With the recognition of diurnal phenomena, one can tell the time from the changing height of the Sun or, at night, of certain stars above eastern or western horizons.
Example: Aboriginies and European peasants (before the development of church clocks) were able to determine the time from the Sun (except on cloudy days!). Sailors, too, might learn to tell time at night by the height of the stars.
A stick in the ground that is vertical and placed so that the Sun can cast its shadow on the ground is called a gnomon. The shadow will fall on the ground in the opposite direction to the Sun, so that if the Sun rises to the southeast of the gnomon, then the gnomon's shadow will fall to the northwest. Its length will be greatest when the Sun is closest to the horizon; i.e., at sunrise or sunset it would reach infinity. Its length will be shortest when the Sun reaches its highest altitude in the sky; i.e., when the Sun crosses the meridian, which is "local noon." At that moment the Sun will be due south of the gnomon, and the gnomon's shadow will point due north.
Each hour a given celestial body such as the Sun (or a star) should shift its position by about 15 degrees (360 degrees ÷ 24 hours = 15 degrees/hr) due to the daily motion alone. This angle can be determined (and found to be roughly constant) for the Sun with a sundial. Note that since the angular diameter of the Sun is about 1/2 degrees, the Sun shifts its position by an angular distance equal to its own diameter every two minutes (1/2 degrees x 60 min/15 degrees = 2 min).
