Azimuth is the compass bearing of the sun: 90° due east, 180° due south, 270° due west — where to look, and where a window faces.

Measured clockwise from north. At the equinoxes the sun rises at 90° and sets at 270° everywhere; at midsummer in London it rises near 50° and sets near 310°. The window instrument reads a facing the same way, which is how it knows a north-east pane sees the early-summer sun for hours.

The sun rises north of east in summer and south of east in winter, and the difference grows with latitude.