What is an Eclipse

Aside from being a clever name for a new type of chewing gum, and eclipse is a naturally occurring phenomenon in which a celestial body such as the moon covers or obscures another celestial object such as the sun. While the moon can obscure other objects such as far off planets or distant stars, this phenomenon is referred to as an occultation while the term eclipse is reserved for occasions when the objects eclipsing and being eclipsed are the sun, moon and Earth.

Eclipses occur in two types, lunar and solar. A lunar eclipse occurs when the moon passes through the Earth’s shadow in space. Eclipses of this type can be partial, total or penumbral depending on which portion of Earth’s shadow the moon passes through. Every object that casts a shadow casts both a lighter outer shadow, the penumbra, and a darker inner shadow, the umbra. When the moon passes through the Earth’s penumbra only the eclipse is referred to as a penumbral eclipse. If a portion of the moon passes through the darker umbra, it is known as a partial eclipse. If the moon completely passes through the dark umbra of Earth’s shadow the eclipse is total. Because the Earth’s shadow is considerably larger than the moon’s visible diameter, a lunar eclipse can last for several hours and is visible over a much larger area of the globe. Lunar eclipses are only visible at night and only occur when the moon is at full phase.

A solar eclipse occurs when the moon passes between the Earth and sun and casts its shadow onto the Earth’s surface. Solar eclipses can be partial, total or annular. A partial solar eclipse occurs when the moon only partially eclipses the sun’s disk while a total eclipse occurs when the moon completely covers the sun. Yet occasionally the moon’s disk does not completely cover the sun even though it passes directly in front of the sun, this is an annular eclipse. Why does an annular eclipse occur? The explanation lies in the fact that the moon’s orbit around the Earth is not a perfect circle, it is an ellipse. Because of its elliptical orbit, the moon’s distance from the Earth changes as it goes from aphelion to perihelion. At aphelion the moon is slightly further away from Earth than it is at perihelion. An object that is further away from the observer appears smaller. Therefore the moon does not completely cover the sun’s disk. While an annular eclipse is a rare and fascinating phenomenon, by far the most spectacular is a total solar eclipse.

A total solar eclipse is truly a wonder of celestial geometry. The diameter of the moon is approximately 400 times smaller than that of the sun. However, the moon is also approximately 400 times closer to Earth than the sun is. This relationship means that when the moon passes in front of the sun it can exactly cover the sun and allow observers on Earth to observe the solar corona. Yet because the moon is so small it’s shadow is visible over a much smaller area, which makes a solar eclipse visible to far fewer people. Solar eclipses are only visible during daylight hours and only occur when the moon is at new phase.

If solar eclipses are only visible when the moon is new, and lunar eclipses are only visible when the moon is full why do we not see an eclipse every month? Theoretically, there should be both a lunar and solar eclipse every 29.5 days, since that is the orbital period of the moon about the Earth. Yet eclipses are only observed infrequently. This is due to the orbital plane of the moon. If the moon orbited in exactly the same plane as the Earth, the three objects would be in line every month. However, the moon’s orbit is tilted about 5 degrees relative to the plane of Earth’s orbit. When the moon is above or below the Earth’s orbital plane no eclipse can occur. Only when the moon crosses the Earth’s orbital plane, known as the line of node, AND when the three objects are in line can an eclipse occur.

An eclipse, whether it be lunar or solar, partial or total, is an amazing sight in the sky. Both types have been revered and feared in human society for thousands of years. While astrologers attempt to attach special significance to these events, astronomers use them as tools to study the sun, the moon and our earth. For armatures, they are a rare and wonderful chance to witness the universal clockwork in action.