02-29-2016, 12:05 PM
(This post was last modified: 02-29-2016, 12:07 PM by jeremyparker.)
Fred you're correct, 'v' stands for the variation in the rate of change of GHA between the mean rate of 15* per hour, as tabulated in Increments and Corrections, and the actual rate of change for that particular planet at that particular time.
The earth rotates relative to the sun at a fairly fixed rate of 15* per hour. There is very little variation in this rate and what there is is negligible within the tolerances of small craft navigation. The Increments and Corrections for the sun in the NA are based on this rate. We see that the same corrections are used for the planets, but, because, as Stargazer correctly explains, these have their own orbital path independent of earth, the rate at which they appear to circle the globe is variable and this is allowed for by the v factor.
We can prove this by comparing the GHA of a planet over two consecutive hours.
For example:
Venus - Sat 27 Feb, 2016
00 hrs 201* 30'.5
01 hrs 216* 29'.8
diff. 14* 59'.3, which is 0'.7 less than 15*
v correction at foot of page -0'.7
You'll notice that the moon's GHA also has a variable rate, and the moon is so erratic that this is tabulated for each hour! Increments and Corrections for the moon's GHA are based on a mean hourly rate of change of 14*19'.0 but we see that in the first hour of 27 Feb 2016 the actual increase is 14*34'.0, a value 15'.0 more than 14*19'.0 - and we see the v correction tabulated for that hour: 15'.0.
We apply the v correction to GHA in the same way as the d correction for declination: by finding its value for the appropriate whole minute in Increments and Corrections and appending that to the GHA correction:
Venus 003215
GHA 00h 201*30'.5
32'15" 8*03'.7
v -0'.7 -0*00'.4
209*33'.8
Moon 003215
GHA 00h 312*38'.6
32'15" 7*41'.7
v +15'.0 +0*08'.1
320*28'.4
Jeremy
PS - apologies for the asterisk - I'm not sure where to find a degree symbol on this keyboard!
The earth rotates relative to the sun at a fairly fixed rate of 15* per hour. There is very little variation in this rate and what there is is negligible within the tolerances of small craft navigation. The Increments and Corrections for the sun in the NA are based on this rate. We see that the same corrections are used for the planets, but, because, as Stargazer correctly explains, these have their own orbital path independent of earth, the rate at which they appear to circle the globe is variable and this is allowed for by the v factor.
We can prove this by comparing the GHA of a planet over two consecutive hours.
For example:
Venus - Sat 27 Feb, 2016
00 hrs 201* 30'.5
01 hrs 216* 29'.8
diff. 14* 59'.3, which is 0'.7 less than 15*
v correction at foot of page -0'.7
You'll notice that the moon's GHA also has a variable rate, and the moon is so erratic that this is tabulated for each hour! Increments and Corrections for the moon's GHA are based on a mean hourly rate of change of 14*19'.0 but we see that in the first hour of 27 Feb 2016 the actual increase is 14*34'.0, a value 15'.0 more than 14*19'.0 - and we see the v correction tabulated for that hour: 15'.0.
We apply the v correction to GHA in the same way as the d correction for declination: by finding its value for the appropriate whole minute in Increments and Corrections and appending that to the GHA correction:
Venus 003215
GHA 00h 201*30'.5
32'15" 8*03'.7
v -0'.7 -0*00'.4
209*33'.8
Moon 003215
GHA 00h 312*38'.6
32'15" 7*41'.7
v +15'.0 +0*08'.1
320*28'.4
Jeremy
PS - apologies for the asterisk - I'm not sure where to find a degree symbol on this keyboard!