Monday, 17 October 2016

Hunter's Moon and supermoon

The Moon was unearthly orange when we were driving home at 7pm last night so I looked to see if we were missing an eclipse. Articles called it a Hunter's Moon, though that seems to be a folklore name for a full Moon in October, perhaps when the poachers were out. It turns out it was also a supermoon. This is not a scientific term since it originated with an astrologer not an astronomer, The Moon does not go on a perfectly circular orbit around the Earth. It follows an ellipse. A supermoon is when it is at its nearest point, which is more accurately called the perigee. That's about 362600km away. By contrast, at its most distant or apogee, it is at about 405400km which is 12% further. They talk about how big the Moon looks. This will be connected to the angle subtended at your eye. In radians, that is the diameter of the Moon divided by the distance to the Moon. For perigee, the angle subtended is 0.00959 rad and for apogee its 0.00857 rad by doing the calculation using the radius of the Moon as 1738km. The apogee angle is 0.88 x the perigee angle so the perigee angle is 12% bigger. Squaring to give area would leave the Moon looking 23% bigger at perigee. I read figures that say 30%.