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 154.Solar Ephemeris
for May 1985
Observatory, is shown in table 154. The Solar
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 made at
the meridian, record the local apparent time as 12^{h}.
2. Add the longitudinal equivalent time to obtain Greenwich
apparent time (GAT).
3. Add or subtract the equation of time (true solar time
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 O^{h} (midnight) daily at Greenwich for the whole year.
4. Correct the apparent declination for the date for the
elapsed GMT from O".
5. In case the local standard time of the observation is
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:
Local apparent time Longitudinal equivalent
time
Greenwich apparent time
Equation of time
Greenwich mean time (GMT)
Declination at (table 154)


NOTE: 23.94" =
Difference for 1 hour (table 154)
Correction for elapsed 7’03.7"
Corrected declination 21°36’56.4"
From the computation shown above, you see that the
declination is positive, so it is a north declination.
The transit was pointed south, so this is a case in which the
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"
