The Orionid shower is a long-lasting display of meteors that peaks about October 21st. With moonlight not a factor, an observer under clear dark skies might see an Orionid every 5 minutes in the hours before dawn.
Just about everyone has heard of Halley's Comet. It last cruised through the inner solar system in 1986, and now it's nearing aphelion well beyond Neptune's orbit.
Every year we get to witness not one but two reminders of this celebrated celestial visitor. The Eta Aquariid meteor shower in May and the Orionids in October are caused when Earth comes near the outbound and inbound legs, respectively, of comet's orbit.
Technically, the paths of Earth and Halley are separated by about 14 million miles (22 million km) at their closest. So why do these two showers occur at all? Over time the radiation pressure of sunlight and gravitational tugs from various planets have caused the cometary particles to drift far off track. So Halley must have been looping around the Sun in its current orbit for thousands of years. Clearly, the Orionids are quite old as showers go.
They've been seen for a long time too. The first known Orionid shower was recorded by the Chinese in AD 288, when "stars fell like rain." The shower has been well observed ever since astronomers first recognized its radiant in 1864.
Where and When to Look for Orionid Meteors
The Orionid shower is a long-lasting affair that runs from roughly October 17th to 25th and peaks around the morning of the 21st. This shower is actually a complex of several threads of debris that arrive over several days. These radiants (the points in the sky from which the meteors appear to radiate) are grouped near Orion's club, as shown on the accompanying chart.
Since Orion doesn't rise in the east until around midnight, you'll have to stay up at least that late (or get up very early) to see any Halley bits flashing across the sky. For observers around 40° north latitude, the shower's radiants rise high in the eastern sky (at least 45° up) by about 2 a.m. So that's about when the meteor activity is best. Dawn's twilight begins stealing into the east about four hours later.
There's good and bad news concerning this year's Orionid meteor shower. The good news is that it won't have to compete with moonlight, thanks to a thin waning crescent.
The bad news, according to the International Meteor Organization, is that we're likely near the low point in a 12-year cycle of activity. The shower boasted up to 70 meteors per hour during 2006 through 2009. This year, however, even under ideal conditions, you'll be lucky to catch 20 meteors per hour.
Like the Eta Aquariids, the Orionids tend to be faint and swift, as they strike our atmosphere at 41 miles (66 km) per second (only November's Leonids come in faster) — and they often leave briefly glowing trains.
Track a year's worth of sunsets and sunrises — and all the celestial highlights that happen in between — with our annual Skygazer's Almanac.