Zodiacal Light

Every clear, moonless February and March night in the Northern Hemisphere, just after evening twilight and before morning twilight, the sky is faintly illuminated above the sunside of the horizon.* That thin lucence is called the Zodiacal light. Over the opposite side of the night horizon is another just perceptible glow rarely perceived called the Gegenschein, or counterglow. In the tropics the Zodiacal light appears every clear night, except when the moon outshines it. The light of the milky way blots out the Gegenschein during December, January, June and July. Other times it matches the Zodiacal light. Last week before the annual meeting of the American Physical Society at Columbia University Dr. Edward Olson Hulburt of the Naval Research Laboratory at Washington, gave the latest, refined theory of these phenomena. The round earth lies, in Dr. Hulburt's theory, within an egg-shaped ion cloud of its own making. This cloud is the out-spraying of the earth's spherical atmosphere which hugs the earth like a skin about 100 miles thick. Neutral atoms and molecules at the outer rim of the atmosphere dart further away from the earth into space. Sunlight ionizes them, creating an extremely tenuous cloud of ions and electrons. These radiate a faint light of their own, comparable to the light of the sun's corona. Zodiacal light and Gegenschein are the earth's coronal light visible from within. It is without any doubt too faint to be discerned even from the moon./-

If light did not exert pressure this ion and electron cloud would be spherical in form. But because light does press, sunlight forces the cloud into its egglike shape. The butt side is towards the sun. It is in that direction 30,000 miles thick and appears as the Zodiacal light. From the opposite side of the earth where the cloud's resistance to sunlight pressure is less it is squeezed a million miles or more from earth into a thin taper. That is where the Gegenschein glows. Through that taper the earth's atmosphere very, very slowly escapes into universal chaos, a millionth part in a million years.

*Evening twilight ends this week in the U. S. a little before 7:30 p. m. mean, or sun time. The difference between mean and standard times must be calculated for every community. Morning twilight begins this week in the U. S. just about s arn. mean time. /-A man on the moon could of course see the round of the earth because the earth, like the dead moon, reflects the sun's light across the intervening mean distance of 238,857 miles. A man on the earth can see the earth's reflected shine on the moon when "the old moon is in the arms of the new," that is, just as the thin crescent of the first quarter moon becomes visible. That will next happen in the Northern Hemisphere March 7, at 11 p. m. Eastern Standard Time.

This file is automatically generated by a robot program, so reader's discretion is required.