DAVIS Mark 15, 25 Sextant - Manual (Page 16)

Next, you find the Greenwich Mean Time (GMT) of your local noon by listening
to the radio time signal, correcting any error you watch may have had. In this
example, you tune in the time signal and find that GMT is now 22:10:00. Your
watch reads 12:10:00, so it has no error. You now know that your local noon
occurred at GMT 21:43:30 (26 minutes 30 seconds ago).
You can now work out your noon sight: the date, the time of meridian passage
(local noon), the altitude of the sun at meridian passage, the height of your
eye above the surface of the sea, and the index error of the sextant you are
using.
Finding Longitude
Meridians of longitude are measured east or west from the prime meridian
(zero degrees) at Greenwich, England. Because the ground position of the
sun moves around the earth at an average speed of 15° per hour (15 nautical
miles per minute), longitude may be calculated by comparing local noon with
Greenwich Mean Time (refer to Fig. 13, longitude).
For example: If local noon occurred at 2:00 GMT, your longitude is approxi-
mately 30° west of Greenwich (2 hours x 15°/ hour = 30°).
While the method already described gives your approximate location, you
must apply the equation of time to determine your exact position. The earth in
its orbit around the sun does not travel at a constant speed. Clocks and watch-
es, therefore, keep the time of a fictitious or mean sun which travels at the
same average speed throughout the year. Furthermore, the position of the true
sun (as seen from the northern half of the earth) is not always due south or
180° true at noon by the clock. The difference in time between the true sun
and the mean sun is called the “equation of time.” The equation of time for any
given day may be found in a Nautical Almanac. An approximate value may be
found in the student tables at the end of this booklet.