A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2017 Mar 27, 02:25 -0700
Mike. Navigational tables can only give values in convenient steps, e.g. hourly or every degree. If they had to give a value for every second of every day and/or every second of arc, the tables would be a mile thick. Corrections like d and v are to help fill in the gaps. For most astral bodies, ie the Sun, stars, and planets, GHA varies at a fairly constant rate, i.e. 15 degrees per minute, so you can use a simple arc to time table to work out GHA for intermediate times. Because the Moon is a relatively close planet of the Earth, its change in GHA with time isn’t quite so constant, so if you’re using a simple arc to time table, this variation in rate has to be allowed for as an additional correction, hence the v correction. When using the increments and correction tables (Appendix C in Mary Blewitt) to work out GHA Moon for an intermediate time, first look in the Moon column to get the basic arc to time for the Moon, then enter the last three columns with the v Moon value from the daily page to get the actual v correction to be added to the basic arc to time value. Because the tabulated basic Moon arc to time value is its lowest possible value, the actual v correction should always be added, so Moon v is always positive.
The best way to work most of this out is to download the full Nautical Almanac and HO249/AP3270 from the links given by Gary, because the explanations therein not only explain the tables but give worked examples of their use. Also, having the whole thing in front of you enables you to explore any ‘what ifs’ which might crop up when you read them. As Gary says, some of the corrections made necessary by using the Nautical Almanac can be avoided by using the Air Almanac, which has ten minute rather than hourly values in the daily pages. DaveP