Moon Myths Busted

Today’s instalment on the countdown to #observethemoon.

Different countries see different phases of the Moon on the same day.

Everyone sees the same phases of the Moon, but we who live south of the equator who face North to see the Moon when it is high in the sky will see the Moon upside down from our Northern friends so that the reverse side of the Moon is lit.

The Moon does not rotate.

The Moon does in face spin on its axis. It completes a rotation once every 27.3 days.

The confusion is caused due to the fact that it also takes the same period of time to orbit the Earth, so that it keeps the same side facing us.

The Moon takes 27 d 7 h 43 min to revolve around the Earth, considering the turning with respect to background stars (sidereal revolution), but 29 d 12 h 44 min when seen from the Sun (synodic) and this is because in this period the Earth has revolved around the Sun (view month). 

This latest revolution period is governing Moon phases, eclipses and lunisolar tides. Since the Moon takes the same time to circle in on itself around the Earth, it always presents the same face. This is because the Earth, due to an effect called the gravitational gradient, has slowed the Moon completely.

The fact that we always see the same face of the Moon means that the Moon must rotate once in the same amount of time it takes to orbit Earth once. You can see why by walking around a model of Earth while imagining that you are the Moon.
Credit: The Cosmic Perspective.

Donna the Astronomer

I am a keen astronomer lucky enough to live and work in Coonabarabran the Astronomy Capital of Australia! I am a ‘Drover’s Brat’ and discoverer of a couple of comets and asteroids. I operate Milroy Observatory and can show you how to best integrate dark sky experiences into your tourism, farm stay or AirBnB business.

This Post Has One Comment

  1. Ggreybeard

    It’s surprising how little the average person thinks about the Moon. So many people still talk about the “dark side” when they really mean the far side – and my head nearly explodes every time someone casually talks about the Moon being made of cheese, which happens at most public nights.

Leave a Reply to Ggreybeard Cancel reply