To read the civil hour directly (wristwatch time) the following corrections should be included on a sundial:
- For LONGITUDE:
The earth is divided into 24 time zones, where each zone corresponds to one hour of time and 15º (360º/24h) of longitude. The official time is taken at the middle of each time zone, however, the time is the same 7º30' east or 7º30' west of the middle. This is a standard for almost the entire
globe.
- For EQUATION OF TIME:
This correction is required to account for the earth's elliptical orbit and the inclination of the plane of the equator to the ecliptic. The movement of the sun during the year is not uniform, and these corrections are different for every day of the year. The maximum required correction is approximately 16.5 min.
- For DAYLIGHT SAVINGS TIME:
For the purpose of saving energy, some governments adopt a particular time schedule in summer that advances civil time one hour compared to true solar time.
The sum of these three corrections gives us the exact civil hour. They can be incorporated into the hour lines on the sundial although the reading can sometimes be complicated if the hour lines are closely spaced. This complication is more usual during summer.