What …an is… Eclipse?

The Eclipsed PastThroughout the history of this earth there

have been recorded instances where the sun’s rays have been hidden for a brief time when they should otherwise not be. Ancient civilizations did not yet fully understand the circumstances that brought about this rare display. Often these events were attributed to an act of intervention from the gods and foretold some form of imminent doom.

Today we know that eclipses are one of the most awe inspiring natural events that can be witnessed by the inhabitants of this earth. They are neither scary, nor ominous. They can occur in any part of the world, and therefore are generally available for everyone to see personally at some point in their lifetime. They are more common than most comet appearances, like the famous Halley’s Comet which comes around once every seventy-five years or so. Eclipses, while amazing to watch, are not as frequently occurring as sunsets, or shooting stars, two other natural wonders.

Anatomy of an EclipseAn eclipse happens when a light source

is either obscured for the viewer by another object, or when the shadow of the viewer is

projected upon another object. Eclipses are most commonly referred to as being in relation to the Sun, Earth, and the Moon, but can certainly happen elsewhere as well.

Because of the 23 ½ degree tilt of the earth, the moon does not often cross directly between the earth and the sun or fall directly behind the earth from the sun. The moon follows the tilt of earth’s axis like the hammer, or weighted ball at the end of the cable that is thrown Olympians. All of the planets orbit on an ecliptic, or tilted plane to some degree, and any moons that those planets might have will do much the same thing.

Through the course of one year there are at most only seven instances out of the twenty five or so positions where the moons orbital path crosses directly between or behind the earth and the sun. This limited number of actual eclipse causing instances happens because most of the times when the path presents an opportunity, the moon’s position is actually far away from being in the right place to make it happen. Everything needs to be just right for even a partial eclipse to happen.

Pieces PartsThere are parts of the shadow that is cast

by the planetary bodies that must be discussed in order to more fully understand what causes an eclipse. The diagram below will be used to show these features.

When a shadow is cast by the moon, and that shadow reaches the earth at any point, an eclipse occurs. The darkest cone shape, directly behind the moon is called the umbra. No direct sunlight exists in this area of the shadow. The Penumbra encircles the umbra, being itself part of the overall shadow of the moon, but also containing at least a part of the visible and direct light from the sun within its zone. Beginning directly behind the focal point of the umbra starts a zone where a part of the direct rays of the sun are always visible. This zone is called the antumbra. Which type of eclipse occurs will depend on these four zones, which are directly affected by how close the moon is to the earth when it passes between the earth and the sun.

Three Major TypesA lunar eclipse happens when the moon

crosses either partly or completely through the shadow of the earth. It may happen around the time when the moon is in its fully lit up phase, called the full moon phase. The moon does not have to be completely full for a lunar eclipse to happen, however. Small crossings of the earth’s shadow can still happen when the moon is in a gibbous, or nearly full phase. While lunar eclipses take place mostly during the night time, they can be viewed during twilight hours as well.

A solar eclipse happens when the shadow of the moon falls on the earth such that a portion of the umbra cone reaches the earth. This creates a small circular area on the face of the

earth that receives no direct sunlight. All who view the eclipse within the area of the umbra will view a total solar eclipse. These usually last a few minutes long. The shadow moves across the face of the earth at around1000 miles per hour, so it goes away all too quickly.

The last type of eclipse that will be mentioned is the annular eclipse. This eclipse is experienced when the viewer is within the region of the antumbra. The sun’s outside diameter is then visible, because the moon is farther away from the earth, making it appear smaller than the apparent diameter of the sun. This phenomenon is commonly referred to as the “Ring of Fire.”

Phases of an EclipseThere are four phases of an eclipse. The

first, called “first contact” happens when the moon’s outside diameter (OD) touches the sun’s OD. Second Contact is when the moon’s OD first completely enters the sun’s OD. Third Contact is when the moon’s OD once again touches the exiting edge of the sun’s OD. Lastly, fourth contact is when the moon leaves the sun’s OD altogether. It should be noted that the second and third contacts do not always happen during every eclipse event. During a partial eclipse, only the first and last contacts might be the only visible events.

Finally, at no time should anyone look directly at the sun during one of these events. The annular eclipse, even though as little as six percent of the sun is visible, is still fully able to cause permanent retinal damage. Alternate viewing methods like special solar viewing glasses, or homemade pinhole projection boxes are readily available and highly encouraged.