Courtesy of EarthSky
A Clear Voice for Science
www.EarthSky.org

January 1 mid evening eastern sky.

Here is the moon again, in line with the the constellation Gemini’s brightest stars, Castor and Pollux, and not that far from the planet Mars. The moon will be even closer to Mars tomorrow night.

Many people write to ask about the fact that, from Earth, we see only one side of the moon. The moon rotates about once a month, in contrast to the once-a-day rotation of our own planet Earth. It’s the moon’s monthly rotation that keeps us from seeing the back side of the moon.

The moon spins on its axis once in the same amount of time it takes to orbit Earth. If you place a chair in the middle of a room, having it represent Earth, and pretend to be the moon – always keeping one face toward the chair as you walk entirely around it – you’ll see that your body has to spin once in order to keep your face toward the chair. So it is with the moon and Earth. The moon has to spin once each month to keep one face toward Earth, as it orbits once a month around Earth.

This relationship between the Earth and moon is no coincidence. Over the long history of our solar system, Earth’s stronger gravity locked onto the moon and forced it into the one-sided relationship we see today.

Our moon isn’t the only moon in the solar system share that has this property. In fact, nearly all the known moons of planets in our solar system are gravitationally locked with respect to the planets they orbit!

Written by earthsky