1885.] On Certain Errors in Lunar Observations. 585 
Again, suppose we have three stars vertically above one 
another, as shown in the following diagram, — viz. x v z 
, an A-^ ese three stars ai- e located on the meridian*’ the^angu- 
lar distance xy measured with the sextant, plus the angular 
distance yz measured with the sextant, will be equal to xz 
measured with the sextant. 4 
o 
A 
/ 
Y 
z 
lake 0, a point about 2° to the left of y, the points xo z 
are not in a straight line, and the angular distance xo, plus 
the angular distance 0 z, measured with the sextant, will be 
greater than the angular distance xz measured with the 
sextant, two sides of the triangle being greater than the 
third. 
Now, as regards sextant measurements, these results 
would be the same, no matter how we moved these points, 
so long as they retained their exacft relative position. Conse- 
quently we will keep the point z fixed, and rotate the other 
points round z, when they may be shown as seen below. 
As before, z y + y x = z x, z 0 + 0 x will be greater than zx. 
Consequently ox must be greater than y x, and zo greater 
than z y, when measured with the sextant. 
The point z is on the meridian, and the apparently curved 
line zo x represents the apparent curve of the Equinoctial ; 
VOL. VII. (THIRD SERIES.) 2 R 
