Mercury Retrograde Triangles
Every Mercury retrograde for twenty years, on one zodiac wheel. Each bright arc is one retrograde, running from the degree where Mercury turns back to the degree where it turns forward again, coloured by the element of the sign where it began. Time runs outward from the centre, so you can watch the retrogrades walk backwards through the elements, from fire to water to air to earth and round again.
| Retrogrades per year | about 3.15 |
|---|---|
| Average length | 22 days |
| Step between retrogrades | about 114° |
| Triangle drifts back | about 18° a year |
What you are looking at
The ring is the zodiac, with 0 degrees Aries on the left as on a birth chart, each sign tinted by its element: fire, earth, air and water. Every bright arc is one Mercury retrograde. It starts at the degree where Mercury stations retrograde and runs backwards along the zodiac to the degree where it stations direct, about 13 degrees later on average. Its colour is the element of the sign where it began.
Distance from the centre is time. The first retrograde after the start date sits close to the middle and the last one near the wheel, with faint rings marking the years. The thin spiral underneath is Mercury's whole path: it winds outward about once a year and doubles back at every retrograde.
The fine lines join each retrograde to the next. Three in a row land roughly a third of the zodiac apart, so each year's retrogrades trace a rough triangle, and you can see the triangles turning slowly backwards as they spiral out.
The math
Mercury turns retrograde each time it laps the Earth, every 115.88 days on average, so about 3.15 times a year. In that time the Sun moves on about 114 degrees, and so does the place where the next retrograde starts: about 6 degrees short of a third of the circle, a trine. That is why three retrogrades in a row usually fall in signs of the same element, each one starting on average about 6 degrees earlier in its sign than the one before.
Those shortfalls add up. Each set of three lands about 17 degrees further back than the last, about 18 degrees a year. So the retrogrades spend about a year and two thirds in one element before moving on to the element of the signs just behind: from fire signs to water, then air, then earth. The whole cycle through all four takes about six and a half years.
Mercury's orbit is the most elongated of the planets, so the steps are uneven, from about 105 to 127 degrees. Each retrograde lasts about 20 to 24 days, 22 on average, and over twenty years the four elements share them almost evenly. The pattern nearly repeats after 13 years: 41 retrogrades later, each one comes back about 3 days later in the calendar and about 3 degrees further along the zodiac.
How it is computed
Stations are found by following Mercury's apparent motion along the zodiac, as seen from the Earth, every two days and catching each moment its speed passes through zero, then narrowing that moment down to about a minute. The degrees are apparent tropical longitudes of date, the same degrees an ephemeris prints, from astronomy-engine, an open-source library built on the VSOP87 planetary theory that agrees with the Swiss Ephemeris to well under an arcminute.
Each arc follows Mercury's real position every half day between its two stations, so its length and curve are the real retrograde. The element is read from the sign at the retrograde station. The drift in the caption is an average over the span you choose: the shift between each retrograde and the one three after it, divided by the time between them.
Questions
Does Mercury really go backwards?
No. It only looks that way from the Earth. Each time Mercury overtakes us on its faster inner orbit, it appears to slide backwards against the stars for about three weeks. The dates and degrees here are those apparent stations, the ones ephemerides print and astrologers use.
Why do Mercury retrogrades stay in one element for a while?
Successive retrogrades start about 114 degrees apart, close to the 120 degrees between signs of the same element, so three in a row usually land in fire signs, or water signs, and so on. The 6 degree shortfall moves the set back into the next element after about a year and two thirds. Astrologers often read the element of a retrograde as colouring its themes; this piece shows only where in the zodiac each one falls.
Which element are the next Mercury retrogrades in?
Set the start date to today and look at the arcs nearest the centre. Their colour is the element of the sign where each coming retrograde begins, and the caption gives the date and the degrees of each one as the picture builds.
Does the pattern ever repeat?
Nearly, after 13 years. Thirteen years hold 41 retrogrades, and each one returns about 3 days later in the calendar and about 3 degrees further along the zodiac than its match 13 years before.