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Fred Hebard asked-

>I use the parallax of the sun in clearing lunar distance observations,
>and haven't done any for a while, until this evening (about 1.5-2.5
>minutes out).  It's usually 0.1' of arc.  Is it tabulated or mentioned
>in the Nautical Almanac?  I couldn't find any mention of an actual
>value.

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For what it's worth, the average value of the Sun's Horizontal Parallax
(the parallax the Sun would show if it was on the horizon.) is 0.146
arc-minutes. It's given (in arc-seconds) in Bowditch. This value varies
slightly between Northern Winter (when the Earth is slightly nearer to the
Sun, due to its elliptical orbit, so parallax is slightly greater) and
Northern Summer (vice versa), but not enough to matter.

If you want to calculate a value for the Sun's HP from the almanac, it's
the Sun's semidiameter multiplied by .00916 (which comes from the ratio of
the radius of the Earth to the radius of the Sun).

But the actual parallax of the Sun depends on its altitude, reducing with
altitude prorortionally to cos alt until it's zero when vertically above.

The Nautical Almanac tabulates angles only to the nearest 0.1 arc-minutes,
so you can see that a correction of 0.1 arc-minutes is about the best shot
it can make, better than saying "just forget Sun parallax".

George.

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