The hour at which the sun crosses the meridian must be taken for the special observation now to be described. This hour can always be learnt from 'Dietrichsen's Almanac'; but noon, civil time, is near enough for practical purposes. Now it is necessary first to know the position of the ecliptic with reference to the celestial equator. Of course, at noon a horizontal line across the sun's disc is parallel to the equator, but the position of that diameter of the sun which coincides with the ecliptic is not constant: at the summer and winter solstices this diameter coincides with the other, or is horizontal at noon; at the spring equinox the sun (which travels on the ecliptic) is passing towards the north of the equator, crossing that curve at an angle of 23-1/2 deg., so that the ecliptic coincides with that diameter of the sun which cuts the horizontal one at an angle of 23-1/2 deg. and has its _left_ end above the horizontal diameter; and at the autumn equinox the sun is descending and the same description applies, only that the diameter (inclined 23-1/2 deg. to the horizon) which has its _right_ end uppermost, now represents the ecliptic. For intermediate dates, use the following little table:-- -------------------------------------------------------------------------- Date. |Dec. 22|Jan. 5|Jan. 20|Feb. 4|Feb. 19|Mar. 5 |Mar. 21 (Circiter.) | |June 6|May 21 |May 5 |Apr. 20|Apr. 5 | -------------------+-------+------+-------+------+-------+-------+-------- Inclination of |Left |Left |Left |Left |Left |Left |Left Ecliptical Diameter| | | | | | | of Sun to the |0 deg. 0' |6 deg.24' |12 deg.14' |17 deg.3' |20 deg.36' |22 deg.44' |23 deg.27' Horizon.[17] |Right |Right |Right |Right |Right |Right |Right -------------------+-------+------+-------+------+-------+-------+-------- Date. | |Dec. 7|Nov. 22|Nov. 7|Oct. 23|Oct. 8 | (Circiter.) |Jan. 21|July 7|July 23|Aug. 6|Aug. 23|Sept. 7|Sept. 23 --------------------------------------------------------------------------