The Alpha Capricornids meteor shower
Key facts
- Peak
- around 30 July
- Active
- about 3 July to 15 August
- Rate at peak (ZHR)
- about 5
- Speed
- 23 km/s
- Radiant
- Capricornus
- Parent body
- Comet 169P/NEAT
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 Alpha Capricornids are not a numerous shower. Even at the peak you may see only a handful each hour. Yet they have a loyal following, because what they lack in numbers they make up for in drama: this shower is famous for slow, bright fireballs that drift majestically across the summer sky.
A fireball is simply a very bright meteor, usually defined as one brighter than the planet Venus. Alpha Capricornids are among the slowest meteors of any annual shower, entering the atmosphere at about 23 km/s, less than half the speed of a Perseid. Slower meteors stay visible for longer, sometimes a second or two, and the Alpha Capricornids are often yellowish. When a bright one appears, you have time to point and shout before it is gone.
The shower is named after Alpha Capricorni, a star in the constellation of Capricornus, the Sea-Goat, close to the radiant. It is active through much of July and into August and overlaps with both the Southern Delta Aquariids and the early Perseids.
The parent comet. In 2002 the near-Earth object survey NEAT (Near-Earth Asteroid Tracking) discovered an object that was first catalogued as an asteroid. It later showed faint signs of comet activity and is now known as Comet 169P/NEAT. Its orbit matches the Alpha Capricornid stream. The comet is weakly active today, and researchers have suggested that the meteoroid stream came from a larger object that broke apart several thousand years ago, with 169P/NEAT as the main surviving fragment.
What to expect. Do not expect a storm. Even under perfect conditions the rate is about 5 an hour, and real counts are often lower. The rewards are quality rather than quantity: a single long-lasting fireball is worth the wait. The peak is broad, so the last days of July all give similar chances.
Telling them apart. On a late-July night you might see meteors from three showers. Alpha Capricornids are slow and often bright and point back to Capricornus in the south. Delta Aquariids are faster and fainter and point back to nearby Aquarius. Perseids are fast and point back to Perseus in the north-east.
Viewing from the UK. Capricornus is a southern constellation and stays low from Britain, rising in the south-east during the evening and reaching its highest point in the south around midnight. That makes the Alpha Capricornids one of the few showers that can be watched well before the early hours. Find a site with a clear view to the south and settle in from late evening until about 2 a.m. The short summer nights in Scotland limit the darkest hours.
Viewing worldwide. The shower can be seen from both hemispheres, but the radiant is higher from the tropics and the southern hemisphere, where it passes almost overhead. Observers in Australia, South America and southern Africa have the best view, but anyone with a clear southern sky can catch the odd fireball.
Peak dates vary slightly from year to year, and a bright Moon will hide the fainter members, though not the fireballs.
Viewing tips
- Watch from late evening, because Capricornus is highest in the south around midnight.
- Expect only a few meteors an hour, but look out for slow, bright fireballs.
- Choose a site with a clear view low towards the south.
- If you see a slow, yellowish meteor coming from the south, it is probably an Alpha Capricornid.
- Keep a camera on a tripod pointed south to try to catch a fireball.
Read about the parent: Comet 169P/NEAT, and the other comets behind meteor showers.