ZODIACAL LIGHT. 141 



body alcove its equator, that is to say, the point at v.hich 

 gravity and centrifugal force are in equilibrium, must be the 

 same as the altitude at which a satellite would rotate round 

 the cential body simultaneously with the diurnal revolution 

 cf the latter.* This limitation 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 Gtars. 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 I", we find that the outermost 

 nebulous layer of ?uoh a star must be 150 times further from 

 the central body than our Earth is fron^ 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 tlie narrow limitation of the Sun's atmosphere, 

 v/hich 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 cf Venus 

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



* Laplace, Expos, dn. Syst. du Monde, p. 270 ; Micanique Celeste, 

 t. ii., p. 1G9 aud 171 ; Schubert, Astr., bd. iii., $ 206. 



t Arago, ill the Aivniaire, 1842, p. 408. Compare Sir John Her- 

 pcViel's considerations on the vokime and faintness of light of planetary 

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

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

 j)l]ere presents the phenomenon of zodiacal liglit, did not originate with 

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

 de VAurore Bar., p. 47 and 263; Arago, in the Annnaire, 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 Mercuiy 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." (Mdm. de VAcad., t. viii., 1730, p. 218, 

 and Biot, in the Comptes liendus, 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 ita 

 connection with the November phenomenon — a connecticn which 01 

 bei-3 doubts. (Schum., Jakrh., 1837, s. 281.) Regarding the question 

 whether the place of the zodiacal light perfectly coincides with that 

 f>f the Sun's equator, see Houzeau, in Sebum., Astr. Nachr-, 1843, No 

 i32, s. 190. 



