Eclipses: The Science And The Pseudoscience -- By: Rodger C. Young
Eclipses: The Science And The Pseudoscience
Eclipses, whether of the sun or the moon, are not rare. There are from two to five solar eclipses that can be viewed from somewhere on earth each year, and every year there is at least one lunar eclipse of one type or another. “Total” lunar eclipses are those for which the entire moon is covered by the earth’s full shadow (the umbra) at some time during the eclipse. “Partial” eclipses are those for which only a portion of the moon’s surface is covered by the earth’s full shadow, and “penumbral” eclipses are those for which no part of the moon is covered by the earth’s umbra, but a portion of the moon is slightly darkened by being in the earth’s partial shadow, or penumbra. Penumbral eclipses are usually too faint to be observed by the unaided eye. The other two types can be seen by anyone who is on the night side of the earth when the eclipse occurs, unless their visibility is obscured by clouds.
It might be thought that during a total lunar eclipse the face of the moon could not be seen at all, since the moon’s entire surface is in the geometric shadow of the earth. However, the accompanying graphic from NASA shows how some sunlight is bent (refracted) as it passes through the earth’s atmosphere, and thereby reaches the moon’s surface. The shorter blue wavelengths are scattered by the earth’s atmosphere and the light that does reach the moon during a lunar eclipse therefore takes on a reddish tinge. The reddening becomes more pronounced and deeper when the moon is viewed from those longitudes of the earth where it is just after sunset or just before sunrise. In these cases, the light returning to the earth again travels horizontally through the earth’s atmosphere and undergoes further reddening, taking on a dark red color. An additional phenomenon in effect for such observers is that the moon, when observed at the horizon, appears to be enlarged. NASA (2011) states that “For reasons not fully understood by astronomers or psychologists, low hanging moons look unnaturally large when they beam through trees, buildings, and other foreground objects.” This phenomenon has been known for a long time, and an explanation was offered by the Christian philosopher George Berkeley (1709) in his insightful book on the theory of vision. These two effects—the apparent enlargement of the moon when seen on or near the horizon, and its reddening during an eclipse—combine to make a lunar eclipse viewed at sunset or sunrise an impressive sight. The accompanying article touches on the psychological effect this must have had on the people of Jerusalem at the time of the lunar eclipse immediately after the death of Christ.
NASA diagram showing the diffraction of ...
Click here to subscribe