth's shadow blocks any
direct sunlight to the Moon, occurs when the Moon is at the Full Moon phases as it passes
through an orbital node, such that the Sun, Earth and Moon are perfectly aligned.
During a lunar eclipse the Moon takes on a dull, red coppery
appearance because the Earth's atmosphere has observed the blue component of all
refracted sunlight.
- A solar eclipse, during which the Moon's shadow blocks
direct sunlight to the Earth, occurs when the Moon is at the New Moon phase as it
passes through an orbital node, such that the Sun, Moon and Earth are perfectly
aligned. This blocking of the Sun's light is possible because
the Moon's apparent or angular size is approximately the same as the Sun's angular
size as viewed from Earth. A total solar eclipse is observed from within the
Moon's umbra shadow region, a partial solar eclipse is observed from within the Moon's
pentumbra shadow region and an annular eclipse is observed from within the Moon's antumbra
or "negative shadow" region when the Moon is sufficiently far from the Earth.
Interesting Online "Lunar" Resources
The purpose of this website discussion was to capture the phenomenology of the lunar
phases in easy to understand, but accurate scientific terms. However, a more
interesting and perhaps more entertaining aspect of the lunar phases can be found in
the "mythology" that has been created around the Moon, and especially around
solar eclipses, by civilizations all across the globe and throughout history. The
online resources are some of many available that describe, in text and images, many
aspects of this lunar and eclipse mythology.
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Basics of Celestial Motion. Copyright 2006
S. E. Scruggs
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