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Frank Reed wrote, on 22 Sept.

>Consider especially the case of Sun-Moon lunars tables. If you
>stay in ecliptic coordinates, the calculation is very short since the Sun's
>ecliptic latitude is zero. The lunar distance, LD, is calculated from the
>difference in ecliptic longitude and latitude using the simple rule
>  cos(LD) = cos(diff_longitude)*cos(diff_latitude)
>With the stars and planets, the ecliptic latitudes are low enough that
>certain approximations can be applied which also reduce the task of
>computation.
>Rotating to RA and Dec would lengthen the calculations quite a bit.

In a reply, I copied that text but omitted the first two sentences, then
objected that the "simple rule" would give inexact answers. But in the
special case of the Sun and Moon, the Sun's ecliptic latitude is always
zero (as near as dammit) as he said. As a result, Frank's simple rule is
indeed correct, in the case of the Sun. What he wrote was clear enough, but
I had (as he says) "missed the point". Sorry about jumping to confusions.

George.

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