The Antihelion Source meteor shower
Key facts
- Peak
- no single peak; slightly richer in late March to April and late June to July
- Active
- about 10 December to 20 September (the Taurids take over for the rest of the year)
- Rate at peak (ZHR)
- about 2–4
- Speed
- 30 km/s
- Radiant
- A broad patch on the ecliptic, about 12° east of the point opposite the Sun, moving through the zodiac constellations during the year
- Parent body
- Many bodies, especially Jupiter-family comets and their scattered dust
Dates shift slightly from year to year. This year's peak is in the events calendar, and the meteor-shower planner shows how high the radiant climbs from where you live, night by night.
The Antihelion Source is not a shower in the usual sense. It is a steady drizzle of meteors from a large, oval area of sky that sits roughly opposite the Sun and moves around the zodiac as Earth orbits. On any night outside the Taurid season, a few of the meteors you see belong to it.
Why opposite the Sun? Countless meteoroids orbit the Sun in the same direction as the planets, on orbits only slightly tilted to Earth's. Many have their closest point to the Sun inside Earth's orbit. Earth meets them while they are still falling inward towards the Sun, before their closest approach, so they arrive from the direction opposite the Sun, shifted a little east by Earth's own motion. This direction is called antihelion, meaning "opposite the Sun". Much of this dust comes from Jupiter-family comets, short-period comets whose orbits are shaped by Jupiter, and from older streams that planets have spread out until they lost their identity.
Old names. Before the 2000s, observers gave names to many parts of this activity as the radiant crossed different constellations: the Delta Cancrids in January, the Delta Leonids in February, the Virginids from March to April, the Scorpiids and Ophiuchids in May and June, and the Sagittariids and Capricornids later in summer. Better observations showed that most of these were not separate streams at all but the same broad source seen at different times of year. In the mid-2000s meteor observers replaced them with the single Antihelion Source. A few real streams inside the area, such as the Alpha Capricornids, are still counted separately.
What to expect. Two to four meteors an hour under a dark sky, of medium-slow speed. The radiant is about 30 degrees long and 15 degrees wide, so meteors seem to come from a general region rather than a point. It is highest around local midnight, because it lies opposite the Sun. Its meteors can be bright, and northern spring in particular brings a fair share of slow fireballs.
Viewing. Anywhere on Earth can see it, although the radiant is better placed from southern latitudes in the middle of the year, when it lies in Sagittarius and Capricornus. Observers use its position to decide whether a meteor belongs to it or is a sporadic meteor, one from no recognisable shower.
Viewing tips
- Look roughly opposite where the Sun set, at the zodiac constellation that is highest at midnight.
- Expect slow to medium meteors; very fast ones are probably not from this source.
- Plot meteors on a star chart if you want to tell them from sporadics.
- Use it as a reason to watch on quiet nights between the big showers.
- Try a session around midnight in late March or April for the richest activity.