Every Mercury Retrograde is accompanied by what is known as the ‘Shadow Period’. The Shadow Period refers to the time period in which Mercury travels over the area of the chart it will retrograde in.
So, for example, the last Mercury Rx started from 26° 55′ Pisces on 2 March, back to 13°11′ Pisces, going direct on 25 March. The Shadow Period for this Retrograde began when Mercury started going into 13°11′ Pisces on 17 February. This refers to the ‘Back-end Shadow’, i.e. the shadow preceding the retrograde.
The time it takes Mercury to return to the point it was at before it retrogaded is known as the ‘Front-end Shadow’. So the March retrograde saw Mercury reach 26° 55′ Pisces again only on 13 April.
For a visual representation of the retrograde and shadow periods, see this article in Astroprofile. Cafe Astrology also has an effective graphical representation of retrograde cycles and stations.
Some claim that the retrograde effects can be felt subtly, as strongly, or even more strongly than during the actual retrograde period itself. While the views on its intensities vary, all seem to agree that Mercury retrograde effects can nevertheless be felt during the shadow periods, effectively extending it from three weeks to nine!
I haven’t thought about Shadow periods myself, but will try to pay more attention during the next retrograde in July (and I’m travelling then too!). I will update my iCal calendar to reflect shadow periods from today.
2006 Mercury Retrograde Cycles and Calendar. Accessed 6 May 2006.
‘Retrograde Cycles and Stations’. Cafe Astrology. Accessed 6 May 2006.
Shawvan, Jim. Mercury Retrogrades in 2006, with Shadows and the Storms. Accessed 6 May 2006.