On times and shadows: the observational analemma

Abstract

The observation that the shadows of objects change during the course of the day and also for a fixed time during a year led curious minds to realize that the Sun could be used as a timekeeper. However, the daily motion of the Sun has some subtleties, for example, with regards to the precise time at which it crosses the meridian near noon. When the Sun is on the meridian, a clock is used to ascertain this time and a vertical stick determines the angle the Sun is above the horizon. These two measurements lead to the construction of a diagram (called an analemma) as an extremely useful resource for the teaching of astronomy. In this paper we report on the construction of this diagram from roughly weekly observations during more than a year.