2. Find (using on-line tables of ephemerides) what two locations have
the
sun and moon respectively at zenith at that moment
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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).