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Saturday, October 16, 2010

Lunar Distances with Jupiter

Tonight and over the next few nights (if the weather holds) should be excellent for lunar distance practice to obtain Universal Time (formerly known as Greenwich Mean Time) by measuring the distance between the moon and Jupiter. Remember to align the moon's limb to the center of the planet, as Jupiter's present apparent diameter is large enough to affect your sextant readings.

The easiest method to compute UT by lunar distance is of course with a calculator or computer, but most of the point of computing them in the first place is to be able to derive UT at sea without any electronic means whatsoever, such as might occur after a lightning strike of a sailboat's mast (an all-too-common occurrence at sea). So instead, a set of tables such as those created by Bruce Stark are probably the best method of clearing lunar distances and deriving UT from them.

The other primary purpose of measuring lunar distances is to afford sextant practice to the landlocked celestial navigator. No visible horizon is necessary, so the sights may be taken from any location (and at any time) from which the moon and another bright object in the ecliptic may be seen.

An interesting astronomical application for this method which requires no special tables at all, but does require about a month's worth of observations, is comparing the moon's apparent diameter with it's hourly eastward rate of motion across the sky. With a simple marine sextant and a little patience, you can correlate the moon's daily relative distance based on apparent diameter (or actual distance, based on a mean lunar diameter of 3474 km, but this is in no way necessary) with its daily rate of movement relative to the sun, a planet or a star in the ecliptic. Armed with a months worth of data, you can easily demonstrate the elliptical nature of the moon's orbit and prove Kepler's three laws of planetary motion.

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