The stage is set for a meteor-shower peak in the predawn hours Friday morning – thanks to Halley's Comet.
The famous comet itself is approaching its most distant point from the sun, where it will make a broad U-turn and head back into the inner solar system for its closest approach to the sun in July 2061.
But over the past few days, Earth has been plowing through debris the comet has left behind – one of two encounters the planet has each year with detritus from Halley.
Overnight Thursday night, Earth will pass through the most dense portion of this debris, in a meteor shower known as the Eta Aquarids.
The shower draws its name from the constellation Aquarius. The meteors that this shower sends streaking across the sky appear to originate near one of the constellation's brightest stars, eta Aquarii.
Where is the best place to watch?
Aquarius never rises very high in the skies over the northern hemisphere, explains Rick Fienberg, an astronomer and former editor in chief of Sky and Telescope magazine. So if you live in the north, have big bucks, and access to really fast transportation, anywhere south of the equator will do for the most intense shower.
Because the constellation is largely a southern-sky feature and the southern hemisphere is heading into winter, with hours of darkness increasing as the June solstice approaches, people in the southern hemispheres will have a mix of viewing conditions – including a new moon – that give them the best seats in the house, this time around.
Still, the event won't be a complete wash for residents of the northern hemisphere. In general, the closer your latitude is to the equator, the higher Aquarius rises – and the more of the sky you'll see meteors in – before sunlight begins to make meteor-spotting difficult.
Meteors are most likely to appear between 11 p.m. local time tonight to shortly before sunrise tomorrow. Depending on latitude, light pollution, and clear skies, peak rates range could from about as few as 15 to as many as 60 or more an hour. The most intense part of the shower will take place an hour or two before sunrise, when the radiant is above the horizon.
"Folks north of about 40 degrees will see practically nothing," cautions William Cooke, who tracks meteor showers for NASA at the Marshall Spaceflight Center in Huntsville, Ala. That latitude roughly corresponds to a line from Philadelphia to Eureka, Calif.
"People down around 35 degrees will see about 10 to 15 per hour. Father south is better," he says. For site-specific viewing information, click here.
The Eta Aquarids may not be the most spectacular annual meteor shower for people in the northern hemisphere, but it's got the imprimatur of the comet Mark Twain is said to have arrived and departed with.
For northerners, the shower is "definitely not spectacular, but the rate is not what makes Eta Aquarids special," Dr. Cooke adds. "It's where they came from – Halley's comet. How often do folks have a chance to see a chunk of that?"
And if in the end you don't see a chunk of that?
"There's a better meteor shower coming this summer," Dr. Fienberg says: the Leonids.
One final incentive for an early rise tomorrow, especially if you live in a "big sky" landscape with low horizons: Four, count 'em, four of the five easily visible, naked-eye planets are in a rough conjunction at sun-up – Mercury, Venus, Mars, and Jupiter.