Why Planets Loop

Hold the Sun still and run two years: the stars stream by, and that is the Earth going round. Mercury and Venus swing from side to side of the Sun. The outer planets drift through with the stars — and now and then one stops.

Nine months around Mars's next opposition, when it is opposite the Sun, up all night and brightest. Mars runs east, slows, stops, runs back west for about eleven weeks, and carries on. Ptolemy needed epicycles to explain it.

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 and watch the line: it swings one way, stops, swings back, then carries on. Mars never reverses — the Earth, on the inside track, overtakes it. That was Copernicus's answer, and it cost the Earth its place at 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.