Why can we not see the backside of the Moon from Earth? And, I was reading about 581 c, and have a question regarding the theory that the newly discovered planet possibly may NOT rotate, meaning one side of the planet is always light and the other side of the planet is always dark.  I guess I assumed that all planets rotate…but sometimes in opposite directions (like Earth and Venus, I think?)  Are there any planets in our solar system that don’t rotate?

All planets (and asteroids and moons) that we know of rotate, although some (like Venus) rotate very slowly.  Perhaps there is some confusion between “no rotation” and “rotation that keeps the same side toward the parent object.”  The Moon, for example, rotates once in each orbit around the Earth, thus keeping the same side toward the Earth.  Its rotation period and its orbital period are the same.  Thus we cannot see the far side of the Moon; if it did not rotate, we could see all parts of it from Earth over the course of a month.  Some scientists have also suggested that the new planet Gliese 581c rotates in the same period that it orbits its star, keeping one side in eternal light and the other in eternal night.  The cause would be the same for this planet as for our Moon: the friction of tides that gradually slows a fast-rotating object until its rotation period equals its orbital period.  This is likely for the new planet, but its rotation period has not actually been measured.  The chances that it is not rotating, of that its rotation is slower than its 13-hour orbital period, are negligible.

David Morrison

NLSI Interim Director

May 2, 2007

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Did you know?

On July 20, 1969, Neil Armstrong became the first person to set foot on the Moon.

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