Saturday night was a fine evening for witnessing the geometry of the night sky.

This is sunset on the evening of the autumnal equinox, September 22, 2012, with a first quarter Moon in the sky. The image illustrates the geometry of the quarter Moon’s position, which is always 90° away from the Sun, a quarter of the way around its orbit in its monthly cycle.

In this case, because the Sun (at right) was on the equinox position on the ecliptic, it was setting due west this night (something it does only on the dates of the fall or spring equinox). This put the quarter Moon (at left) 90° away, due south at sunset.

Draw a line from the Moon to your eye (the camera) and then back out to the Sun and it forms a 90° right angle. This geometry holds true for any quarter Moon, but being equinox the Sun and Moon were nicely aligned due west and south, making their right angle arrangement more obvious.

I took this shot from the grounds of the Rothney Astrophysical Observatory on the occasion of their monthly Open House.

– Alan, September 23, 2012 / © 2012 Alan Dyer