FOOTNOTES:
[748] Airy, Month. Not., vol. xvii., p. 210.
[749] Mars comes into opposition once in about 780 days; but owing to the eccentricity of both orbits, his distance from the earth at those epochs varies from thirty-five to sixty-two million miles.
[750] J. D. Cassini, Hist. Abrégée de la Parallaxe du Soleil, p. 122, 1772.
[751] The present period of coupled eccentric transits will, in the course of ages, be succeeded by a period of single, nearly central transits. The alignments by which transits are produced, of the earth, Venus, and the sun, close to the place of intersection of the two planetary orbits, now occur, the first a little in front of, the second, after eight years less two and a half days, a little behind the node. But when the first of these two meetings takes place very near the node, giving a nearly central transit, the second falls too far from it, and the planet escapes projection on the sun. The reason of the liability to an eight-yearly recurrence is that eight revolutions of the earth are accomplished in only a very little more time than thirteen revolutions of Venus.
[752] Die Entfernung der Sonne: Fortsetzung, p. 108. Encke slightly corrected his results of 1824 in Berlin Abh., 1835, p. 295.
[753] Owing to the ellipticity of its orbit, the earth is nearer to the sun in January than in June by 3,100,000 miles. The quantity to be determined, or "mean distance," is that lying midway between these extremes—is, in other words, half the major axis of the ellipse in which the earth travels.
[754] Month. Not., vol. xv., p. 9.
[755] The Distance of the Sun from the Earth determined by the Theory of Gravity, Edinburgh, 1763.