NavList:

The measurement sensitivity is not too great to begin with. One must choose a comparing object that is as close to the lunar orbital plane as possible to get the maximum sensitivity (~ 0.5 arc min per minute of time). Best to stick with planets or the Sun. If the comparing object has any significant out-of-plane component, then the closer it gets to the moon the slower the arc angle will change between the two, lessening the sensitivity and increasing the error bars of time estimate. So even for objects like the Sun or planets the angle between them and the moon should be chosen to be in the range of 45-90 degrees apart. But I would say there is also a best moment to make this measurement. The altitudes are required to determine the refraction corrections necessary to get a good arc distance. When a celestial object is at its meridian passage the altitude is not changing, and for a good bit of time about the passage the altitude changes slowly. If you wait until the mid point between the moon and the comparing object is at a virtual passage, then the two objects will be at the same altitude. One will be rising, the other sinking. The refraction corrections between the two will be the same, and if you are taking your time making the observations then the increasing refraction of one will be nullified by the decreasing refraction of the other. This may be too fine a point to bother with however. Rodger Farley
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