Mary Murphy, Age 10, Of St. Paul, Minn., for her question:
How long is a day on the moon?
The gravity of the earth reaches out into space like an invisible net, and its biggest catch is the golden moon. Our satellite is a captive, held forever in an orbit around its parent planet. As it makes each orbital trip around us, it turns around once like a spinning top.
It takes our captive satellite 27 days, 7 hours, 43 minutes and 11.5 seconds to travel one orbit around the earth. In exactly the same number of moments it rotates on its axis. With each rotation, all sides of the moon are turned to face the smiling sun and then around to face in the opposite direction where the velvety heavens sparkle with starlight. The lunar day and night period is equal to about 27 1/3 earth days. To a spaceman standing on any spot of the moon's surface, the length of each sunlit day would equal almost two earth weeks.
The pull of the earth's gravity forces the moon to keep the same side always facing toward us. With each lunar orbit, we can watch a day and night period creep across our side of the moon. In fact, we cannot miss this monthly spectacle, for the moon is the most outstanding lamp in the starry night. In our skies, its disk is exactly the same size as the dazzling sun's.
As each side of the moon turns from the lunar day to the lunar night, it is lit with brilliant sunshine and then plunged into midnight shadow. From the earth we see only that part of the moon which happens to be washed with golden sunshine. With the dawn of each lunar day we see a thin sickle of sunshine around one side of the dark disk. The moon is new. In a week the lunar dawn creeps halfway across our side of the
Moon. The other half still slumbers under the lunar night. In another week the lunar dawn reaches the opposite side of the disk and our side of the moon is washed with golden sunshine, the moon is full.
At full moon, our side of the lunar landscape is enjoying its noontime with the sun beaming down from its highest point in the sky. When we see the new moon and, the old moon, the sun is just above the horizon, casting long shadows across the lunar landscape.
The mare Vaporum is a flattish plain near the center of our side of the moon. An astronaut stationed on this spot would welcome the lunar dawn while we were watching the phase of the half moon from the earth. A week later, as we watched the full moon, he would be under the scorching lunar midday. After another week, as we watched the old moon wane, he would see the evening sun set in the lunar skies.