Keen stargazers hoping to catch a glimpse of the Orionid meteor shower this week may need to set an early alarm — and lower their expectations.
The annual meteor shower, caused by debris left behind by Halley's Comet, graces our skies every October and is expected to peak in the early hours of Thursday morning.
But a bright moon may dampen this year's shower, though astronomers say patient sky watchers could still catch meteors streaking across the pre-dawn sky.
"This year will probably be a quiet one," University of Southern Queensland astronomer Jonti Horner said.
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The Orionid meteor shower occurs each October as Earth passes through a stream of debris left behind by Halley's Comet.
Halley’s Comet is a famous comet of ice and dust that orbits the sun every 75 to 76 years. It will next be visible from Earth in mid-2061.
This year's shower coincides with the Northern and Southern Taurid meteor showers, which contribute a handful of extra meteors to the night sky.
The different showers can often be distinguished by their speed.
Orionids streak quickly across the sky, while Taurids tend to move much more slowly.
"If you are up in the early hours of the morning before sunrise and you see a meteor and it's quite slow, that was probably a Taurid," Horner said. "If you see one that was really fast, that was probably an Orionid."
The shower also has a remarkably long history.
Horner said the Orionids have been observed for at least 2,500 years, with some reports believed to date back to around 800 BC.
Under ideal conditions, the Orionids normally produce about 20 to 30 meteors an hour.
But those figures assume dark skies, clear weather and little interference from moonlight.
This year, a bright waxing gibbous moon is expected to wash out many of the faint meteors.
"The moon doesn't set until 2 or 3am," Horner said. "While the moon's high in the sky, there's a lot more light pollution, which hides all but the brightest meteors."
Horner said moonlight could reduce the number of visible meteors by as much as half, particularly before the moon drops below the horizon.
"What that means is that I would not be suggesting to anybody that they go out until the moon has set."
Anyone hoping to catch the shower should look to the skies in the hours before dawn.
The Orionids are effectively invisible before midnight because the point in the sky from which they appear to emerge, known as the radiant, has not yet risen high enough above the horizon.
The best viewing window on Thursday morning is expected to be between 3am and 4am (AEDT).
"Wednesday night is formally forecast to be the best night, but there's a little bit of uncertainty in that because the numbers wiggle around up and down," Horner said.
"I'd say basically anytime this week, even as far as Friday or Saturday, you've got a chance of getting reasonable numbers, but the moon is getting worse, not better."
City viewers may still be able to spot the same bright meteors folks in the rural areas will see.
"The brighter meteors, you'll see whether you're in the city or in the country, so long as you're away from direct streetlights."
But he said there were no guarantees you'll actually see any meteors.
"If you're someone getting up early, getting up before dawn and you're having your morning coffee, going and sitting out under the stars, you might get to see something and that'll be nice."
Anyone disappointed by this week's display won't have to wait long for another celestial show.
Horner describes the Orionids as more of a warm-up act for December's Geminid meteor shower, widely regarded as the year's best.
While the Orionids typically produce around 20 to 30 meteors an hour under ideal conditions, the Geminids can generate as many as 150.
"The Geminids are everything the Orionids are not," Horner said. "They're much stronger, much better, and the moon's out of the way."
The Geminids are expected to peak around 14 December, with strong viewing conditions forecast either side of the peak.
Horner's advice for that shower? Use your annual leave.
"I would actually be planning a camping trip and getting somewhere dark for the Geminids," he said.
And if this year's Orionids leave you underwhelmed, there seem to be only two real alternatives: a time machine, or a lot of patience.
"The Orionids were probably a lot better around 500 BC than they are now," Horner said.
"But there are some predictions that the Orionids could actually be very spectacular in the year 2070."
