Position Angle

Position angle (PA) gives a direction around a point on the sky, such as the centre of the Sun or the Moon. It is measured from the direction of the north celestial pole, through east, from 0° to 360°. Because we look at the sky from inside it, the compass is mirrored compared with a map: with north up, east is on the left. For a satellite transit the predicted chord across the disc is given by two position angles, where the satellite enters and where it leaves, and both change as you move across the transit corridor: on the centreline the chord passes through the middle of the disc, and towards the edges of the strip it moves out towards the limb. Position angles are also used for the bright limb of the Moon, the direction its sunlit edge faces, which tells you whether a lunar transit will cross the lit part, and for double stars and features on the Sun. One practical catch: position angle is tied to the celestial pole, not to the horizon, so on a camera or an alt-azimuth telescope the field appears rotated by an amount that depends on the object's position in the sky and the time. Near the horizon at low elevation that rotation can be large, so check the orientation of your field before the event.