Re: sun's distance as lab exercise?

[Leigh Palmer <palmer@SFU.CA>]

> By definition it is always the local sidereal time at zenith.  So what
> you want to do is to take the sun and moon right ascensions, and find
> the corresponding longitudes where the local sidereal time is equal to
> the right ascension.  For instance, at this moment, it is 20:52 LST
> (local sidereal time) in Eugene Oregon (my location).  Therefore, at
> zenith, the right ascension is also 20:52.  Right ascension increases to
> the east.  So if an object has RA of 21:52, then it is at zenith one
> hour to the east of me (112 degrees longitude at my location and 97
> degrees longitude where the moon is at the zenith - 1 hr east of me).

This works for stars, but not for the Sun and Moon, at least not to
the accuracy required here. The reason is that the right ascension of
the Moon is only equal to the sidereal time at the specific longitude
at which the Moon is in the zenith or elsewhere on the meridian. The
right ascension of the Moon at any instant depends upon the observer's
longitude. The effect is the parallax, of course.

Leigh