Oh, Mr. Moon
Oh, Mr. Moon, Moon,
Bright and silv’ry moon,
Oh, won’t you please shine down on me?
(Civil War Campfire Song)
The moon is our nearest celestial neighbor (excluding man-made satellites, occasional meteoroids, comets, or asteroids which wander too close for anyone’s good), and is the most prominent object in our night sky (and excluding the sun, in our day sky as well). These are the only two bodies which exert any significant influence on the earth, primarily in the form of tides. Being closer and shining brighter than any other heavenly body, the moon is also more prominent than all of the stellar objects, which causes us humans to exaggerate its astronomical importance. To illustrate: Which of these objects, held at arm’s length, would barely cover the full moon; a) a dinner plate; b) a basketball; c) a baseball; d) a golf ball; e) an English pea?
The answer is e), an English pea. Don’t take my word for it. Go outside and try it yourself.
I have witnessed the full moon on the White Sands in New Mexico, and on snow fields elsewhere. On a clear night the visual impact is enormous! It seems as bright as day, but of course it is not. Full moonlight measures about 0.1 lumen per square meter, while full afternoon sunshine measures 10,000 lumens per square meter, a ratio of 100,000 to 1.
The apparent brightness of the moon depends not only on atmospheric conditions, such as clarity of the air, its height in the sky, presence of water vapor, etc., but also on the distance of the moon. Since the moon orbits the earth, one would expect the distance to remain the same from one revolution (i.e., one month) to the next, and it would, if the moon’s orbit were a perfect circle. But the moon travels in an ellipse, so that the distance from earth to moon varies slightly with each month. Last month the moon was closer than it has been in many years. But unless you are an experienced astronomer who spends much time staring through an eyepiece at the moon, the odds are that you would not notice the difference. And who among us can recall the exact apparent size of the full moon a month or six months or five years ago?
The sky has been divided into 360; the apparent diameter of the moon is about º. The difference in the apparent size of the moon at its closest is only a small percentage of that tiny º circle; it is hard to imagine such a small change even being noticeable. Local conditions, such as the presence of small clouds, the moon’s height in the sky, and local scenery, such as mountains, all can add visual drama, and thus influence the way we are affected by our nearest neighbor. When the moon is full, not more than a half dozen to a dozen stars are bright enough to be visible, but in any event, the full moonlight is almost always so bright that it overwhelms all but the brightest stars. So, for the most part, the moon has the stage to herself. At such times she owns the sky!
© March 25, 2011, by Nathan B. Miron, Ph.D.
