Eclipse

From ArticleWorld

The word, eclipse is generally used to described what occurs when an astronomical body blocks the light of another. All eclipses are in actuality occultations. The word “eclipse” comes from the ancient misconception that the moon was actually covering the sun. It is known now that the moon during a solar eclipse is simply between the earth and the sun, and blocking the sun’s light from hitting a certain part of the earth. During a solar eclipse, the moon is occulting the sun and its shadow is eclipsing a small part of the earth.

A lunar or solar eclipse can only occur when the moon, sun and earth are all intersecting the earth’s ecliptic orbital plane. This can happen during two periods each year, when the sun is at this intersection in relation to the earth and the moon. The moon must also be in position for eclipses to occur, and this happens when the moon hits this intersection during the course of its draconic monthly cycle.

Umbra, penumbra

The shadow of any large body cast by the light of another large body has both an umbra (inner shadow) and a penumbra (outer shadow). In the case of a solar eclipse, a total eclipse can only be seen within the moon’s umbra, or inner shadow cone. This shadow is relatively small, corresponding to the apparent size of the moon as seen from earth. Viewers within the penumbra will only see a partial eclipse, and viewers outside of the moon’s shadow will see nothing unusual at all.

In the case of a lunar eclipse, the penumbra cast by the earth is much larger than the moon itself where the shadow intercepts it. Thus, a total eclipse of the moon by the earth does not require exact alignment between the three celestial bodies involved. However, partial umbral and penumbral eclipses of the moon can and do occur.

Stages of the eclipse

While solar, lunar and other eclipses are all qualitatively different events, there is a similar set of stages that can be seen in all of them. They are divided into moments of contact between the eclipsed body and the umbra of the occluding body. The act of entering a part of the shadow is called “first contact,” and the act of exiting this shadow is called “second contact.”

In the case of a solar eclipse, the stages are:

1. First exterior contact: Where the shadow of the moon begins to cover the sun as seen from earth. In actuality, those on earth are watching their location slip into the moon’s umbra.
2. First interior contact: This occurs when the moon appears to completely cover the sun. In many cases, the very edges of the sun can still be seen surrounding the moon. This is known as an annular eclipse. In the absence of direct sunlight during an annular eclipse, the sun’s corona can be seen from earth with the naked eye.
3. Second interior contact: Where the total eclipse is broken and the moon begins to move away from the apparent center of the sun. In reality, the moon’s umbra is now moving away from the viewer’s location on earth.
4. Second exterior contact: This is the point where the eclipse ends completely. The moon appears to move away from the sun and full daylight conditions return. It is at this point in time that the moon’s umbra has completely moved from the viewer’s location.

Lunar eclipses follow the same pattern. However, it is important to note that the stages seen on the moon are direct events between the moon and the earth’s shadow, whereas the stages seen in a solar eclipse are the result of the moon’s shadow on the earth, not the moon’s shadow on the sun.