[923] Harvard Annals, vol. xxxii., p. 81.
[924] Astr. and Astrophysics, vol. xi., p. 778.
[925] Neison, The Moon, p. 25.
[926] Knowledge, vol. xvii., p. 85.
[927] Neison, The Moon, p. 104.
[928] The combination of a uniform rotational with an unequal orbital movement causes a slight swaying of the moon's globe, now east, now west, by which we are able to see round the edges of the averted hemisphere. There is also a "parallactic" libration, depending on the earth's rotation; and a species of nodding movement—the "libration in latitude"—is produced by the inclination of the moon's axis to her orbit, and by her changes of position with regard to the terrestrial equator. Altogether, about 2/11 of the invisible side come into view.
[929] Cel. Objects, p. 58 (4th ed.).
[930] Astr. Nach., No. 1,631.
[931] Cf. Leo Brenner, Naturwiss. Wochenschrift, January 13, 1895; Jour. Brit. Astr. Ass., vol. v., pp. 29, 222.
[932] Respighi, Les Mondes, t. xiv., p. 294; Huggins, Month. Not., vol. xxvii., p. 298.