This is sunset on the evening of the Autumnal Equinox, Sept 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. In this case, because the Sun was on the equinox position on the ecliptic, it was setting due west this night, putting the quarter Moon due south at sunset. Draw a line from the Moon to your eye (the camera) and then out to the Sun and it forms a 90° right angle. This was taken from the grounds of the Rothney Astrophysical Observatory on the occasion of their monthly Open House.
The Amazing Sky 