Finding the planets

Venus never strays more than about 47° from the Sun, so it is always an evening or a morning star.

The wanderers

Watch the sky for a few weeks and five bright points move among the stars while everything else keeps its place. The Greeks called them planētai, wanderers, and the name stuck. Without a telescope you can see Mercury, Venus, Mars, Jupiter and Saturn. Uranus is just at the limit of the naked eye under dark skies, and Neptune needs binoculars. In Korean the names tie each planet to one of the five elements: Suseong (Mercury) water, Geumseong (Venus) metal, Hwaseong (Mars) fire, Mokseong (Jupiter) wood and Toseong (Saturn) earth.

The planets are always found near the ecliptic. All of them orbit the Sun in nearly the same flat plane as Earth, so from Earth they appear along the same band the Sun follows. That is the first rule for finding them: a bright "star" near the ecliptic that is not on your star map is probably a planet. A second clue is that planets twinkle less than stars. A star is a point, so every ripple in the air makes it flicker; a planet shows a tiny disc, and the ripples average out.

Inner planets: evening and morning stars

Mercury and Venus orbit inside Earth's orbit, so from Earth they can never appear far from the Sun. The largest angle between the Sun and Venus, its greatest elongation, comes when our line of sight just grazes Venus's orbit. With Venus at 0.723 of Earth's distance from the Sun, that angle is arcsin(0.723) ≈ 46°, and slightly more when the orbits' small ellipses line up; the model gives about 47° for early January 2027. So Venus is either in the west after sunset, the evening star, or in the east before sunrise, the morning star, and never in the middle of a dark night. Mercury, at 0.387 of Earth's distance, stays within about 28° of the Sun and is hard to catch in the twilight.

Outer planets: opposition and backward loops

Mars, Jupiter and Saturn orbit outside Earth, so they can appear anywhere along the ecliptic, including opposite the Sun. When Earth passes between the Sun and an outer planet, the planet is at opposition: it rises at sunset, is up all night, and is at its closest and brightest. Because Earth laps them on the inside track, oppositions repeat at intervals called synodic periods. From the orbital periods, Mars comes to opposition every 780 days or so, Jupiter every 399 days and Saturn every 378 days.

Around opposition something odd happens. Night by night, a planet usually drifts slowly eastward against the stars, but for a few weeks near opposition it stops, moves westward, stops again and carries on east. This retrograde loop puzzled astronomers for two thousand years. The Sun-centred picture explains it simply: Earth on its faster inner orbit overtakes the outer planet, and for a while our line of sight to it swings backwards, like a slower car seeming to drift back as you pass it on a motorway.

Slow planets also make good clocks. Jupiter takes 11.86 years to go round the Sun, so it moves about 30° along the zodiac each year, roughly one constellation. Saturn, at 29.46 years, moves only about 12° a year. If you see Saturn in a constellation this autumn, it will still be close by next autumn.