A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Peter B
Date: 2017 Mar 3, 09:28 -0800
Our clocks and The Nautical Almanac keep time by UTC, which is for our purposes the same as GMT. GMT is based on the meridian crossing of the mean sun, not the stars. The sun "backs up" to the ewastward against the starry background just about 1 degree each day. Therefore the GHA of Aries listed in the almanac increases just about 1 degree each 24 hours. Check it out in the daily pages.
Stated another way around the sun back slides a little eastward against the steady westward motion of the sarry background each day by about 1 degree. The time for the sun to make the complete circuit back to where it started is the 365.25 days of a solar year. It is also why the sun moves from one zodiac sign to another throughout the year. It turns out that all of the solar system objects back slide in the same manner, each at their own particular rates...except when a planet is in retrograde.
If you were to go out each night at midnight as indicated by your clock, which tracks the sun, not the stars, you would find that the stars that were on your meridian last night will have passed your meridian 4 minutes (about 1 degree) earlier tonight. Well, that is "4 minutes early" as indicated by your clock.
When the planets migrate eastward against the starry background they are in "direct motion." When they migrate westward they are in "retrograde motion." Moving furter westward implies a bigger GHA.
Venus would appear to be standing still against the stars when it was just matching the 1 degree increase in GHA of Aries each day. This is a " v " value of + 2.5. Venus would go into in retrograde when its GHA increases by more than 1 degree per day or v > 2.5. In fact any planet is in retrograde when its v > 2.5
(60 arc minues/degree)*(1degree/24hours) = 2.5 arc minutes/hour
Of course the abbove is all just "thumbnail calculations." If you really wanted to refine this you would compare the hourly change in GHA Aries to the hourly change in the GHA Venus. When the Venus value is greater it is in retrograde.