Sun observations, as compared with star observations, provide the surveyor with a more convenient and economical method for determining an accurate astronomic azimuth. A sun observation can be easily incorporated into a regular work schedule. It requires little additional field time, and when reasonable care is exercised and proper equipment is used, an accuracy within 10 seconds can be obtained.

Two methods are used for determining an azimuth by observing the sun: the altitude method and the hour angle method. The basic difference between these two is that the altitude method requires an accurate vertical angle and approximate time, whereas the hour angle requires a very accurate time but no vertical angle.

To apply the hour angle method, you measure the horizontal angle from a line to the sun. Then, knowing the accurate time of observation and your position (latitude and longitude), you can compute the azimuth of the sun. This azimuth is then combined with the horizontal angle to yield the azimuth of the line.

To compute the azimuth of the sun, use the following equation:

The above equation is derived using spherical trigonometry to solve the pole-zenith-star (PZS) triangle for azimuth

Coordinated universal time (UTC) is another name for Greenwich mean time and is broadcast by the National Bureau of Standards on radio station WWV. (Inexpensive receivers that are pretuned to WWV are available.) The correction factor (designated DUT) that you must add to the coordinated universal time is also obtained from WWV by counting the number of double ticks following the minute tone. Each double tick represents a tenth-of-a-second correction and is positive the frost 7 seconds (ticks). Beginning with the ninth second, each double tick is a negative correction. The total correction, either positive or negative, will not exceed 0.7 second. By adding DUT to UTC, you get time (designated UT1) that is based on the actual rotation of the earth.

A stopwatch with a split (or lap) time feature is ideal for obtaining times of pointings. The stopwatch is set by starting it on a WWV minute tone and then checking it 1 minute later with a split time. If a significant difference is observed, start and check the stopwatch again. Split times are taken for each pointing on the sun and added to the beginning UTC time, corrected to UT1. To enter the ephemeris tables, you must know the Greenwich date for the time of observation. For an afternoon observation (local time) in the Western Hemisphere, if the UT1 is between 12 and 24 hours, the Greenwich date is the same as the local date. If the UT1 time is between 0 and 12 hours, the Greenwich date is the local date plus 1 day.

For a morning observation in the Western Hemisphere and an afternoon observation in the Eastern Hemisphere, Greenwich and local dates are the same.

Both the observers latitude and longitude are required for the hour angle method. Usually these values can be readily obtained by scaling from a map, such as a USGS 7.5-minute quadrangle sheet. For sun observations, locating the observers position on the map and scaling must be performed to a reasonably high degree of accuracy.