Retrograde motion is a viewing angle
Mercury never reverses direction. It orbits the Sun in one direction at all times, taking about 88 days to go round. What changes is our line of sight to it.
Earth and Mercury are both moving, and when Mercury swings between us and the Sun it is travelling in the opposite direction across our field of view. For a few weeks it appears to slide backwards against the fixed stars, then it appears to stop, then it resumes. The same thing happens when a faster train overtakes a slower one and the slower one seems to roll backwards.
The two moments where the apparent motion is zero are the stations. Station retrograde begins the period, station direct ends it. Everything in the table is derived by measuring the change in Mercury's ecliptic longitude either side of a candidate time and narrowing in on the sign change, which is what makes the times exact rather than copied.
Why it happens three or four times a year
Mercury laps the Earth roughly every 116 days, the synodic period. Each lap produces one retrograde, so three of them fit comfortably in a calendar year and occasionally a fourth is caught at one end. The retrogrades in a given year tend to fall in signs of the same element, because 116 days is close to a third of a year and the pattern rotates slowly.
The duration is not constant. It ranges from about twenty to twenty five days, depending on where Mercury is in its noticeably eccentric orbit when the alignment happens. The days column shows the real figure for each period rather than the usual quoted average.
The shadow periods and the stationary days
Astrological practice usually extends the period at both ends. The pre-retrograde shadow starts when Mercury first reaches the degree it will later retrograde back to, and the post-retrograde shadow ends when it finally passes the degree where it turned. Together those add roughly two weeks either side, which is why some calendars make the whole thing sound close to permanent.
The stations themselves are the part most astrologers watch. On those days the apparent motion is near zero, so Mercury sits at one degree for an unusually long time, and the convention is that whatever it is touching gets emphasised. Whether that convention holds is not something this page asserts. What it gives you is the exact day, which is the part you can check against your own experience rather than take on trust.
What the reputation is worth
Mercury retrograde has become the one piece of astrology that people who dislike astrology still use, mostly as a joke about technology failing. It is worth being clear about what is claimed and what is not.
There is no mechanism by which a change in the apparent direction of a planet affects electronics, contracts or travel, and no study has found the effect. The honest description is that this is a traditional interpretation attached to a real and precisely computable astronomical event, and that the tradition is old and widely held.
The event is real. The dates are exact. Everything after that is interpretation, and this site tries hard to keep the two apart.
Questions people ask about this
How long does Mercury retrograde last?
Between about twenty and twenty five days, with three weeks being the usual rough figure. The exact length varies because Mercury's orbit is eccentric, and the days column above gives the real number for each period rather than an average.
How often does Mercury go retrograde?
Three times in most years and occasionally four, once per synodic lap of roughly 116 days. That is far more often than the outer planets, which is one reason it is the retrograde everyone knows about.
What is a Mercury retrograde shadow period?
The stretch before the station when Mercury is crossing the degrees it will later go back over, and the stretch afterwards when it is crossing them for the third time. Each runs about two weeks and the practice of counting them is a convention rather than an astronomical boundary.
Does Mercury retrograde affect everyone the same way?
In the astrological framework it is taken to matter most when the retrograde degrees contact something in your own chart. The sky is the same for everyone, so anything specific to you comes from that contact rather than from the retrograde itself.
Is Mercury actually moving backwards?
No. It continues forward around the Sun the whole time. The apparent reversal is what happens when Earth and Mercury pass each other on the inside, and it is purely a matter of the angle we are looking from.
What does it mean if I was born during Mercury retrograde?
It means Mercury was in apparent retrograde motion in your birth chart, which affects nothing about the date arithmetic and is read as a distinct style of thinking in traditional interpretation. A birth chart will tell you whether it applies to you.