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(1) If only an approximate position is required, the best means of determining it is by fixing the direction of the sun or a star when it has the greatest altitude. The instrument to be used for this purpose would be a small theodolite with both a vertical and horizontal circle, and provided also with tangent screws to give slow motion to each of the circles as required.

By using stars of both high and low altitudes, a greater exactness can be obtained, but, after all, the method only gives a first approximation, as its weakness lies in the very slow change of altitude as the meridian is approached.

(2) A much more accurate method is that of observing with an altitude and azimuth instrument the azimuth (i.e., its angular distance east or west of the north or south) of a star when at the same altitude east and west of the meridian. If the mean of the two readings given by the azimuth circle be taken, the resulting reading indicates the direction of the meridian.

If we employ the sun in place of a star, its change of declination during the interval between the observations must be taken into account.