Planetary Retrograde Tracker Screensaver

Seen from Earth, planets generally drift eastward against the background stars night after night. Occasionally one appears to slow, stop and move westward for weeks or months before reversing again. This apparent backwards motion is called retrograde, and the turning points are known as stations. It puzzled ancient astronomers, who explained it with epicycles, circles riding on circles.

Copernicus’s heliocentric picture gave a simpler answer: retrograde is a perspective effect. The planets orbit the Sun at different speeds, and when Earth passes an outer planet, or an inner planet passes Earth, the line of sight shifts, like a car overtaking a slower one and seeming to slide backwards against the scenery. The centre of an outer planet’s retrograde falls at opposition, when it is opposite the Sun and closest to Earth. Mercury and Venus never reach opposition; their retrograde is tied to inferior conjunction, when they pass between Earth and the Sun.

Duration grows with distance: roughly 22 days for Mercury, 41 for Venus, 72 for Mars and 121 to 158 days for Jupiter through Neptune. The effect is fully real and predictable; what astrology attaches to it is a cultural tradition with no scientific support. Exact dates depend on the position calculation used.