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Weird Science: Eclipses

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<p><strong>SF Fig. 6.2.</strong> The plane of orbit of the moon around the earth (solid line) is offset from the plane of orbit of the earth around the sun (dotted line). This figure is not drawn to scale.</p><br />


The key to understanding eclipses is to realize that the orbit of the moon around the earth is not in the same plane as the orbit of the earth around the sun (SF Fig. 6.2). An eclipse can only occur when the moon is near the Earth-sun orbital plane and the moon, the sun, and the earth are aligned. Most of the time during a full moon or new moon, the moon is actually above or below the Earth-sun orbital plane.

 

<p><strong>SF Fig. 6.3.</strong> Diagram of solar eclipse (not drawn to scale)</p><br />

A solar eclipse occurs when the sun, the moon, and the earth are lined up in space, with the moon between the sun and Earth (SF Fig. 6.3). The alignment of the sun and the moon casts a full, dark shadow on the earth, called an umbra, and also a lighter, partially blocked shadow, called a penumbra. From Earth, the moon, and sun appear to be nearly the same diameter in the sky, because even though the sun is much bigger, it is much farther away from the earth. During a total solar eclipse, it looks like the new moon is covering the sun (SF Fig. 6.4 A). Total solar eclipses are rare because the sun, the moon, and the earth not only have to be lined up (which only happens about every six months), but the moon has to be closer to the earth than average in order to completely block the sun. On average, a total solar eclipse occurs somewhere on earth every 18 months. However, when you take into account the fact that the moon’s umbra is, on average, under 150 km wide, a given location only experiences one every 360 to 410 years (SF Fig. 6.4 B). Partial solar eclipses (SF Fig. 6.4 C), caused by the much larger penumbra, or partial shadow, are more common.

<p><strong>SF Fig. 6.4.</strong>&nbsp;(<strong>A</strong>) Photo of a total solar eclipse from a location on Earth within the umbra</p><p><strong>SF Fig. 6.4.</strong>&nbsp;(<strong>B</strong>) Solar eclipse looking down on Earth from space</p><p><strong>SF Fig. 6.4.</strong>&nbsp;(<strong>C</strong>) Photo of a ring-like annular partial solar eclipse from a location on Earth within the penumbra</p>


 

A lunar eclipse occurs when the sun, the moon, and the earth are lined up and the earth is between the sun and moon (SF Fig. 6.5). Thus, an observer on Earth can see the full moon passing through Earth’s umbra or penumbra. During a total lunar eclipse, the moon often appears red because the light that reaches the earth has traveled through the Earth’s atmosphere and been bent (or refracted) toward the moon. The earth’s atmosphere acts to filter out blue light, leaving proportionally more red light, causing the moon to appear red (SF Fig. 6.6). (Similar reasoning can be used to explain why the sun appears redder near sunrise and sunset than at midday.) Lunar eclipses are much more common than solar eclipses and last longer because the earth is much larger than the moon. When they occur, lunar eclipses can be seen from half of the earth, weather permitting.

<p><strong>SF Fig. 6.5.</strong> Diagram of lunar eclipse (not drawn to scale)</p><p><strong>SF Fig. 6.6. </strong>Picture of a total lunar eclipse as the moon enters the earth’s umbra dark shadow.</p>


 

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.