Dates of mercury turning Retrograde can vary due to time zones (for example, western USA is 8hrs ahead of UK for example), and also because of differences in the way the ‘Mercury Stationery’ period is handled: the period when Mercury is no longer direct, but has not yet turned Retrograde so has ‘stationery’ motion in the heavens.
And it’s worth understanding the exact dates, because the effects of a planet are especially strong the slower its motion is.
And of course we feel the effects of a planet being retrograde often in a very difficult ‘crooked’ way (vakri) – both of these for our Divine Learning around the principle the planet represents.
So what exact date does mercury turn Retrograde in January 2015?
The Western astrology program Astrocalc’s Ephemeris lists the date of start of Mercury Retrograde as 23.1.15. Shri Jyoti star gives 22.1.15 (for London), and my astrological diary gives the date of the start of Mercury Retrograde as 21.1.15.
Joakim Schramm, the holder of the Astrocalc program, gives the following helpful illumination:
Astrocalc is actually correct. The real or exact moment Mercury turns from direct motion is during the 22nd January. However, you have to keep in mind the r,d and s marks (retrograde, direct and stationery). So, while Mercury ceases direct motion during the 22nd, at Midnight the 22nd it’s NOT yet retrograde. And remember, Ephemerises take their data from Noon or Midnight. The Ephemeris gives planet positions – but for these exact moments.
In practical terms this basically means that if you use the Midnight Ephemeris and it show r,d or s for a planet, it means the change took place during the previous day.
As for your 21st date and your astrological diary, I think it’s because it wrongly counts in the stationary phase as being part of the retrograde.
Note that Mercury is stationary for about 30 hours, starting after midnight the 21st January 2015, but it doesn’t start to move retrograde until about 5 AM GMT on the 22nd January.
If you want to exactly pin point the various changes in movement of planets, it’s better to use the movable wheel in the Astrocalc program.