ZODIACAL LIGHT. 141 



body above its equator, that is to say, the point at which 

 gravity and centrifugal force are in equilibrium, must be the 

 same as the altitude at which a sateUite would rotate round 

 the central body simultaneously with the diurnal revolution 

 of the latter.* This Umitation of the solar atmosphere in its 

 present concentrated condition is especially remarkable when 

 we compare the central body of our system with the nucleus 

 of other nebulous stars. Herschel has discovered several, in 

 which the radius of the nebulous matter surrounding the star 

 appeared at an angle of 150". On the assumption that the 

 parallax is not fully equal to 1", we find that the outermost 

 nebulous layer of such a star must be 150 times further from 

 the central body than our Earth is from the Sun. If, there- 

 fore, the nebulous star were to occupy the place of our Sun, 

 its atmosphere would not only include the orbit of Uranus, 

 but even extend eight times beyond it.t 



Considering the narrow limitation of the Sun's atmosphere, 

 which we have just described, we may with much probability 

 regard the existence of a very compressed annulus of nebulous 

 matter,| revolving freely in space between the orbits of Venus 

 and Mars, as the material cause of the zodiacal light. As 



* Laplace, Expos, du Syst. du Monde, p. 270 ; M6canique C6leste, 

 t. ii., p. 169 and 171; Schubert, Astr.,hd. iii., § 206. 



t Arago, in tlie Annuaire, 1842, p. 408. Compare Sir John Her- 

 schel's considerations on the volume and faintness of li^ht of planetary 

 nebulae, in Mary Somerville's Connection of the Physical Sciences, 1835, 

 p. 108. The opinion that the Sun is a nebulous star, whose atmos- 

 phere presents the phenomenon of zodiacal liglit, did not originate with 

 Dominicus Cassini, but was first promulgated by Mairan in 1730 ( Traits 

 de VAurore Bor., p. 47 and 263 ; Arago, in the Annuaire, 1842, p. 

 412). It is a renewal of Kepler's views. 



X Dominicus Cassini was the first to assume, as did subsequently 

 Laplace, Schubert, and Poisson, the hypothesis of a separate ring to 

 explain the form of the zodiacal light. He says distinctly, " If the 

 orbits of Mercury and Venus were visible (throughout their whole ex- 

 tent), we should invariably observe them with the same figure and in 

 the same position with regard to the Sun, and at the same time of the 

 year with the zodiacal light." {Mim. de I' Acad., t. viii., 1730, p. 218, 

 and Biot, in the Comptes Rendus, 1836, t. iii., p. 666.) Cassini be- 

 lieved that the nebulous ring of zodiacal light consisted of innumerable 

 small planetary bodies revolving round the Sun. He even went so 

 far as to believe that the fall of fire-balls might be connected with the 

 passage of the Earth through the zodiacal nebulous ring. Olmsted, 

 and especially Biot (op. cit., p. 673), have attempted to establish its 

 connection with the November phenomenon — a connection which 01 

 bers doubts. (Schum., Jahrb., 1837, s. 281.) Regarding the question 

 whether the place of the zodiacal light perfectly coincides with that 

 of the Sun's equator, see Houzeau, in Schum., Astr. Nachr., 1843. No 

 492, s. 190. 



