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Comets

Why does a comet’s tail point away from the Sun?

What a comet is, why its tails point away from the Sun, and where the dust goes — on Hale–Bopp's best night and Halley's, seen from where you are.

You’ll be able to say which way a comet’s tails will point, and why.

8 steps · about 6 min

Where and which night

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

Where

Which night

Date

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

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Sources

Minor Planet Center orbits of 1P/Halley (1986) and C/1995 O1 Hale–Bopp, with their magnitude parameters, as carried in Stellarium's ssystem_minor.ini
Meeus, Astronomical Algorithms (2nd ed.), ch. 33–34 — positions from orbital elements; the engine is tested against Example 33.b
Bill Gray, Project Pluto — the rule of thumb for a comet's coma and tail length used to draw the tails
NASA Science: Comets — ion and dust tails, and why both point away from the Sun
NASA Science: 1P/Halley — its 76-year period, the 1986 apparition and the 2061 return
International Meteor Organization — Meteor Shower Calendar: the Orionids' peak, ZHR and radiant

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Comet Hale–Bopp, 1997: a lump of ice and dust. Near the Sun its ice turns to gas, the glowing coma, and tails grow.

Two tails. The Sun is below the horizon, so the straight one, gas blown off by the solar wind, points up, away from it. The broad one is dust, pushed more gently by sunlight: each grain lags behind along the comet's orbit, and together they curve.

The line is Hale–Bopp's path among the stars over the two weeks before, a night at a time. A comet wanders like a planet, but on an orbit of its own, often steeply tilted from the planets' line, so it can turn up anywhere. Hale–Bopp stayed visible to the eye for eighteen months — a record.

↺ Planets keep to the line, from The Ecliptic

Hale–Bopp's orbit: 2,500 years round, tilted 89° to the planets', out to 371 (Earth–Sun distances). That is the next lap: Jupiter cut it in 1996 from about 4,200. Some never return at all: A Visitor from Another Star.

Halley's Comet in 1986, at its best from where you are. In 1705 Edmond Halley saw that the comets of 1531, 1607 and 1682 were one, and predicted its return in 1758. He died sixteen years too soon. It came on time.

Halley's orbit: back every 76 years, seen at every return since 240 BC. It creeps out past Neptune to 35 AU, then whips round the Sun. The sliver in the middle is all the time it was bright enough to see: 6 months.

Those months, opened out; the dashed line is its distance from us. At its closest to the Sun it was 1.55 AU away, on the far side. From the north 1986's best was a smudge low in the dawn; from the south it showed a tail. Next: 2061.

Each October the Earth runs through Halley's dust: the Orionids, by Betelgeuse. Before dawn on 21 October, each streak is a grain of it. More: Meteor Showers.

Near the Sun, ice grows a coma and tails
Both tails point away from the Sun