ANOTHER METHOD.
It is a little more accurate to find the altitude by taking the complement of the observed zenith distance, if the vertical arc has sufficient range. This is done by pointing first to Polaris when at its highest (or lowest) point, reading the vertical arc, turning the horizontal limb half way around, and the telescope over to get another reading on the star, when the difference of the two readings will be the double zenith distance, and half of this subtracted from 90° will be the required altitude. The less the time intervening between these two pointings, the more accurate the result will be.
Having now found the altitude, correct it for refraction by subtracting from it the amount opposite the observed altitude, as given in the refraction table, and the result will be the latitude. The observer must now wait about six hours until the star is at its western elongation, or may postpone further operations for some subsequent night. In the meantime he will take from the azimuth table the amount given for his date and latitude, now determined, and if his observation is to be made on the western elongation, he may turn it off on his instrument, so that when moved to zero, after the observation, the telescope will be brought into the meridian or turned to the right, and a stake set by means of a lantern or plummet lamp.
It is, of course, unnecessary to make this correction at the time of observation, for the angle between any terrestrial object and the star may be read and the correction for the azimuth of the star applied at the surveyor's convenience. It is always well to check the accuracy of the work by an observation upon the other elongation before putting in permanent meridian marks, and care should be taken that they are not placed near any local attractions. The meridian having been established, the magnetic variation or declination may readily be found by setting an instrument on the meridian and noting its bearing as given by the needle. If, for example, it should be north 5° east, the variation is west, because the north end of the needle is west of the meridian, and vice versa.
Local time may also be readily found by observing the instant when the sun's center[1] crosses the line, and correcting it for the equation of time as given above--the result is the true or mean solar time. This, compared with the clock, will show the error of the latter, and by taking the difference between the local lime of this and any other place, the difference of longitude is determined in hours, which can readily be reduced to degrees by multiplying by fifteen, as 1 h. = 15°.
[Footnote 1: To obtain this time by observation, note the instant of first contact of the sun's limb, and also of last contact of same, and take the mean.]
APPROXIMATE EQUATION OF TIME.
_______________________
| | |
| Date. | Minutes. |
|__________|____________|
| | |
| Jan. 1 | 4 |
| 3 | 5 |
| 5 | 6 |
| 7 | 7 |
| 9 | 8 |
| 12 | 9 |
| 15 | 10 |
| 18 | 11 |
| 21 | 12 |
| 25 | 13 |
| 31 | 14 |
| Feb. 10 | 15 |
| 21 | 14 | Clock
| 27 | 13 | faster
| M'ch 4 | 12 | than
| 8 | 11 | sun.
| 12 | 10 |
| 15 | 9 |
| 19 | 8 |
| 22 | 7 |
| 25 | 6 |
| 28 | 5 |
| April 1 | 4 |
| 4 | 3 |
| 7 | 2 |
| 11 | 1 |
| 15 | 0 |
| |------------|
| 19 | 1 |
| 24 | 2 |
| 30 | 3 |
| May 13 | 4 | Clock
| 29 | 3 | slower.
| June 5 | 2 |
| 10 | 1 |
| 15 | 0 |
| |------------|
| 20 | 1 |
| 25 | 2 |
| 29 | 3 |
| July 5 | 4 |
| 11 | 5 |
| 28 | 6 | Clock
| Aug. 9 | 5 | faster.
| 15 | 4 |
| 20 | 3 |
| 24 | 2 |
| 28 | 1 |
| 31 | 0 |
| |------------|
| Sept. 3 | 1 |
| 6 | 2 |
| 9 | 3 |
| 12 | 4 |
| 15 | 5 |
| 18 | 6 |
| 21 | 7 |
| 24 | 8 |
| 27 | 9 |
| 30 | 10 |
| Oct. 3 | 11 |
| 6 | 12 |
| 10 | 13 |
| 14 | 14 |
| 19 | 15 |
| 27 | 16 | Clock
| Nov. 15 | 15 | slower.
| 20 | 14 |
| 24 | 13 |
| 27 | 12 |
| 30 | 11 |
| Dec. 2 | 10 |
| 5 | 9 |
| 7 | 8 |
| 9 | 7 |
| 11 | 6 |
| 13 | 5 |
| 16 | 4 |
| 18 | 3 |
| 20 | 2 |
| 22 | 1 |
| 24 | 0 |
| |------------|
| 26 | 1 |
| 28 | 2 | Clock
| 30 | 3 | faster.
|__________|____________|