Reading Time and Date


· For horizontal, vertical, oblique, equatorial, multi-face, cylindrical and internal cylindrical type sundials, the procedure for reading the time and date are similar:

Since the sun's position only marks true solar time at a location, it is necessary to make corrections to give actual civil time.

The season of the year must also be considered. As noted in basic concepts a clock may be designed for an entire year with a complete analemma. In this case the observer must be careful to select the appropriate semi-analemma for the season. To avoid errors, some sundial analemmas are multi-colored.

These sundial types can include a calendar (See options) to approximate the date.



· For a shepherd type sundial the procedure is:

The shepherd sundial can include the mentioned corrections. To make a reading, the gnomon is rotated to the correct date. Once the gnomon is located, the entire clock is turned about its vertical axis toward the sun until the shadow of the gnomon is completely vertical. When in this position, it is easy to take a reading, the observer only needs to remember the type of lines drawn; true solar time / summer- autumn / winter-spring ) and any applied corrections.

This clock type doesn't have calendar, it must be known to read the correct time.



· For a ring type sundial the procedure is:

The ring sundial is aligned at the instant of reading while rotating it about the vertical axis. The ray of light passing through the hole should fall exactly on the hour circle corresponding to the date of the reading. Once this is done, it is easy to read the hour, and make any time corrections desired if necessary.

This sundial type doesn't have a calendar; it must be known to read the correct time.