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    Re: Lunar distance measurement in ideal conditions: attainable accuracy.
    From: Frank Reed
    Date: 2013 Jan 16, 17:20 -0800

    This is a somewhat different issue. Pointing the telescope at a single body is not normally a problem since the scope magnification on a sextant is usually not high. Though sextants from the 19th century did include 12x scopes (specifically for lunars) that's the highest you're likely to find and modern sextants rarely include anything above 7x magnification. So for a lunar, you just point the instrument at the Moon and find it in the horizon view. But the tricky part is getting the other body in the reflected view. Actually, it's not all that difficult if you know two tricks: as Alex said, you preset the sextant for the approximate predicted lunar distance (not cheating since you always know the approximate true time and tables of the predicted distance are a prerequisite for the method anyway), and rotate the sextant until the Moon's "horns" are horizontal in the field of view. The latter works perfectly when the Sun is the other body since the points at the ends of the terminator (the horns of the crescent moon) are necessarily on a line that is perpendicular to the direction to the Sun. It works reasonably with other bodies for traditional lunars since the method is only useful for stars and planets that are more or less along the Moon's path through the sky, which is near enough to the ecliptic that the horns will still be perpendicular to that direction, too. These two tricks, by the way, were well-known historically. This is another case where lunars are, in a way, less stressful than standard altitude sights in celestial navigation. The altitudes of celestial bodies change too rapidly for presetting so navigators learn various methods for bringing the body down to the horizon. For lunars, as long as you know the true time (GMT/UT) within an hour, the angle will not change by more than half a degree. The parallax correction amounts to as much as one degree, but even with these two differences, you're guaranteed that the other body will be in the sextant's field of view.

    You could try out the above tonight if the weather is good. Around 7pm PST, the Moon-Jupiter distance is just about 60.9 degrees. Two hours later, it's 59.8 degrees. Preset your sextant to the right angle. Aim at the Moon. Then rotate the sextant, keeping the Moon centered in the field of view, until the horns are horizontal (that is, the line through the horns will be perpendicular to the frame of the sextant). The reflected image of Jupiter should pop right into view near the direct image of the Moon. Make it "kiss" right on the limb of the Moon (if Jupiter's tiny disk is visible, you should bisect it on the limb) and then get out your magnifier and read that vernier! You should also record the time (UT/GMT) so that you can test your results. And of course, you should try again a few minutes later. That's probably the most impressive thing to see when you first shoot lunars: you can detect the Moon's motion right away.

    You can get your own predicted distance tables and also clear your lunars using my online tools here: http://reednavigation.com/lunars/


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