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Thursday, December 30, 2010

Excellent Lunar Distance Link

Here's a link to a website with some amazingly cool on-line calculators for lunar distance applications. I've long been a fan of the Bruce Stark tables for clearing the lunar distance to obtain UTC (Universal Coordinated Time, previously called GMT) at sea, but they are admittedly a bit arcane, and require tables and forms not typically carried for routine celestial navigation. For emergency navigation I've long preferred David Burch's lunar altitude method, which utilized both standard sight-reduction methods and standard sextant observations to derive UTC. Because if you're in a lifeboat and your only source of UTC is the moon, you probably don't actually have the means of clearing lunar distances with you, unless you happen to have the calculations for such already loaded onto a programmable calculator like StarPilot

This website has a number of calculators both for clearing lunar distances and predicting them. It's this latter which I think is huge. Because other than a hobbyist, nobody who has access to the internet is going to be sighting the moon and a star with a sextant to determine what time it is. But if you can look at your planned route before a voyage and precompute a number of lunar distances for each day of your journey, then with the addition of a perpetual almanac such as the Kolbe tables you effectively have a wholly self-contained system of navigation to cross an ocean without reliance on any external source. And, by having the lunar distances precomputed, instead of doing all (or any!) of the computations typically needed, you just dial in the correct distance into your sextant, point and wait. When the moon kisses the other object, mark the time on your hack-watch, and then you know the difference between it and UTC for all of the rest of your celestial sights for that day. Pretty darned cool.

Here's the link, and if this happens to be your website, great work and thank you!!

Longitude by Lunars

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