How do I calculate whether a planet is retrograde?
Retrograde motion isn't esoterica — it's geometry. Planets never truly move backward; they only appear to, seen from Earth. Here's how that apparent motion arises and how to calculate it exactly.
Calculation done — just get reminded
Free · Double opt-in
Never miss a retrograde
We'll send you a short email the moment Mercury, Venus or Mars goes retrograde — with the exact window. Free, unsubscribe anytime.
Why planets appear to move backward
Earth and the other planets orbit the Sun at different speeds. When Earth "overtakes" an outer planet — or an inner planet like Mercury overtakes Earth — an apparent backward motion against the starry background appears from our viewpoint. An optical effect, much like passing a slower train.
Physically, no planet ever moves backward. From a geocentric view, however, its motion along the zodiac appears to reverse for a few weeks — that's what we call retrograde.
The exact calculation
The key is ecliptic longitude speed: for a given moment we compute the planet's position along the zodiac and check whether its longitude is increasing or decreasing. If it's decreasing, the planet is retrograde. The moment the speed passes through zero is called the station.
We use the Swiss Ephemeris — a high-precision planetary table. A day-by-day scan locates the stations, and bisection refines them to within seconds. It's all deterministic: same input, same result — no randomness, no AI guess.
From calculation to meaning
Astronomy gives you the when, your codex the what-for-you. The free reading connects the math to your chart — across 22 more systems.
Calculated with Swiss Ephemeris — astronomically precise data
23 cosmic systems · No sign-up
FAQ
Does a planet really move backward?
No. Retrograde is an apparent motion caused by Earth and the planet orbiting the Sun at different speeds. Physically, all planets travel in the same direction.
What is a "station"?
The moment a planet appears to change direction — its longitude speed passes through zero. There's a retrograde station (the start) and a direct station (the end).
Can I calculate this myself?
In principle yes, with an ephemeris like the Swiss Ephemeris. We do the work for you and show status and dates live — and send you an alert if you like.