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How Lunar Eclipses Work


Classifying Lunar Eclipses
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There are many dynamic interactions going on when the shadowy Earth eclipses her celestial companion. The color and brightness of the moon during a lunar eclipse varies according to this planet's atmospheric conditions.

None of the sun's light can slip past the bulk of the Earth to shine in the umbra. However, Earth's atmospheric particles (like volcanic ash, dust and water vapor) refract the light and send it on to illuminate the moon. The refracted, indirect light is on the redder side of the spectrum, which is why the moon often emits between a deep brown and bright orange hue. For more information on how this light trick happens, read How Light Works.

The work of French astronomer André Danjon brings us what's known as the Danjon Scale, a way to classify the lunar luminosity during an eclipse.

  • L=0: Eclipses with this level of luminosity are typically very dark. The moon will be difficult to identify against the sky. Earth's atmosphere is dense with particles at this end of the scale.
  • L=1: These eclipses are also dark, but the moon may appear with dark brown or dark gray hues. Identifying lunar features is challenging.
  • L=2: The moon will appear a deep red or rusty shade. The central umbra is dark, but may appear lighter around the edges.
  • L=3: This luminosity is characterized by a brick-red moon, with definite, possibly yellowish brightening around the edges of the umbra.
  • L=4: Eclipses at this end of the scale appear a bright copper-red or orange color. The rim of the umbra is very bright with a bluish tint. These eclipses are the result of less atmospheric density.

Lunar eclipses happen infrequently. On average, they occur up to three times a year, if at all (although three in one year is rare). About a third of these occurrences are the faint penumbral lunar eclipses. Partial lunar eclipses take place about another third of the time and are worth checking out. The rest of the time, total lunar eclipses are wowing crowds of fascinated spectators and astrophotographers.

The lunar eclipse that began Oct. 27, 2004 was fairly bright due to the relative transparency of Earth's atmosphere.
The lunar eclipse that began Oct. 27, 2004 was fairly bright due to the relative transparency of Earth's atmosphere.
Roger Ressmeyer/Science Faction/Getty Images

You can view a lunar eclipse from anywhere that night has befallen (at varying times throughout the evening), and it appears somewhat uniformly to people throughout a specific region. The amount of time an eclipse lasts depends on the type of eclipse and what part of the shadow the moon is passing through. The longest lunar eclipses may last a few hours, from start to finish.

Are you ready to pull out a lawn chair and spend the evening watching the majestic movement of the next lunar eclipse? Impressive lunar eclipses don't happen every day (or every night, to be more specific), but with the links on the next page, you can find out when the next one will be visible in your area.

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