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
Table 15-4.-Solar Ephemeris for May 1985
Observatory, is shown in table 15-4. TheSolar Ephemeris is issued (on request) each year by major engineering instrument makers.
Find the sun’s declination as follows:
1. Accepting the observation as having been madeat the meridian, record the local apparent time as 12h.
2. Add the longitudinal equivalent time to obtainGreenwich apparent time (GAT).
3. Add or subtract the equation of time (true solartime minus local civil time) from GAT to obtain GMT (Greenwich mean time). The equation of time is given in the Solar Ephemeris or Nautical Almanac for the instant of Oh (midnight) daily at Greenwich for the whole year.
4. Correct the apparent declination for the date forthe elapsed GMT from O".
5. In case the local standard time of the observationis recorded, find the GMT at once by simply adding the time zone difference. Then, after all the necessary corrections are made, substitute the value to one of the formulas enumerated above, analyzing carefully to see which formula is appropriate.
EXAMPLE:Suppose that on 28 May 1985 in the Northern Hemisphere, you obtained a corrected meridian altitude (h) of the sum of 67°37'06" at longitude 86°08’W. The sun bears south of the observer. The computation to get the corrected declination is as follows:
NOTE:23.94" = Difference for 1 hour (table 15-4)
Correction for elapsed 7’03.7"
Corrected declination 21°36’56.4"
From the computation shown above, you see thatthe declination is positive, so it is a north declination.
The transit was pointed south, so this is a case in whichthe body observed was between the zenith and the equator. This is, then, a Case I situation, in which the latitude equals declination plus zenith distance (90 - 67°37'06" or 22°22’54"). Therefore, the latitude is equal to 21°36’56.4" + 22°22’54" or 43°59'50.4"
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