← Wheel of the Sky

Planet Paths

Each planet goes steadily round the Sun, but we watch from a moving Earth. Here is the same motion both ways at once.

Around the Sun. Looking down on the solar system, the Sun in the middle: Earth (blue) and the planet on their real orbits, to scale. The line is our line of sight from Earth to the planet, carrying on to the stars behind it. The fading tail is the last four months of the path from Earth (the right-hand drawing), pinned to where Earth is now.
From Earth. The same motion with Earth held still in the middle: the path the planet traces against the zodiac, and the Sun's yearly circle (gold). Pink marks the stretches when it seems to move backward (retrograde).

Loading…      

About Venus →

Why the loops?

Mars, Jupiter and the planets beyond them go round the Sun on bigger, slower tracks than Earth's. Every so often Earth, orbiting closer to the Sun and moving faster, overtakes one of them, and for a while it seems to turn round against the stars: retrograde motion. Mars, for example, is passed about every two years, and its backward motion is greatest at the middle of the loop, when Earth goes by.

Mercury and Venus are inside Earth's orbit. They never appear far from the Sun, and shuttle back and forth across its position, their motion sometimes reversing too.

Nothing really turns back. In the left-hand drawing every planet keeps going the same way round the Sun; the backward stretches on the right come only from where we're standing.

Things to try

  • Pause in the middle of a pink stretch and look at the left-hand drawing: where are Earth and the planet?
  • Watch the line of sight in both drawings. It points the same way in each, to the same place in the zodiac.
  • Try Venus over 8 years, Mars over 15, and Saturn over 30, and compare the patterns.

Sources for the explanation: David P. Stern, "The Planets" (NASA Goddard Space Flight Center, From Stargazers to Starships), and NASA's Astronomy Picture of the Day, "Retrograde Mars" (06/13/2010). Positions come from the Astronomy Engine library, as in the Planet Tracker: Earth's and the planet's real positions round the Sun, every day (every few weeks over the longest spans), with the zodiac as it was on the first day. The drawings are to scale, so over long spans Earth's orbit is small next to an outer planet's.