Only 41% of the moon is dark


in Daily updates, Science

Eagle owl

After having coffee with Louise today, who I was glad to see before my departure, I saw an Eagle Owl being displayed as part of a fundraising drive for the Barn Owl Centre of Gloucestershire. Naturally, the sight of the enormous eyes of this majestic bird made me think about moonlit nights.

If you were to sit on the surface of the sun and look at the earth through a telescope, you would observe it rotating both around the centre of the solar system and around its own axis. The side of the sphere facing you (the side experiencing daylight) would consist of a constantly shifting selection of Earth’s surface as it rotates: with new areas becoming lit in the west as sections in the east fall into shadow. You could watch as the eastern seaboard of North America came into illumination, then passed back into darkness as it spins away to the shadowed side of the planet once again.

By contrast, when you look up from Earth at the moon, the same face is basically presented all the time. Just as one side of the moon is always in view, there is a ‘dark side’ that is always hidden from the vantage point of an observer on Earth. This is because of a phenomenon called tidal locking. The moon rotates on its own axis at just the right rate so that, as it orbits the Earth, the same side is presented. There are, however, minor oscillations in this presentation. This is called libration, which derives from the Latin word meaning ‘to sway.’ You can see an animation of the phenomenon here. It derives both from the fact that the moon’s axis is slightly inclined when compared to its orbit around the Earth and because the moon’s orbit around the Earth is slightly eccentric. Because of the cumulated rocking motion, it is actually possible to see 59% of the moon’s surface from the Earth.

I’ve always wondered how people were able to make celestial observations of such incredible detail and precision in the period prior to modern instruments and measuring systems. It is an enormous tribute to the vigilance and dedication of early astronomers that prior civilizations knew as much about observable astronomical phenomena as they did: knowledge that found application in essential tasks like predicting the chance of the reasons and recurrent episodes of rains or flooding.

{ 4 comments… read them below or add one }

Anonymous September 2, 2006 at 10:49 pm

Young people nowadays might understand tidal lock more easily if you say it’s ‘just like circle strafing.’

Milan September 2, 2006 at 10:51 pm


I partly got the idea for this post as I was looking through this collection of amazingly good images from Wikipedia.

B September 3, 2006 at 12:33 am
Milan September 4, 2006 at 1:06 pm

This entry is now an Everything2 node

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