The Mercury retrograde
shadow period.
The shadow period is the stretch before and after a Mercury retrograde when Mercury crosses the same degrees it will retrace. For the next cycle, pre-shadow begins January 25, 2027 at 18:58 UTC and post-shadow ends March 23, 2027 at 14:58 UTC.
What the shadow is
During a retrograde Mercury backs up over a stretch of the zodiac, typically 12 to 16 degrees. It crosses those degrees three times: forward before the retrograde, backward during it, and forward again after. The two forward passes are the shadow periods. Pre-shadow begins when Mercury first reaches the degree where it will later station direct. Post-shadow ends when Mercury, moving forward again, passes the degree where it had stationed retrograde. For the coming cycle, pre-shadow begins January 25, 2027 at 18:58 UTC, the retrograde runs from February 9, 2027 at 17:36 UTC to March 3, 2027 at 12:32 UTC, and post-shadow ends March 23, 2027 at 14:58 UTC.
How long the shadows last
Each shadow usually lasts two to three weeks, slightly longer than the retrograde itself, because Mercury moves more slowly near its stations. Counted from the start of pre-shadow to the end of post-shadow, the whole cycle spans about eight to nine weeks: for the next one, 57 days. In that view Mercury spends more of the year in a shadow or retrograde than out of them, which is why the shadow is better read as a geometric fact than as a warning.
Pre-shadow
In pre-shadow Mercury moves forward at full speed but is already inside the degrees it will revisit. It begins January 25, 2027 at 18:58 UTC and lasts 15 days, until the station retrograde on February 9, 2027 at 17:36 UTC. Traditionally this is the window for finishing what should not be reopened later: closing agreements, sending the final version, backing up what matters.
Post-shadow
After the station direct Mercury is moving forward again, but slowly, and it is still covering degrees it passed twice already. Post-shadow for the coming cycle runs from March 3, 2027 at 12:32 UTC for 20 days, until March 23, 2027 at 14:58 UTC. Only then is Mercury on new ground. Tradition treats this as the time to restart what was paused during the retrograde and to send the revised version rather than the first draft.
Why the shadow is not in every calendar
Many calendars list only the two stations because those are unambiguous astronomical events. Shadow boundaries are a convention: they depend on reading the station degrees from an ephemeris and finding when Mercury first and last occupied them. Cazimi computes them from the same JPL DE421 ephemeris as the stations, to the minute in UTC, so the pre-shadow start and post-shadow end on each cycle page are as precise as the stations. How the moments are timed.
The shadow and the cazimi
The shadow has nothing to do with the Sun's shadow or an eclipse. The word describes the degrees Mercury "casts" ahead of and behind its retrograde. The Sun enters the picture only at the cazimi, the inferior conjunction in the middle of the retrograde: for this cycle, February 18, 2027 at 16:39 UTC. What the cazimi is.
Questions
When does the next Mercury retrograde shadow period start?
Pre-shadow for the coming cycle begins January 25, 2027 at 18:58 UTC, 15 days before the station retrograde on February 9, 2027 at 17:36 UTC.
When does the post-shadow end?
Post-shadow ends March 23, 2027 at 14:58 UTC, when Mercury passes the degree where it stationed retrograde on February 9, 2027 at 17:36 UTC.
Is the shadow period part of the retrograde?
No. Mercury moves forward during both shadows; only the 22 days between the stations, February 9, 2027 at 17:36 UTC to March 3, 2027 at 12:32 UTC, are the retrograde.