v for the sun

[albyreto]

hi,
why there's not v correction for the sun GHA?
In italy the "Nautical Almanac" have the v correction for the sun.
Maybe its not important but sometimes give me a correction of 0.2'/0.3' on the sun GHA

thank you!
Alberto

[c_davidson]

hi,
why there's not v correction for the sun GHA?
In italy the "Nautical Almanac" have the v correction for the sun.
Maybe its not important but sometimes give me a correction of 0.2'/0.3' on the sun GHA

thank you!
Alberto

Alberto,

The Nautical Almanac, either on this site or from the USNO doesn't list a v correction for the Sun.  Given the accuracy of a position determined using a sextant isn't exact or precise a difference in position by 1 nm is no great amount.

If you'd like to, it's easy enough to obtain the v correction by comparing the Sun's GHA of the time of the sight to the next hour's GHA of the Sun.  For example, today, March 30, 2017, the Sun's GHA is increasing per hour by 15° 00.2'    The v figure is positive and is increasing about 0° 0.2' minutes per hour.  For this date only, that's not very much of an increase in GHA or a correction that is necessary to get a good LOP.  It would be more precise to correct for the Sun's GHA v figure but that takes more time and would probably be considered unnecessary. 

That's also why we don't correct for the Sun's "d".  It won't make that much difference in your position and your pencil width, even with a pencil of 0.5 mm, when drawing the LOP will often render even the most accurate figures to be incorrect (though not by an amount that one should be worried about).

Clen

[albyreto]

Thank you so much for your answer,

Alberto

[BigBill]

I understood that the v correction was built in to the GHA for the sun.
True or no?

[craigsl]

I understood that the v correction was built in to the GHA for the sun.
True or no?

Not that I have superior knowledge of these things but, yes, the GHA of the Sun is augmented enough to account of the v correction.

Regardless, as Clen said above, any correction for v or d of the Sun is insignificant and, for our purposes, isn't done.

[jeremyparker]

hi,
why there's not v correction for the sun GHA?
In italy the "Nautical Almanac" have the v correction for the sun.
Maybe its not important but sometimes give me a correction of 0.2'/0.3' on the sun GHA

thank you!
Alberto

Alberto
The advance-per-second of a body's GHA, as indicated by the table of Increments and Corrections, is based on a mean or least rate of change for that body, i.e Sun 15º/hr; Aries 15º02'.5/hr; Moon 14º19'/hr.
For the planets we use the Sun's mean rate, but because their apparent orbital rate varies there is a further correction for that variation - given as the v correction and tabulated either daily or the mean over 3 days, depending on the Almanac. This variation may be more or less than the Sun's mean rate.
We have a similar correction for the Moon - the rate of change tabulated is its least rate, but at times the Moon appears to move more rapidly through our heavens and as this variation is quite erratic, its v correction is tabulated hourly.
But for the Sun, whilst it is true that, at various times of year, its apparent orbit advances or retards, the amount of variation is generally negligible. Any such variation is accounted for by the whole values of GHA for each hour and, if we wish to be so precise, we may compare those values and determine the actual rate of change - but, as Clen points out, this will only amount to decimals of a minute over the hour so can be considered insignificant within the tolerances of sextant navigation.

However, there certainly is a correction for 'd'. The value of d in the Almanac is simply the hourly rate of change of the body's declination. For the Moon this is tabulated hourly, as the Moon is such an erratic beast, but for the Sun an average over 3 days is given, its rate of change being quite slight - it takes the Sun six months to get from Capricorn to Cancer! You can confirm the truth of this by comparing one hour's declination with the next, and indeed we do this to check whether d is added or subtracted.

Jeremy

[CelNav57]

Albyreto,


Maybe the following will be of some help. Bowditch in Chapter 19- THE ALMANACS page 3 (Bowditch overall page 289) explains why there's no v correction for the Sun.  It's negligible.  You can find the part to explain this in the left hand column beginning with the words, "In the Nautical Almanac, the permanent table for"

See here- Sun GHA, v, dec, etc.

CelNav57