Analemmatic Sundial

Ancient Observatory, Beijing

The horizontal scale of this sundial (see previous page) measures the sun's right ascension (angle from the equinox); the vertical scale measures half a year; and the "figure eight" shows how the point of sunrise swings back and forth during the course of the year. The gnomon's shadow at noon will intercept the figure eight at a specific point, where the day of the year can be read off from the vertical scale.