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.