A sundial is a device that uses the position of the sun to tell time. Sundials work by casting a shadow on a marked surface, with the length and direction of the shadow indicating the time of day. The earth’s declination, or the angle at which the earth’s axis is tilted relative to its orbit around the sun, plays a key role in the accuracy of a sundial.
The earth’s declination changes throughout the year as the earth orbits the sun. This means that the sun appears to move north and south in the sky, reaching its highest point at the summer solstice and its lowest point at the winter solstice.
Because of the earth’s declination, the sun’s position in the sky varies throughout the day, and a sundial’s accuracy depends on the angle at which it is oriented relative to the sun. To make a sundial as accurate as possible, it is important to align it with the earth’s declination for your location.
For example, if you are located in a place with a high latitude (such as near the North Pole), the sun will be relatively low in the sky for much of the year, and a sundial will need to be tilted at a steep angle to accurately track the sun’s position. In contrast, if you are located near the equator, the sun will be almost directly overhead for much of the year, and a sundial will need to be oriented more horizontally.
Understanding the earth’s declination is important for designing and using sundials accurately. By taking into account the earth’s tilt and the sun’s position in the sky, you can create a sundial that accurately tells time throughout the year.
K.P. Cheung, Department of Architecture, The University of Hong Kong,
Pokfulam Road, Hong Kong
This page started in July 1997
TABLE 1 – Mean value of the solar declination (for l99l, noon UT (GMT), adapted from The
Nautical Almanac l99l, HMSO, UK, p.10-253)