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Astronomy

Lunar and Solar Eclipses

Most people on the planet are aware that both the sun and moon can appear to be blocked out at certain times. In both forms the event is known as an eclipse and is a type of syzygy, which is a term that is used to describe the nearly straight-line arrangement of three bodies in a single gravitational system. Eclipses can occur in many kinds of gravitational configurations and are not limited to our observations from Earth, though the terms lunar and solar eclipse are almost universally understood to be in reference to our own moon and sun respectively. Perhaps somewhat surprisingly, the similarities between solar and lunar eclipses are virtually non-existent beyond these fundamental characteristics. There are many important differences between these observable occlusions of the sun and moon that demonstrate the characteristics that are unique to each event Chaisson & McMillan, 2011).

            A lunar eclipse occurs when the moon passes through the shadow of the Earth. The gravitational arrangement in this situation has the moon on the far side of the Earth in reference to the sun. The Earth casts a shadow that obscures the sunlight that would otherwise illuminate the moon for regular viewing. Only a relatively small and especially dark central portion of the shadow known as the umbra can cause a total eclipse, while the larger outer segments of the shadow form the penumbra which is not completely free of solar radiation. A partial lunar eclipse occurs when only part of the moon is in the umbra and a penumbral eclipse refers to the darkening of the body while in the penumbra. A total penumbral eclipse is possible when the moon is still completely in the outer shadow, though the side closest to the umbra can still appear to be darker than the rest. Lunar eclipses are the most common to be observed by humans. This prevalence is based on many factors including the availability of viewing access from the planet, as lunar eclipses can be seen from the entire nighttime side of the planet. Also, the total eclipse can last for nearly two hours dependent upon the positioning of the planet, with partial coverage being present for up to four.  

            Solar eclipses are a much rarer observation than their lunar counterparts. A major reason for this scarcity is the fact that the moon is responsible for the occlusion when the relatively tiny body passes between the sun and Earth. The moon casts a shadow in this situation and a total solar eclipse occurs when the umbra reaches the surface of the Earth. The total eclipse will then only be seen by those who are within the area of the Earth covered by the umbra, which lasts for only a short time in any given spot. Should the small area fail to reach the planet then viewers directly in line with the moon would experience an annular eclipse where the sun appears as a bright ring around the smaller circular darkness of the moon. An especially rare solar eclipse is the hybrid type that describes a situation where the eclipse may appear to be annular from one vantage point but total from others. A partial solar eclipse is the most common form to be viewed because it can occur in the penumbra during total eclipse events and it can be the only observable result of a total eclipse when the umbra passes beyond the Earth’s poles. One of the most commonly known differences between solar and lunar eclipses is that you can safely look directly at an occluded moon while doing the same for a solar eclipse could cause serious eye damage.

            It can be tempting to assume that eclipses should occur on a monthly basis due to the orbit of the moon around the Earth. However, these events are far rarer in reality because our planet is not orbiting the sun on the same plane as the moon orbits the Earth. Accordingly, the three bodies will not form a straight line every pass as the moon may be above or below the plane of reference, and an eclipse of either type will not occur on a monthly basis.

Reference

Chaisson, E., & McMillan, S. (2011). Astronomy: A beginner’s guide to the universe. (6th ed.).             Benjamin-Cummings Pub Co.