October 19, 2010 > TechKnow Talk: Moon Myths and Misconceptions
TechKnow Talk: Moon Myths and Misconceptions
The sun dominates the heavens with a fiery, blinding brilliance. It brings warmth, light, and life itself. Yet no celestial body has inspired more poets, songwriters, and lovers than the cold, gray moon. Though we are all very familiar with the shapes and cycles of this romantic and mystical sphere, it is the subject of some common misconceptions as well.
First, contrary to some popular literature and songs, there is no dark side of the moon. All areas of the moon's surface receive an equal amount of sunlight. But there is a "far side" of the moon, at least from our perspective on Earth. This is because the same "side" of the moon always faces the Earth. This is not a coincidence; it is the result of millions of years of gravitational interaction between Earth and the moon.
Actually, there is only one surface to a sphere, so it is more accurate to say that one hemisphere of the moon always faces toward the Earth and the other hemisphere faces away. But "near side" and "far side" are in common usage and easily understood, so let's stick with that.
Man's first look at the far side of the moon was from the Apollo command modules orbiting the moon in the late 1960s. Unfortunately, the astronauts saw no little moon people living in green cheese cottages along custard creeks. In fact, the far side looks a lot like the near side: a gray, desolate, heavily-cratered landscape.
The various forms in which the moon appears when observed from Earth are called the phases of the moon. Everyone understands the moon looks different at different times of the month, but how these shapes are formed is confusing to many.
With a little imagination, you can easily understand the phases of the moon that result from its orbit around Earth. Imagine you have a pencil and an orange. Mark an 'X' on the orange, then pierce the orange with the pencil and use that as a handle to hold it out at arm's length with the 'X' facing you. This is the near side of the moon and your head is the Earth.
Now imagine standing outside just before sunset, with the sun low in the sky. Face the sun with the orange held in front of you. The part of the orange facing you, the near side, is in shadow. All the sunlight is on the far side. This is known as the new moon. On such nights there is no visible moon from Earth.
Now move your feet to rotate 90 degrees, keeping the 'X' facing you. You can now see about one-quarter of the orange. Half of it is illuminated by the sun, as always, but half of the lit part is hidden from your view on the far side. This phase is called the first quarter. Continue rotating another 90 degrees and the entire near side is now in sunlight and the far side is dark. This is the full moon. Another 90-degree rotation brings you to another quarter moon, called the third quarter (or sometimes the last quarter). Finally, another 90 degree rotation returns to the new moon, with the near side in full shadow.
The first half of this rotation the moon is waxing. This means simply that we see more of it each day. After the full moon, it is a waning moon, meaning we see less of it each day. The moon orbits the Earth and completes a full cycle, new moon to new moon, in about 29.5 days. So the new moon waxes for about 15 days, reaches full, then wanes for 15 more, becoming a new moon once again. The days before and after the new moon we see the crescent moon. The days before and after it is full, the moon appears as a bulbous shape; this is known as a gibbous moon.
There is a rather esoteric astronomical fine point that is worth mentioning. The time required for the moon to complete an orbit of Earth as observed from far away in space is actually only about 27.3 days. This is called a sidereal month. But because the Earth is orbiting the sun, the moon must travel more than 180 degrees each time to reach the new moon position, interposed between the sun and the Earth. This 29.5 day cycle of new moon to new moon is called a synodic month, and represents a full lunar orbit from our perspective here on Earth.
One movement not mentioned yet is the rotation of the Earth. As the Earth spins on its axis, the moon appears to rise in the east and set in the west, just as the sun does. Since the moon is orbiting in the same direction the Earth is rotating, we are constantly "chasing" it. After 24 hours of rotation the Earth has come back to the same orientation, but the moon has moved on. Thus, the moon rises later each day. How much later varies, depending on the season and the latitude of the observer. This is why, during the month, the moon rises and sets at all hours of the day and night.
Occasionally, the sun, Earth, and moon align exactly. This is an eclipse. Recalling your mental experiment with the orange, it is easy to see that this can happen only at a new moon or a full moon. If the moon is exactly between the Earth and sun at the new moon, the sun is blocked from our view and a solar eclipse occurs. If the Earth is exactly between the sun and moon at the full moon, we see the shadow of the Earth pass across the moon in a lunar eclipse.
By the way, it is definitely not a myth that watching a solar eclipse can cause permanent eye damage. Never look directly at the sun under any circumstances.
Finally, what about the pervasive belief that the full moon is associated with an increase in emergency room visits, traffic accidents, or psychological problems? Study after study has failed to find such a correlation. Though many of us may say, "It must be a full moon," when faced with unusually violent or bizarre behavior, the promulgation of this myth is probably mostly a case of selective memory.
Now about the werewolves . . .