“Along the skies the Sun obliquely rolls, Forsakes, by turns, and visits both the poles; Diff’rent his track, but constant his career, Divides the times, and measures out the year.”
The Sun is not an object comprehended in the title of this volume. But to have omitted reference to a body of such vast importance, and one displaying so many interesting features to the telescopic observer, would have been inexcusable. We may regard the Sun as the dominant power, the controlling orb, and the great central luminary of our system. The phenomena visibly displayed on his surface assume a particular significance, as affecting a body occupying so high a place in the celestial mechanism.
The mean apparent diameter of the Sun is 32′ 3″·6, and his real diameter 866,000 miles. The apparent diameter varies from a minimum of 31′ 32″ at the end of June to a maximum of 32′36″; at the end of December; and the mean value is reached both at the end of March and September. The Sun’s mean distance from the Earth is about 92,900,000 miles, computed from a solar parallax of 8″·8, which appears to agree with the best of recent determinations. At this distance the linear value of 1″ of arc is 447 miles.
The Sun’s apparent diameter is as follows on the first day of each month:—
′ ″ Jan. 1 32 36·0 Feb. 1 32 31·8 Mar. 1 32 20·4 April 1 32 3·8 May 1 31 48·0 June 1 31 36·4 July 1 31 32·0 Aug. 1 31 35·8 Sept. 1 31 47·0 Oct. 1 32 2·6 Nov. 1 32 19·2 Dec. 1 32 31·6
Solar observations may be pursued with a facility greater than that attending work in some other departments of practical astronomy. The Moon, planets, and stars have to be observed at night, when cold air, darkness, and other circumstances are the cause of inconvenience; but the student of the Sun labours only in the light and warmth of genial days, when all the incidentals to observation may be agreeably performed. There are, however, some drawbacks even in this pleasant sphere of work. The light of the Sun is so great that much persistent observation is apt to have an injurious effect on the eye, and will certainly deaden its sensitiveness on faint objects. In the summer months the observer experiences discomfort during a lengthy observation from remaining so long in the powerful rays of the Sun, some of which must fall upon his face unless measures are adopted to shield it. During the progress of solar work the student should always provide for himself as much shelter as possible from the glare, which must otherwise disturb that equanimity of feeling in the absence of which no delicate research is likely to be successfully conducted.