NavList:
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Re: Double Altitudes: Prelude to Sumner's line?
From: Bruce Stark
Date: 2005 Feb 28, 17:38 EST
From: Bruce Stark
Date: 2005 Feb 28, 17:38 EST
Jim,
You wrote:
So my question now is, what is (was, more correctly), the
meaning of "double altitudes"?
Although the term has been used other ways, Sumner would have been talking about the technique of finding latitude from two altitudes of a body, together with the elapsed time between them. One of the oldest and best known of these was the one invented by Douwes. Maskelyne included the tables for it in his Requisite Tables: Those "Half Elapsed Time," "Middle Time," and "Rising" tables remained in most navigation manuals for at least a century and a quarter.
Years ago I was working it with my WW II era navigation tables. Just add the log of 1/2 in one spot and you can use regular trig-log and log haversine tables in place of the special tables. It's an interesting technique, but I certainly don't recommend it for general use!
Part way through the calculation you come up with "rising." It's the interval of time between the greater altitude and noon. You need it to take out "log rising" for the next part of the calculation. But, in case you are interested, it also gives you the error of your watch on local time. Not the most accurate way of getting time, but worth using if you missed the morning time sight.
Bruce
You wrote:
So my question now is, what is (was, more correctly), the
meaning of "double altitudes"?
Although the term has been used other ways, Sumner would have been talking about the technique of finding latitude from two altitudes of a body, together with the elapsed time between them. One of the oldest and best known of these was the one invented by Douwes. Maskelyne included the tables for it in his Requisite Tables: Those "Half Elapsed Time," "Middle Time," and "Rising" tables remained in most navigation manuals for at least a century and a quarter.
Years ago I was working it with my WW II era navigation tables. Just add the log of 1/2 in one spot and you can use regular trig-log and log haversine tables in place of the special tables. It's an interesting technique, but I certainly don't recommend it for general use!
Part way through the calculation you come up with "rising." It's the interval of time between the greater altitude and noon. You need it to take out "log rising" for the next part of the calculation. But, in case you are interested, it also gives you the error of your watch on local time. Not the most accurate way of getting time, but worth using if you missed the morning time sight.
Bruce
