Saturday, April 26, 2014

Sight Reduction Tables - Not So Obselete

Someday my children; or perhaps grandchildren; will come across these volumes of carefully arranged numbers and wonder just what in the hell they are; and what they were used for. That will explain this post.

I used these books while in the Navy; and later as a Merchant Mariner; to find where we were while at sea. They were commonly called “lattice tables” if my memory holds correct. When shooting the sun or stars for a position these books were invaluable since they saved you hours of the computations necessary to obtain the information already contained in the book.

The book itself   is the finished product of about 3,000 years of observations and calculations made by mariners who were sometimes representative of nations who were at war with one another. But still the knowledge was shared. That’s how important these books were. Now, of course, everything is on computer and smartphones, navigation satellites, etc. but at one time you had to actually know what the numbers represented. And how to use them.

Basically the numbers are published in six volumes, with each spanning a 15 degree segment of latitude. The one at top would be used in latitudes from Northern Brazil to the lower southern United States.; or any other location in the world within those boundaries. They are used for navigating by sextant while crossing the ocean and out of sight of land.

Their use is outlined in the beginning pages so that anyone can open the book up and teach themselves. That’s what I did. You take your longitude in relation the Aries; this is called your Local Hour Angle. That, along with your assumed latitude and sextant reading will yield you the precise mathematical location of the star, or planet, which you have “shot” with your sextant. Converted, this will give you a line of position. Ideally, you do this for several stars.

These lines of position will then be used to lay lines on the chart to the points where the stars would be if they were at sea level. Just like a lighthouse, or bouy. The point where all of your lines cross represents the position you are at on the terrestrial plane.The satisfaction of making these computations is enormous, as they give you a sense of just how small you, and your ship, are in comparison to the larger picture. Nowadays there is a place to go online and the computer will make these calculations for you.

You may have seen these for sale on e-bay. They run anywhere from $9 to $20. I suppose many of the people snatching them up are survivalists, waiting for the end times when all the technology will come crashing down and  knowledge will be magic. More power to them. I’m holding onto mine because when I hold it in my hands I am 26 years old again, “punching pubs” at dawn; or dusk; beneath a red light to preserve my night vision. The ship is rolling and the air smells like diesel; or maybe that was me. 

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