C *57 ] 
book ; in which he has demonftrated, that if an 
obferver on the Earth makes a correCt obfervation on 
the Sun’s place, when the Moon is in one quadra- 
ture, it will differ from a like obfervation, taken in 
the oppofite quadrature (according to a mean elliptic 
motion) by an angle which the diameter of this epi- 
cycle will fubtend at the Sun. The fame learned 
author has alfo (hewn, in the fcholium to the fame 
propofition, that this quantity, or parallax, will be 
twice greater to Mars in oppofition, and three times 
greater to Venus, in her inferior conjunction with 
the Sun. 
The difference thus produced in the apparent place 
of the Sun, and of all the primary planets being- 
governed by the Moon, and having it’s period the fame, 
may perhaps be not unaptly called the menjlrual 
parallax. 
Now if, with Sir Ifaac Newton, the relative gravi- 
ties of the Earth and Moon are taken between the 
proportion of 39 and 40 to one; the menftrual 
parallax of the fun will come out 1 3 " upon the ra- 
dius of the Earth’s epicycle, and will affeCt the folar 
obfervations at the oppofite quadratures, by double 
that quantity, viz. 26 //f : in like manner, the mean 
diftance from the Earth of Mars in oppofition, being 
to the Sun’s mean difiance, as 1 to 2. 1 ; and theleaft 
difiance of Mars from the Earth, to the Sun’s mean 
difiance, as 1 to 2 i, the menftrual parallax of Mars 
will affeCt the obfervation upon him in that fituation, 
by 56" and 73 '4, refpeCtively. 
The mean diftance of Venus from the Earth, in 
her inferior conjunction, being to that of the Sun as 
3 4. to 1 nearly, and not very variable, on account 
of 
