Planetary retrograde motion: an effect of perspective
A retrograde planet does not turn around: it only seems to move backward, as seen from Earth, for a few weeks.
retrograde motion
retrograde planet
perspective
Retrograde motion of planetsThe sky and the zodiac
2 sources
Claudius Ptolemy, 2nd century
Nicolaus Copernicus, 1543
The essentials
Retrograde motion of planets at a glance
For you
A retrograde planet seems to move backward against the background of stars for a few weeks: it is an effect of perspective caused by the difference in speed between the Earth and the planet.
It all plays out between two planets orbiting the Sun on unequal orbits.
When the Earth overtakes Mars, or Venus overtakes the Earth, the planet seen from afar appears to leave its usual path, stop, move backward against the background of stars, then set off again. It is the same effect as with a slower car you overtake: it seems to move backward against the landscape, though it is moving forward.
The durations vary. Mercury turns retrograde three to four times each year, for about twenty days each time. Venus returns to it after nineteen months, Mars after twenty-six, for two months or more. Jupiter, Saturn, Uranus, Neptune, and Pluto do so once a year, for four to six months. The Sun and the Moon never turn retrograde; only the mean lunar nodes move backward permanently.
Each change of direction passes through a stationary point, where the planet seems motionless. In astrology, tradition associates these periods with a return to the planet’s themes, rather than with events. The pages devoted to Mercury, Venus, Mars, Jupiter, and Saturn give the exact dates of each phase, calculated by the site. They also state, each time, that these readings are traditions, and that none announces what lies ahead.
Family
The sky and the zodiac
The diagram
Who goes retrograde, and for how long
Read next
The precession of the equinoxes
History, debates, and sources
A little history
Ptolemy explains the loops of the planets with epicycles: each planet would trace a small circle whose center travels along a large circle around the Earth.
This model served for fourteen centuries. Copernicus, in 1543, replaces this mechanism with an effect of perspective, the Earth itself orbiting the Sun. Kepler, in the early 17th century, then gives the orbits their elliptical shape and the calculations gain in precision.
Sources
2
Oldest
Claudius Ptolemy, 2nd century
Most recent
Nicolaus Copernicus, 1543
What is debated
For astronomy, retrograde motion is an optical illusion fully explained.
The debates concern the astrological reading. Ancient authors, including Lilly, saw a weakening in the retrograde planet. Some modern astrologers see rather a phase of return and review, and others draw nothing in particular from it. The effects attributed to Mercury retrograde in popular culture have never been scientifically validated.
Schools do not all agree: the site sets out the positions without deciding for you, and states the setting it uses by default.
Where this concept comes from
The authors and texts that establish or discuss this concept, with the passage when it is known.
2nd centuryClaudius PtolemyAlmagest, Books IX to XIII
Explains the loops of the planets with epicycles, in an Earth-centered world.
1543Nicolaus CopernicusDe revolutionibus orbium coelestium, Book V
Explains retrograde motion as an effect of perspective, with the Earth orbiting the Sun.
The diagram’s figures are calculated by the site’s engine (Astronomy Engine) or taken from the recorded facts, never copied by hand.
Who goes retrograde, and for how long
Retrogrades: usual frequency and length (orders of magnitude)
Planet
Frequency
Length
Mercury
3 or 4 times a year
≈ 3 weeks
Venus
About every 19 months
≈ 6 weeks
Mars
About every 2 years
≈ 2 to 3 months
Jupiter
Every year
≈ 4 months
Saturn
Every year
≈ 4 and a half months
Uranus, Neptune, Pluto
Every year
≈ 5 months
Sun and Moon
Never
—
Frequently asked questions: retrograde motion of planets
Does a planet really move backward?
No. It keeps turning in the same direction, but seen from the Earth, which is also moving, its position against the background of stars seems to reverse for a few weeks. It is an effect of perspective, comparable to a car that seems to move backward when you overtake it.
Which planets can be retrograde?
All of them except the Sun and the Moon: Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The closest ones turn retrograde more often and for shorter periods. The slow planets, like Saturn or Pluto, stay in this phase for several months each year.
Are the Sun and the Moon ever retrograde?
Never. The Sun is the center of our system, and the Moon orbits the Earth without ever seeming to turn back. Only the mean lunar nodes, which are not celestial bodies, move backward permanently through the zodiac, in a cycle of about 18.6 years.