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On Jan 15, 2004, at 8:13 PM, George Huxtable wrote:

> Fred Hebard said-
>
>
>> I suppose right around meridian passage of the moon might be a bad
>> time
>> to take a lunar, especially where it's altitude is about 90 degrees?
>
> Why do you say that, Fred? It isn't so, though it's what I have argued
> until the last few days. Although the AMOUNT of the parallax changes
> little
> around meridian passage of the Moon, the DIRECTION of the displacement
> of
> the Moon by parallax is changing very fast, and so, therefore, is the
> component, along the Moon's path, of the displacement between true and
> apparent Moons. The "clearing" procedure is clever enough to work all
> this
> out, and get from the uneven apparent motion to the regular true
> motion, at
> all altitudes of the Moon.

Meridian passage for the moon on an earth with a 12-hour day was when I
was thinking parallax could get the moon to stand still with respect to
the heavens.  That would also be when the altitude of the moon was
standing still, so that one would be back in the predicament of no
change in either argument to the clearing function (ignoring
refraction).  The arguments would be distance and altitude.

However, I don't have the presence of mind at this hour (or possibly
any other!) to figure out whether the moon would appear to be standing
still when it was near the zenith or near the horizon.

The fact that the derivative of sine is cosine, and vice versa, can be
very confusing.

Fred

   

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