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  • Gloria Hoffner

Does sunlight strike both sides of the moon?

FREE Science for Seniors Moon Activities - July 20th is this Saturday!!! Celebrate the 50th anniversary of the greatest achievement in human history!!

Why do we only see one side of the moon?

According to the moon orbits the Earth once every 27.322 days. It also takes approximately 27 days for the moon to rotate once on its axis. As a result, the moon does not seem to be spinning but appears to observers from Earth to be keeping almost perfectly still. Scientists call this synchronous rotation.

The side of the moon that perpetually faces Earth is known as the near side. The opposite or "back" side is the far side. Sometimes the far side is called the dark side of the moon, but this is inaccurate. When the moon is between the Earth and the sun, during the new moon phase, the back side of the moon is bathed in daylight.

The orbit and the rotation aren't perfectly matched, however. The moon travels around the Earth in an elliptical orbit, a slightly stretched-out circle. When the moon is closest to Earth, its rotation is slower than its journey through space, allowing observers to see an additional 8 degrees on the eastern side. When the moon is farthest, the rotation is faster, so an additional 8 degrees are visible on the western side.

Experiment: Does the dark side of the moon receive sunlight?

Materials: Dark room, basketball, two volunteers and flashlight.

Process: In a dark room, have one volunteer hold the moon out with extended arms. Have the second volunteer stand five feet away from the moon and turn the flashlight so the light strikes the center of the ball.

Result: The basketball represents the moon and the flashlight the sun. Just as light is striking both 'sides' of the ball, sunlight falls on both sides of the moon during its orbit around the Earth. Why?

The Moon is tidally locked to the Earth. This means the time it takes to rotate around its axis is the same as the time it takes perform one orbit. When the Moon’s orbit takes to the sunward side of the Earth the side of the Moon which we can’t see is lit by the sun


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