v for the sun

[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