Comet Hale–Bopp, in 1997, at the twilight it looked best from where you are. A comet is a lump of ice and dust — Hale–Bopp's perhaps 60 km across — on a long orbit that has carried it in from far beyond the planets. Near the Sun its ice turns to gas, and the glowing cloud that makes — the coma — is what you see.
Two tails. The straight blue one is gas, lit by sunlight and blown off by the solar wind at 400 km/s, so it points directly away from the Sun, below the horizon beneath it. The broad white one is dust, pushed more gently by the pressure of sunlight itself: 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 six weeks before, a night at a time. A comet moves against the stars as a planet does, but on an orbit of its own, often steeply tilted, so it can turn up anywhere in the sky. Hale–Bopp stayed visible without a telescope for eighteen months, longer than any comet on record.
Halley's Comet in 1986, at its best from where you are. It is the famous one because it comes back — every 76 years, recorded at every return since 240 BC. This pass was its poorest in two thousand years, with the Earth on the far side of the Sun at the best moment, and its tail stayed short. It returns in 2061.
Halley sheds dust all along its orbit, and the Earth crosses that orbit twice a year. In October the grains stream from this point near
Betelgeuse: the
Orionids, about 23 an hour at their peak. In May the same dust is the Eta Aquariids. Nearly every meteor shower is an old comet's trail.