Reed's Sextant Simplified
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Reed's Sextant Simplified Customer Reviews
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How to use, adjust, and care for a sextant
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There are a number of books that give on advice on what to look at with a sextant. Pike does mention appropriate targets for navigation, such as the Sun, the Moon, Venus, and Polaris, as well as coastal landmarks (with known heights). But the main focus of the book is on how to use the sextant: how to take sights, how to read the sextant, what errors one will encounter and what adjustments one can make to mitigate them, and how to care for the sextant.
The sextant, of course, is employed to measure angles between objects, especially the angular distance of heavenly bodies above the horizon. A single sighting of Polaris, together with a copy of the Nautical Almanac, can give one the latitude outright. Of course, that only works at night (with Polaris not covered by a cloud), and one has to be at least a couple of degrees into the Northern hemisphere to see Polaris. And a sighting of the Sun as it goes through high noon (once again, this works only if the Sun is visible then) also gives one the latitude directly, using that Nautical Almanac. Used in conjunction with an accurate timepiece, it also gives the longitude.
While Pike does not give advice on what to do with the information you get to, say, plot your position on a chart, he does give advice on how to deal with the problems of taking sights on a moving boat. This is by no means an easy task!
This is a fine book, but I wish to comment on the following topic: is a sextant still that useful an instrument in ocean navigation?
Can one navigate without a sextant? Of course: all one needs is a working GPS system. Of course, if one's GPS isn't working, it's not a bad idea to have a sextant as a backup.
Can one navigate with a sextant but without the Nautical Almanac (or an equivalent, such as Bennett's less precise "Celestial Navigator")? Well, yes. A simple program such as "Pocket Stars" will fit on one's PDA and can be used to do the calculations. Of course, that won't help much if your PDA runs out of power.
Can one navigate lacking both GPS and a sextant? That's not so easy. Captain Bligh had to do that when he was denied a sextant after he and some of his men were evicted from the Bounty by the mutineers. And he did a terrific job of it with an improvised but rough angle-measuring device. Even without such equipment, one can use "dead reckoning," which means estimating "course made good," hopefully with the aid of a compass. And you can also get some very useful information by taking sun sights at sunrise or sunset (no sextant is needed for these, but the variation errors due to the atmosphere can be rather significant).
I think this puts the utility of a sextant in perspective. It can be used to determine one's latitude to well within a mile, and it is really useful if your GPS is not available. |
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