Latitude by Altitude of the Sun at Noon

Figure 15-12.-Three possible situations in determining latitude by meridian altitude observation. where: @ = latitude of place 6 = declination of observed body h = corrected observed altitude CASE II. When the body observed is toward the pole from the zenith, which is the case for circumpolar stars, you can get the latitude of the place of observation by using the following formulas: 0 = h f p. Use this formula only for circumpolar star observations, where p is the polar distance (90° - 6). CASE III. When the equator is between the body observed and the zenith, use the following formula to get the latitude: In the above situations, always remember that 6 and @ positive when they are located north of the equator and negative when south of it. Latitude by Altitude of the Sun at Noon You can observe the altitude of the sun by two methods. In the first method, you follow the sun just before it is about to cross the approximate meridian. In the second method, you set the line of sight of the transit in the plane of a known meridian and wait for the sun to cross the line of sight. At this instant take the reading of the vertical angle. In either method your main objective is to measure the sun’s altitude accurately. You should know the exact time so that you can compute the instant of local apparent noon. Then you will know exactly when you should be in the field to have everything ready just before the instant of observation. If the instrument used is not a transit equipped with solar prism attachments, set the horizontal cross hair tangent to the lower edge of the sun’s disk. By the first method referred to above, when you are observing for maximum altitude, follow the sun until it no longer rises. The moment the sun starts going down, record the vertical angle and determine the index error. In the second method above, the setting of the sun’s disk is similar to the first method except that you get the reading at the instant the sun crosses your known meridian. In either method, you should correct the altitude observed in the field for index error, semidiameter, parallax, and refraction. You can eliminate index error in the second method by plunging the telescope and taking another reading as fast as possible. The declination for the Greenwich time corresponding to the instant of local noon is taken from a table of the Nautical Almanac, the Solar Ephemeris, or The Ephemeris. The table for May 1985, taken from The Ephemeris, published by Bureau of Land Management, U.S. Department of Interior, and prepared by the Nautical Almanac office, U.S. Naval 15-15