A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2016 Oct 16, 16:04 -0700
v and d corrections as I see them
The Nautical Almanac (NA) gives GHA and declination for Aries, Sun, Moon, and Planets for every hour of the year.
Mariners might take shots at any second of the year, so increment tables are required.
Unfortunately, hourly changes in GHA and declination for the Sun, Moon and Planets are not constant, because of axis tilt, relative motions and eccentricities of orbits, so we could end up with 60x365 pages of increments.
To reduce these to 60 pages we use average values of GHA and declination changes for the increment tables and add d and v corrections instead, the hourly differences from average being given on the relevant daily pages.
Now whereas d & v for the Planets and the Moon can be quite large, d for the Sun is relatively small.
It’s probably true to say that because of the practical difficulties of using the marine sextant at night; mariners shoot the Sun more than any other body. Also, they would be pleased to reduce the number of corrections by one from the large number they have to do, particularly if the effect of ignoring v is small in comparison to the other errors they might expect.
Therefore, for the Sun alone, it makes sense to do just that, ignore v, but tweak the Sun’s daily page hourly values in such a way that the maximum error throughout the hour when using the increment tables is minimised. If you like to put it that way, yes the NA keeps the Sun’s speed at 15degrees per hour in the increment table and adjusts its GHA at the hour points on the daily pages to minimise the maximum error at any time during the hour. I hope this approach makes sense.
I should look again at your calculation of 0.25 minutes of arc GHA error causing a maximum of 4nm position line error. I make it more like 0.25nm. I think if you follow this now, you’ll see you don’t need to start involving longitude in the explanation or email UKNA again. DaveP