y6 Sidereal Astronomy . [January, 
moon turn quicker, and vice versa. The same occurs in 
respedt to the sun and the earth. If the sun were increased 
in weight, the earth and the other planets would turn more 
quickly round him, and the years would diminish in length. 
If the mass of the sun were diminished, the contrary would 
happen. It is by comparing the adtion of the sun on the 
earth with the adtion of the earth on the moon, that we have 
found that the sun is 324,000 times more energetic, more 
powerful, more heavy than the earth. 
If, then, we had in space a celestial couple, the mutual 
distance of the two components of which was equal to that 
which separates the earth from the sun, or 92 millions of 
miles, the examination of the duration of its revolution 
would immediately give us the mass of the system relatively 
to that of the sun. It is easy to show that if a couple of 
celestial bodies turning round their common centre of gravity 
employs a certain time, T, to accomplish its revolution, 
whilst another couple, of which the components are at the 
same distance from each other, employs another duration of 
time, D, to accomplish its revolution, the mass of the first is 
to the mass of the second in the inverse proportion to the 
square of the time, that is to say as D 2 is to Th 
If the distance is not the same, it is necessary at first to 
reduce it to this equality, by bearing in mind the law which 
regulates the distances. “ The squares of the times are to 
each other as the cubes of the distances.’” 
I have been able to calculate the mass of the stellar 
system of the double star 70 p. Ophiuchi. By the combina- 
tion of all the observations, I have found that the period is 
92 '77 years, or 92 years 283 days. The parallax of this star 
being o' 168" corresponds to a distance from the earth equal 
to 1 ,zj 00,000 times that of the sun. At this immense distance, 
the size of the terrestrial orbit being reduced to the pre- 
ceding angle, the half major axis of 4'88" represents 1075 
millions of leagues. This is a little less than the distance 
from Neptune to the sun. A planet situated at this distance 
from the sun will accomplish its revolution in 156*55 years. 
The proportion between them is then as (i56‘55) 2 is to 
(92 "77)% or as 2*89 is to 1, whence I have concluded that 
the mass of the Ophiuchus system is nearly three times superior 
to that of the sun and Neptune unitedly, or (Neptune having 
only a small mass) to that of the sun alone. Thus there is 
a star, scarcely visible to the naked eye, which weighs 
900,000 times more than the Earth. 
Let us remark, in passing, that the orbital movement of 
the small star round the large one is 191,830 leagues a day, 
