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    Jupiter and the Moon
    From: Frank Reed
    Date: 2013 Jan 20, 11:39 -0800

    Tomorrow night (US time: Monday January 21 and into the early morning hours of Tuesday). the Moon will pass very close to Jupiter again, as seen from North America. In a large portion of South America, as well as the Galapagos Islands and a swathe of the South Pacific, the Moon will occult Jupiter. Closest approach is late on Monday evening for North America.

    This is a fine opportunity to practice lunars, and the observation is nearly identical to measuring the Sun's diameter with your sextant which I've described recently. Since Jupiter is big (more than 0.5' in angular diameter), you will be able to see its disk with a 6x or 7x scope. Traditionally with lunars, the observer is expected to "split" the disk on the Moon's limb when bringing it into contact. The angular distance from Jupiter to the near limb of the Moon will be less than a degree for most observers in the Americas. The exact value depends on your location.

    A "lunar" like this would never have been used historically for determining GMT in the traditional practice of finding longitude by lunar distances. The angle changes too slowly since Jupiter is located off to the side of the Moon's track through the sky, and there are other problems connected with small angle lunars. But there's nothing wrong with it for practice. It's still an excellent test of your sextant and your skill in using it. It also could be used for a position fix assuming that GMT is known (as it always is today). In fact, by observing this distance, you can determine your latitude with reasonable accuracy (one s.d. 6 n.m. by averaging four sights) even if you can't see a horizon at all. Latitude by lunars. What a crazy idea!


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