Sidereal Astronomy. 
5 6 
[January, 
sufficiently considerable to admit of their orbit being calcu- 
lated and the period examined. We have even been able 
not only to measure the light, but also to find the mass of 
many of these stellar couples, and we thus know that our 
sun is but one of the smallest stars, and that a certain 
number of those which shine so modestly under the discreet 
veil of the silent night are in reality more bulky, more 
heavy, more resplendent and hotter than this gigantic focus 
on whose rays the life of our small planet hangs. These 
groups of suns in orbital movement are connected mutually 
by the bonds of a reciprocal attraction, and execute their 
revolutions in closed curves following the laws discovered 
by Kepler and explained by Newton. Before observation, 
in our century, had discovered their existence, we only knew 
similar movements in our solar system, in the translation of 
the planets round the sun, the satellites round the planets, 
the periodical comets round the same focus of attraction. 
We now know that the force which governs our family, 
which extends with decreasing power as far as the last 
planet of our system, as far as Neptune, and even twenty- 
eight times farther, since the solar attraction still governs 
at 131,000 millions of kilometres the great comet of 1680, 
retains it in its orbit, and forces it to return, . . . we 
know, say I, that this force reigns also in other universes, 
that it is truly universal, and that it governs in the depths 
of infinity stellar systems the farthest from us. 
Stars which, until our day, were called fixed, are, on the 
contrary, those which move the most rapidly, and the flight 
of a cannon ball is as the slowness of a tortoise compared 
to them. The knowledge of these partial systems, whose 
movements are accomplished without any outward in- 
fluence, opens to one’s thoughts a field so much larger that 
already these systems appear in their turn like mere details 
in the vast ensemble of movements which animate celestial 
space. 
When one of two associated suns possesses a mass much 
more powerful than the other, it appears to be the centre of 
the movement, as our sun appears to be the centre of the 
movement of translation of the earth and the planets, al- 
though in reality the planets and the sun itself turn together 
round their common centre of gravity, a mathematical point 
the position of which varies constantly, and which is usually 
situated in the interior of the sun. The smaller of the two 
stars revolves round the greater, and no spectacle is more 
imposing than these sidereal revolutions. In some systems 
the revolution is made in less than half a century; for 
