283 
parallax of certain fixed stars . 
puted by the pole star may be uncertain ©",2 or o",3, in July, 
and the same in an opposite direction in January. Add to 
this, it has not been usual for astronomers to consider the 
variable velocity of the earth in its orbit. The effect of this 
in N. P. D. as to stars, the iR s . of which are nearly 3 s or 9 s 
is always insensible, but not as to stars, the M,\ of which 
are nearly o s and 6 s , and are also far from the ecliptic. This 
quantity is nearly 
= 65 ' cos - ( 0 Lon S- ~ 9 s 9 ■) 
and therefore in the pole star ma} r amount in July to ©",34,. 
Hence, from these two causes, the uncertainty in the aberra- 
tion of the pole star in declination in July may be = o",6. 
The joint effect of these causes will be o in October and in 
January. 
The index error computed by the pole star when below 
the pole in July, will be opposite to the above, and thus the 
index errors so computed at the same time may differ i'',2. 
5. As to n t or the nutation, according to some astronomers, 
the nutation in declination, 
= 7", 8.5 Sin. (M — a) + T',15 Sin. ( jR + a) - - (1) 
according to others, 
8", 42 Sin. (M — a)+ 1", 23 Sin. (iR-f- a) - - - (2) 
M. Laplace * made it ( as will appear by the proper re- 
ductions ) so great as 
S' 1 , 76 Sin. (At — a) -f- 1", 35 Sin. (JR + a - - (3) 
( 1 ) is the nutation according to Lambert's tables. The 
maximum of formula (3) was deduced from the mass of the 
moon, =j8~6r> (the earth being unity), as determined by the 
* Mec. Cei. Tom. z, p. 350. 
Pp 
MDCCCXVIII. 
