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Bill wrote:

>I am aware of the almanac corrections, but those are for altitude (up from
>the horizon).
>
>When shooting lunars with Venus recently, Venus can range from the same
>altitude as the moon to lower than the moon, so altitude corrections may not
>apply as the moon-as-horizon would split a different "side" of the partially
>illuminated sphere than a true horizon.
>
>If Frank's program does not take phase into account for Venus and I am
>splitting a body that has an apparent width of 10" instead of 40" (right
>side illuminated?), then my observation would be 15" off.
>
>As we move towards inferior conjunction Jan 13, Venus may become 60" or more
>in diameter, with the area illuminated diminishing.  If not accounted for
>one way or another, we could be looking at an errors up to 1.0'.
>
>

You may be looking at errors of up to 1.0', but you will hardly be
looking at Venus at inferior conjunction before June 6, 2012!

That said, in principle, you are right. But I don't understand your
point about the almanac correction being for altitude only. If the
almanac tabulates center of illuminated area instead of center of mass,
why would this not be applicable to all kinds of optical observation?

Herbert Prinz

   

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