Planets · 2 of 6

Why Planets Loop

Why would a planet ever go backwards?

Mars stops and runs backwards for weeks: the loop on the sky, the loop as a plot, and the view from outside the orbits that explains it.

You’ll be able to explain why Mars runs backwards for weeks at a time.

7 steps · about 5 min

Before this: Spotting the Planets (6 min)

Where and which night

Working out when this is best placed from where you are…

Where

Which night

Date

Time

Kept for this guide only, until you change it here — your map settings, and every other guide, stay as they are.

Print, play outside or present

Present on a big screen

Scan with a phone's camera, or open

Sources

JPL Horizons — the ephemeris convention this page's planetary positions follow
Meeus, Astronomical Algorithms, 2nd ed. (1998), ch. 32 — planetary positions
Kuhn, The Copernican Revolution (1957) — epicycles, and the heliocentric account of retrograde motion

← GUIDES

More below ↓

With a keyboard, focus the sky, aim it with the up and down arrow keys, and press Enter to drop the pin at the centre of the view; or press Show me.

You are here

Settings

Screen

Night mode Turns every page red, to keep your eyes adjusted to the dark.

High contrast Bolder constellation lines, larger and brighter names on the sky, and stronger text and edges everywhere. Starts on if your device asks for more contrast.

Reduce motion The camera cuts to where it is going instead of flying there, and panels and sheets appear without sliding. Starts on if your device asks for less motion.

Battery saver Draws the sky thirty times a second at most: smoothness traded for battery.

Night brightness Dims the screen below your phone’s lowest setting, down to a fifth, while night mode is on.

Clock and voice

Clock How every time is written. Until you choose, your language’s own clock decides.

D describes What D says about the view on the sky: one sentence, or in full with the field, the constellation and five names.

Sounds Quiet tones as you use the map: a tick as you drag the clock past a mark or dusk, a chime for the star you pick — higher for hotter stars — and a low swell while a figure is out of the dome. Made in your browser, and off until you turn them on.

All settings and your progress

Run two years, Sun held: the stars stream by as the Earth goes round, two planets swing to and fro, and one now and then stops.

Nine months round Mars's , opposite the Sun: it runs east among the stars, then does what Ptolemy needed epicycles for.

The same loop as a plot: Mars's place along the ecliptic, night by night. The backward stretch is centred on opposition, the night the Earth draws level with it.

Now from outside. The rings are the orbits of Mercury, Venus, Earth and Mars. The line runs from Earth through Mars and on to the stars behind it: that is what "where Mars is tonight" means.

Run two years: the line swings one way, stops, swings back, carries on. Copernicus saw why, and the Earth lost the centre.

Planets meet each other, too: the next conjunction of Venus and Jupiter, the arc saying how close. It is only a line of sight; Jupiter is several times farther away. Venus and Mercury have lessons of their own.

Mars at opposition, the middle of its loop. Note it against the stars weeks apart to catch it.

Planets drift east among the stars
Outer planets loop back at opposition
The loop is Earth overtaking them