position in regard to the fixed plane, of the orbit of reference, and of the departure- 
point in this orbit, are determined by O', a', <£',—that is, we have for the orbit of 
reference, 
O', the longitude of node, 
a', the departure of node, 
</>', the inclination. 
The position of the planet’s orbit in relation to the moveable orbit of refer 
ence is determined in like manner by ©, X, d>,—that is, we have for the planet’s orbit 
in relation to the orbit of reference, 
©, the longitude of node, 
X, the departure of node, 
( f>, the inclination. 
Hence if, as before, 0, a, </>, belong to the orbit of the planet considered in relation 
to the fixed plane, X — a, © — a, 0 — O', will be the sides of a spherical triangle, the 
opposite angles of which are 0', 180° — cf> and d>. 
Putting for shortness S = X — a, S' = © — a', G = 0 — O', so that these symbols denote 
S, the distance of node, along planet’s orbit, from fixed plane, 
S', the distance of node, along orbit of reference, from fixed plane, 
G, the distance in fixed plane of the nodes on fixed plane, 
the sides of the spherical triangle are S, S', G, and the opposite angles are <£', 180° —</>, <t>. 
c. in. 36