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The Earth and the Moon

Phases of the Moon - A Popular Misconception Relative Positions of Earth, Sun and Moon The Earth's Shadow and Lunar Eclipses The Moon's Angular Size and Solar Eclipses Phases of the Moon - Understood!
Lunar Resources

Phases of the Moon - The Earth's Shadow and Lunar Eclipses
References: [Wikipedia: Lunar Eclipse, Robbins and Jeffreys 1988]

NOTE: Some Images May Not Appear in Internet Explorer

The popular misconception introduced in the opening section held that the lunar phases must be caused by the Earth’s shadow falling across the face of the Moon. In the previous section, however, the Moon’s changing appearance was shown to be a result of how much of its Earth-facing side was illuminated by the Sun during each phase of the lunar orbit. The Full Moon phase is shown to occur when the Moon is positioned on the opposite side of the Earth from the Sun, and its Earth-facing side is fully illuminated. Since it is often very difficult to disengage people from their favorite misconceptions it is necessary at this point to address two very logical questions:

  • Doesn’t the Earth block some degree of sunlight and cast a shadow?
  • If the Earth does cast a shadow does it ever affect how the Moon appears?

The Earth's Umbra and Penumbra

The answer to the first question is, yes, like any other solid object the Earth casts a shadow when illuminated. In astronomy the Earth’s shadow is typically characterized by two regions, the umbra or dark inner portion of the shadow, and the penumbra or outer portion of the shadow. (A third region called the antumbra is discussed a little later.) The diagram below illustrates the Earth’s shadow regions.

The Earth's Shadow Regions
Illustration courstesy of Wikipedia
Globe image courtesy of GraphicMaps.com

Shadows Cast by Backlit Book
Book shadow cast by backlighting
The lines drawn tangent to the Sun and Earth depict how the Earth’s umbra and penumbra are formed. The umbra corresponds to that conical area on the opposite side of the Earth that is darkest because no direct sunlight reaches this area. The penumbra corresponds to those regions that border the umbra that are only partially illuminated by direct sunlight. The picture on the right illustrates how a book, standing on a desk and backlit by light sources, would cast similar darkly shadowed and partially shadowed regions.

The Moon's Tilted Orbit

The second question to address is whether the Earth’s shadow ever affects how the Moon appears. The answer is yes, the Moon sometimes passes into the Earth’s umbra causing a lunar eclipse during which the Moon appears to be a reddish or coppery color. Although the Moon goes through a complete set of lunar phases at least once a month, lunar eclipses happen no more than two or three times a year. The reason these eclipses do not happen more often is that the Moon’s orbit around the Earth is actually tilted about five degrees from the plane of the Earth’s orbit around the Sun as shown in the figure.

5o Tilt of Moon's Orbit
Moon's 5 degree tilted orbit
Globe image courtesy of GraphicMaps.com

Twice during every lunar orbit the Moon passes through the Earth’s orbital plane at two points called the orbital nodes . During most months the Full Moon occurs when the Moon is inclined from the Earth’s orbital plane as shown in the second figure.

Celestial Alignment without Lunar Eclipse
No Eclipse Full Moon not in umbra
 
Celestial Alignment During Lunar Eclipse
Eclipse Full Moon in umbra

However when the Full Moon occurs as it passes through an orbital node and into the Earth’s umbra and orbital plane, direct sunlight from the Sun is blocked resulting in a lunar eclipse. During a lunar eclipse the only sunlight the Moon receives has been “bent” by the Earth’s atmosphere and depleted of its blue component, with the result that sunlight reflected off the Moon gives it a dull reddish appearance. The photographs below of the Moon were taken during two lunar eclipses that occurred in 2003.

Lunar Eclipses Photographed in 2003
Two Lunar Eclipses in 2003
Eclipse images courtesy of Wikipedia

With an understanding of how and why lunar eclipses occur firmly in hand, it is now time to discuss solar eclipses . While a lunar eclipse occurs when the Full Moon passes into the Earth’s orbital plane, a solar eclipse occurs when the New Moon passes into the same plane aligning itself directly between the Sun and the Earth. As will be discussed in the next and final section, there is more to the solar eclipse “story” than just the relative positioning of the Earth, Sun and Moon – now the relative sizes of these celestial bodies will play a role.


Phases of the Moon - A Popular Misconception Relative Positions of Earth, Sun and Moon The Earth's Shadow and Lunar Eclipses The Moon's Angular Size and Solar Eclipses Phases of the Moon - Understood!
Lunar Resources

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