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    Re: Lunar clearance methods
    From: Frank Reed
    Date: 2013 Sep 4, 20:08 -0700

    Francis, you wrote:
    "I've been doing Cnav for years and joined the lunatics 2 years ago, but this is my first posting."

    Welcome. Glad you've uncloaked. :)

    You asked about the method I discovered for clearing lunars that makes a very large number of them exceedingly easy --almost no trig. I'll see if I can write this up fresh within the next week. The short form: you observe the altitudes of the Moon and other body also the lunar arc between them. If a certain condition is met (details to follow), then you throw out the Moon's altitude and replace it with a calculated altitude such that the two altitudes and the measured lunar distance add up to 180° (assuming they're on opposing azimuths which is usually the case when the condition is met). In other words, you throw out a spherical triangle problem and replace it with a vertical circle problem.

    As Örjan Sandström has already noted, you probably should not try using a Bygrave slide rule for clearing lunars by any "direct triangle" method. You can, however, use a slide rule for the trig portions of a series method since you only need three to four significant figures in the resulting values of the factors which I have termed the "corner cosines".

    You also wrote:
    "I've been studying the use of lunars in Cook's Endeavour voyage. His journals say he had 1767-8 nautical almanacs to help with the "short," method of clearance. However, his journals mention lunars taken on the return trip, after Greens's death, in 1771,3 years later."

    Oh that's interesting. I've never looked into this. Alex Eremenko may know more. It's possible they had available "early" or "draft" editions of the Nautical Almanac tables for that period. It's also possible that they simply shelved the observations for later clearing when they returned home. When lunars are taken for mapping purposes, as opposed to live navigation, there's no need to clear them as they're being taken.


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