[Footnote 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.] [Footnote 752: _Die Entfernung der Sonne: Fortsetzung_, p. 108. Encke slightly corrected his results of 1824 in _Berlin Abh._, 1835, p. 295.] [Footnote 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.] [Footnote 754: _Month. Not._, vol. xv., p. 9.] [Footnote 755: _The Distance of the Sun from the Earth determined by the Theory of Gravity_, Edinburgh, 1763.] [Footnote 756: _Opera_, t. iii., p. 326.] [Footnote 757: _Comptes Rendus_, t. xlvi., p. 882. The parallax 8·95" derived by Leverrier from the above-described inequality in the earth's motion, was corrected by Stone to 8·91". _Month. Not._, vol. xxviii., p. 25.] [Footnote 758: _Month. Not._, vol. xxxv., p. 156.] [Footnote 759: _Wash. Obs._, 1865, App. ii., p. 28.] [Footnote 760: _Comptes Rendus_, t. xxix., p. 90.] [Footnote 761: _Ibid._, t. xxx., p. 551.] [Footnote 762: _Ibid._, t. lv., p. 501. The previously admitted velocity was 308 million metres per second; Foucault reduced it to 298 million. Combined with Struve's "constant of aberration" this gave 8.86" for the solar parallax, which exactly agreed with Cornu's result from a repetition of Fizeau's experiments in 1872. _Comptes Rendus_, t. lxxvi., p. 338.] [Footnote 763: _Month. Not._, vol. xxiv., p. 103.] [Footnote 764: _Astr. Papers of the American Ephemeris_, vol. ii., p. 263.] [Footnote 765: _Month. Not._, vol. xvii., p. 208.] [Footnote 766: Because closely similar to that proposed by him in _Phil. Trans._ for 1716.] [Footnote 767: _Month. Not._, vol. xxxviii., p. 447.] [Footnote 768: _Ibid._, p. 11.] [Footnote 769: _Ibid._, p. 294.] [Footnote 770: _Ibid._, p. 334.] [Footnote 771: _Comptes Rendus_, t. xcii., p. 812.] [Footnote 772: _Observatory_, vol. v., p. 205.] [Footnote 773: _Transits of Venus_, p. 89 (1st ed.).] [Footnote 774: _Am. Jour. of Sc._, vol. xx., p. 393.] [Footnote 775: _Month. Not._, vol. xvii., p. 219.] [Footnote 776: _Mem. Roy. Astr. Soc._, vol. xlvi., p. 163.]