★ Orionid Meteor Shower

The Orionids are October's main meteor shower and a yearly link to Halley's Comet. Less rich than the August Perseids or the December Geminids, they are still reliable, and their fast meteors often leave glowing trains. With the radiant not far from the celestial equator, you can watch them from either hemisphere.

Where the meteors come from

The Orionids are dust from Comet 1P/Halley, a very dark nucleus about 15 by 8 km that returns about every 76 years. It last passed the Sun in 1986 and is due back in 2061. Over many centuries its dust has spread along the whole orbit, which is why the shower lasts more than a month.

Earth passes near Halley's orbit twice a year, at the two points where the comet's path crosses the plane of Earth's orbit. In early May we meet the dust at one crossing and see the Eta Aquariids; in October we meet it at the other and see the Orionids. At the October crossing the comet's orbit lies far outside Earth's, which made the link hard to prove.

When to watch

The shower is active from October 2 to November 7 and peaks around October 21–22; the table above gives the expected peak time and Moon phase for the next two years. The peak is broad, so the nights either side can be nearly as good. The radiant rises in the east in the late evening and reaches a useful height around midnight, a little later from the Southern Hemisphere. Rates climb with it, so the best hours run from about 2 a.m. until dawn twilight.

How to find the radiant

The radiant is in northern Orion, near the border with Gemini. Find Orion's belt, then the orange-red star Betelgeuse at the hunter's shoulder. The radiant is about 10 degrees (a fist at arm's length) northeast of Betelgeuse and about 4 degrees west of Alhena, the bright star at the Twins' feet. From northern latitudes Orion is in the southeast after midnight, and the radiant climbs to about 65 degrees above the southern horizon before dawn. From around 35°S it reaches about 40 degrees up in the north, with Orion upside down.

What to expect

IMO gives a ZHR of 20 or more. From a dark rural site with the radiant high before dawn, 10 to 20 an hour is a good night; from the suburbs, expect around 5. Orionids are fast, and many leave trains that last for seconds. Now and then one becomes a fireball. The population index of 2.5 means a reasonable mix of bright and faint meteors.

Viewing tips

• Plan to watch after 1 or 2 a.m.; before midnight the radiant is low.
• Dress for cold: late-October nights get chilly before dawn.
• Face southeast from the north, or northeast from the south, and look halfway up. Meteors 45 to 90 degrees from the radiant show the longest trails.
• If the peak night is cloudy or moonlit, try a night or two either side.
• Watch for the Epsilon Geminids, a weak shower with a radiant about 13 degrees north-northeast of the Orionid one.

History and notable displays

Astronomers have identified Orionid outbursts in Chinese and Japanese records from the years 585, 930, 1436, 1439, 1465 and 1623. The discovery of the annual shower is credited to E. C. Herrick and, independently, Quetelet in 1839; A. Herschel first measured its radiant precisely in 1864 and 1865. Falb proposed the link to Halley's Comet in 1868, but it was debated for decades. The first Orionid photograph was taken at Harvard Observatory in 1922.

Peak rates vary from year to year. IMO found peak ZHRs of about 14 to 31 between 1984 and 2001, and an average of 20 to 30 from 2012 to 2020. Every return from 2006 to 2009 was unexpectedly strong, with ZHRs of about 40 to 70 on two or three consecutive dates, traced to old Halley dust held in an orbital resonance with Jupiter. A 12-year cycle has been suggested; radar data since 2002 seem to show it, but visual counts do not.