How Lunar Eclipses Work

So what is a lunar eclipse? These eclipses are actually remarkably simple considering how interesting and dynamic they can be. Basically, the Earth's shadow blocks most of the sunlight from directly illuminating all — or a portion — of the moon's surface.

Earth generates two cone-shaped shadows: The umbra is the dark, center shadow, and the outer, more diffused one is known as the penumbra. The penumbra encases the umbra. Both these cones are cast out from behind the sunlit side of the planet. Consequently, lunar eclipses only occur during the full moon phase (when the moon and the sun are on opposite sides of Earth). Solar eclipses are possible only during the new moon phase (when the moon plays "monkey in the middle" between the sun and Earth).

It's important to note that lunar eclipses don't occur during every full moon because of two factors. The first has to do with the variations in the orbital planes between the sun, Earth and the moon. The rotational layout of the sun and Earth forms an ecliptic plane between the two celestial bodies. The moon, however, doesn't circle Earth in line with this same plane — instead, its orbit is about 5 degrees off kilter. Any point where the moon happens to cross the ecliptic plane is called a node, and the moon must be near a node for an eclipse to occur.

The occurrence of a lunar eclipse also depends on whether the moon is in the full moon phase when it reaches a node. So the second factor to note is that as the moon orbits Earth, it doesn't complete its full cycle from new moon to new moon as quickly as it returns to the ecliptic plane, creating a disparity between the two occurrences. Three "months" affect the phases of the moon, thus influencing the development of an eclipse. The draconic month lasts 27.2 days and involves the time between the moon's upward passage through a node and back again. The 29.5-day synodic month includes how long it takes the moon to go through all its phases, from new moon to new moon. As Earth travels in its orbit, the moon takes longer to catch up and get back to its original spot in reference to the sun. The anomalistic month, how long it takes the moon's elliptical orbit to bring it from its closest position to Earth (perigee) to its farthest (apogee) and back again, affects the appearance and duration of an eclipse.

These three months work in tandem to create the saros cycle — a pattern discovered by ancient civilizations that can help determine when, where and how a lunar eclipse will appear. Each saros cycle lasts about 6,585 days.

There are three types of lunar eclipses. Penumbral lunar eclipses are difficult to see and occur when the moon grazes or passes through the penumbral shadow only. A partial lunar eclipse occurs when a portion of the moon is obscured by the umbra. The moon fully enters the umbral shadow during a total lunar eclipse.

The time the moon spends encased in the umbra is called totality. If you hear someone referring to the length of totality of a lunar eclipse, they're talking about the middle phase of the overall event. Totality doesn't include the time on either side of the umbra when the moon was passing through the penumbral or partial eclipse phases. The length of totality can range from around 20 to 100 minutes.

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 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.

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.

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 that follow, you can find out when the next one will be visible in your area.

Originally Published: Jun 18, 2008