V and D corrections for Celestial Nav

[Mark_S]

Hi all,

Just a quick question. Been going over celestial navigation stuff recently, and whilst I know how to apply a V and D correction and when to use them, I'm not sure I've ever been told what they actually mean! I've asked around on board and noone seems to be able to give me a definitive answer, google hasn't been too useful either. If anyone could shed some light on the subject it'd be much appreciated.

[Aleorea]

V = Venereal
D = Diseases

[Steve]

I assume you understand the increments for hour angle (HA). d is the equivalent for declination (hence 'd'), applicable to all bodies within our Solar System. v is a fine tuning of the HA increments to account for the differing velocities (hence 'v') of the planets and moon, both with respect to each other and with time. It isn't applied to the sun or moon as they have their own dedicated columns in the increment tables.

You won't ever be asked that in an examination! But it is a good and interesting question. I had to look up the Admiralty Manual of Navigation Vol. 2 for the answer, unfortunately not available new in the shops, though old copies are in circulation.

[Mark_S]

Cheers for that Steve!

[Sdot]

'd' is an extra correction for additional declination movement. 'v' is an extra correction for longitudinal movement of a body.

Steve, I'm pretty sure that the Moon does need 'v' correction, along with the planets.

[Steve]

Steve, I'm pretty sure that the Moon does need 'v' correction, along with the planets.

Yeah, of course it does. Can you tell I haven't done a moon sight in a while?

[Sdot]

Yeah, of course it does. Can you tell I haven't done a moon sight in a while?

Haha me either, I was doing one the other night & thought ****, do I use 'v' correction with the moon!?

[YoungMariner]

I'll try to simplify the answer even more. When we work with celestial navigation, for the stars we assume that they are fixed in space on a 'celestial sphere', and this works fairly well for us because of their vast distance from us and associated apparent movement.

The problem with the moon is that it is in orbit around us, so is a little more complex to determine it's position. In the same way the planets are in orbit around the sun, so they also have a little complexity in determining their position. So we need a simple way to resolve that, and that is through the use of "v" and "d" corrections.

The "v" is an extra correction for additional longitude movement of the celestial body. The "d" is an extra correction for additional declination movement of the celestial body.
The "v" corrections will all be positive, except for venus where it may on occasions be negative. The "d" correction can be positive or negative depending on the trend of the declination (increasing or decreasing).

The "v" and "d" correction that you see on the daily pages is just a reference for the celestial bodys movement in one hour. So if you go into the yellow pages and used that "v" or "d" correction reference to lookup the correction for 30 minutes, it would half of the reference number.

I hope this makes a little more sense.