Why the outer planets are retrograde so often
Retrograde motion happens when Earth passes a slower planet on the inside of its orbit, or when a faster planet passes us. For anything beyond Mars, we do the overtaking, and it happens once per synodic period.
The further out a planet is, the slower it moves and the larger the fraction of the year we spend passing it. Mars is retrograde for about eleven weeks every couple of years. Jupiter is retrograde for about four months a year, Saturn four and a half, and Uranus, Neptune and Pluto for roughly five months each, close to half the time. There is nothing unusual about an outer planet being retrograde, which is worth knowing before reading much into it.
Venus is the exception among the visible planets. It goes retrograde for about six weeks once every nineteen months, the rarest of the ones anyone tracks, and the pattern it traces over eight years is close enough to a five-pointed figure that it has been noticed for millennia.
How the stations here are found
A station is where the derivative of ecliptic longitude with respect to time crosses zero. Rather than look that up in a table, the page samples each planet's longitude on either side of a series of candidate times, watches for the sign of the daily change to flip, and then bisects the interval until the moment is pinned to within a minute.
The step size is finer for Mercury than for the outer planets, because Mercury moves fast enough to reverse quickly and a coarse step could miss it. The positions themselves come from the same model the birth charts use, so the retrograde table and any chart on this site agree with each other by construction.
Reading the sign column
Where a planet turns matters more in practice than the fact that it turned. The sign column shows where each station happens, and where the station retrograde and station direct fall in different signs the column gives both, because the planet has crossed a boundary backwards during the period.
Those backwards crossings are the interesting case. A planet that enters a sign, retreats out of it and comes back covers the same degrees three times over several months, and the traditional reading treats that as a subject returned to rather than passed through. It is also the situation where a transit to your own chart repeats, which a birth chart comparison will show directly.
What retrograde does not mean
It does not mean a planet is closer, brighter in some ominous way, or exerting more force. In the case of the outer planets it does correlate with being closest to Earth, because we pass them near opposition, but the distances involved make that irrelevant to anything but observation.
It does not mean the planet has stopped. Apparent motion at a station is zero in longitude only; the planet continues at tens of thousands of kilometres per hour throughout.
What it means is a geometric relationship between our orbit and theirs, one that repeats with great regularity and can be tabulated for any year. The interpretations attached to it are old and worth knowing, and they are a separate thing from the dates.
Questions people ask about this
Which planets can go retrograde?
All of them except the Sun and Moon, which never do because we orbit one and the other orbits us. Everything from Mercury outwards shows retrograde motion at some point in its synodic cycle.
How long is Mars retrograde?
About ten to eleven weeks, and it only happens roughly every twenty six months. That makes it much rarer than Mercury's and considerably longer, which is why it tends to be noticed as a period rather than an event.
Is it normal for several planets to be retrograde at once?
Entirely normal. The outer planets are each retrograde for close to half the year, so having three or four of them retrograde simultaneously is the common case rather than an alarming one. Pick almost any date in the table and you will find several.
What does a retrograde planet mean in a birth chart?
The traditional reading is that its function turns inward or works less directly. Since the outer planets are retrograde so much of the time, having Neptune or Pluto retrograde natally says very little, while a retrograde Mercury or Venus is less common and carries more weight in that framework.
Why do the station dates differ slightly between sources?
Usually a time zone, occasionally a difference between true and mean positions or the precision of the underlying ephemeris. The times here are UTC and are solved to within a minute from the same positions used for the charts on this site.
Do retrogrades repeat on a cycle?
Yes. Each planet's retrogrades recur once per synodic period, and because those periods are not neat fractions of a year the positions shift steadily round the zodiac. Venus is the clearest case, returning very nearly to the same degrees every eight years.