Determining Latitude From the Ephemeris

his diagram shows graphically what an ephemeris shows in tables. The difference is that the astronomical almanac gives figures for solar (sun), lunar (moon), and astral (star) altitudes for every day of the year along all lines of latitude whereas the diagram indicates only solar altitude at 4 times during the year (equinox and solstice) and only along one line of latitude. What line of latitude? That can be determined from the diagram below as follows: on June 21, at its zenith or highest point in the sky (local noon) the sun is directly overhead (an altitude of 90°) at latitude 23°30' N (the Tropic of Cancer). If the altitude of the noon sun at the point of observation on June 21 is 72°30' (the symbol ' indicates a "minute," and 60 minutes equal one degree), then the latitude of the point of observation is 41°N. This is derived from subtracting 72-1/2 degrees (altitude of the sun at observation point) from 90 degrees (altitude of sun at Tropic of Cancer) and then adding the result (17-1/2 degrees) to the latitude of the Tropic of Cancer (23-1/2 degrees) where the sun's altitude is 90°. To use the ephemeris, all one needs to know is the day of the year and the altitude of the sun, the moon, or one of the key stars such as Betelgeuse, Aldebaran, Antares, or Vega.

--John Logan Allen