Sunday, December 10, 2017

Dark Side Of The Moon

On December 3rd the world was treated to a spectacular supermoon sighting. A supermoon being a full moon that approximately coincides with the closest distance the Moon gets to Earth in its orbit and thus makes it appear to be larger than usual. Because of the eliptical nature of the moon's orbit the distance from Earth to the Moon ranges between 357,000 and 406,000 kilometers; so when that 50,000 kilometers coincides with a full moon it really makes a difference.

But the Moon doesn't emit light it only reflects sunlight and only half of the Moon is lighted at any one time. It doesn't mean there is a dark side it just means we can only see up to half of the lighted side depending on where it is in relation to the earth and sun. The reason we are always looking at the same part of the Moon is because it takes 29 days to make a complete orbit around the Earth which is also the same amount of time it takes to make one rotation around its axis. The two happenings are locked together. 

Also locked together of course are the Earth and Sun as the Earth rotates around its axis while orbiting the Sun. Unlike the Moon, which is relatively upright and therefore having the same season year round, the Earth has quite a tilt to its axis and, over the course of the year, this causes the Sun's radiation to strike the hemispheres at different angles. Whichever hemisphere is tilted away from the Sun experiences winter and when tilted towards the Sun has summer. In the polar regions this can also lead to 24 hour sunlight or 24 hour darkness.

Near and Far Sides of the Moon

Thanks first to the 1959 Soviet Luna 3 probe and later NASA's Lunar Reconnaissance Orbiter in 2009 we now have images of the "dark side" of the moon or "far side" of the moon as it is more appropriately called. Prior to this the only humans to have seen this side of the moon were the Apollo astronauts who orbited it in exploratory missions or before landing on its surface.  Strange how, at this time of year as the days get progressively shorter and darker, it's the moonlight that seems to brighten our existence (even if it's only reflected sunlight from the near side).