Meteors and Meteor Showers

Meteors are caused by small particles ranging in size from a grain of sand to a pea entering the Earth's atmosphere at high speed. Because of the speed, which can be between about 20 km/s and 70 km/s, the particles "burn up" in the upper atmosphere, leaving a visible incandescent trail. When the Earth moves through a stream of such particles left by a comet, a meteor shower may result.

Normally a small number of "sporadic" meteors can be seen each hour of a moonless night. During a shower the number visible may increase considerably with the meteors appearing to originate from a small area of the sky, called the radiant. This is a perspective effect due to the meteors travelling in parallel lines but as they approach the observer they appear to spread out. The shower is named after the constellation in which the radiant lies.

The list of meteor showers shows some showers visible from the southern hemisphere. It includes the range of dates when the shower is active, and the peak date. The possible number of meteors per hour at the peak is also shown. It is known as the Zenith Hour Rate, ZHR, and is the number to be expected with the radiant at the zenith, directly overhead, with a dark sky and no Moon. The state of the Moon at each peak is shown, for the current year. When there is significant Moon in the sky, the number of meteors likely to be seen will be considerably reduced. Also shown is the right ascension and declination of the radiant, and a nearby bright star.

The contents of this page are based on information from the International Meteor Organisation. Further details on meteor observing can be obtained from their web pages which should be consulted by anyone interested in making observations of meteors.

Further information on meteors will also be available at Meteor Showers on Line. The Comet and Meteor Section also has some information.

Notes on the Streams

alpha-Centaurids

Past records of the alpha-Centaurids report many very bright objects, some of fire-ball class (magnitude -3 or brighter), with persistent trains. However recent peaks have shown 5 or less per hour, although bursts in 1974 and 1980 yielded about 25. This group is worth observing for another strong event, particularly when the Moon does not interfere.

gamma Normids

gamma-Normids meteors are similar to the sporadics in appearance, and for most of their activity period are virtually undetectable above this background rate. The peak itself is normally quite sharp, lasting for only a day or two either side of the maximum. The best time for observing is after midnight when the radiant reaches a reasonable elevation.

Virginids

The Virginids radiant has a wide spread, some 15°in right ascension and 10° in declination.

pi-Puppids

The pi-Puppids are produced by Comet 26P/Grigg-Skjellerup. The hourly rate is variable, up to 40 an hour have been recorded. The shower is only occasionally active near the time of a perihelion passage of its parent comet. The last perihelion was in March 2008, the period of the comet is 5.3 years. As the material from the comet may be spreading along the comet's orbit viewing is vital.

eta-Aquarids

The eta-Aquarids is a fine, rich stream associated with Comet 1P/Halley, like the Orionids of October, but it is visible for only a few hours before dawn essentially from tropical and southern hemisphere sites. The fast and often bright meteors make the wait for radiant-rise worthwhile, and many events leave glowing persistent trains after them.

A relatively broad maximum, sometimes with a variable number of submaxima, usually occurs in early May. The zenith hourly rates are generally above 30 for almost a week centered on the main peak. High rates are expected between 2008 and 2010. The radiant rises at about 2 am in New Zealand, so observation is best from about 5 am by which time the radiant has a reasonable altitude. The radiant culminates at about 08h local time.

Sagittarids

The radiant has a wide spread, some 15° in right ascension and 10° in declination.

Bootids

This shower made an unexpected return in 1998 with a peak rate of over 100 meteors per hour over a 7 hour period.. Prior to 1998, only four definite returns of the shower had been detected, in 1916, 1921 and 1927. With no significant visual reports between 1928..1997, it was assumed no longer encountered the Earth.

The shower's parent comet 7P/Pons-Winnecke which was at perihelion in 1996 prior to the 1998 apparition. It was again at perihelion in may 2002 and again in June 2008, so June 2009 will be worth watching. The comet's orbit currently lies around 0.24 astronomical units outside the Earth's orbit at its closest approach, so we have no way at present to predict likely future activity. The radiant is below the northern horizon for the South Island of New Zealand, and very low in the North.

Pisces Austrinids

This is one of a group of 4 or 5 showers with close radiants active in July. The stream is rich in faint meteors, making it well-suited to telescopic work, although enough brighter members exist to make visual and photographic observations worth the effort too, primarily from more southerly sites.

alpha-Capricornids

The alpha-Capricornids are noted for their bright - sometimes fireball-class - events, which, combined with their low apparent velocity, can make some of these objects among the most impressive and attractive an observer could wish for. A minor enhancement of alpha-Capricornid Zenith hour rate to about 10 was noted in 1995 by European IMO observers, although the Southern delta-Aquarids were the only one of these streams previously suspected of occasional variability.

delta-Aquarids and iota Aquarids

Like the Piscis Austrinids, the Aquarids are all streams rich in faint meteors, making them well-suited to telescopic work, although enough brighter members exist to make visual and photographic observations worth the effort too, primarily from more southerly sites.

Such a concentration of radiants in a small area of sky means that familiarity with where all the radiants are is essential for accurate shower association for all observing nights. Visual watchers in particular should plot all potential stream members seen in this region of sky rather than trying to make shower associations in the field. The only exception is when the Southern delta-Aquarids are near their peak, as from southern hemisphere sites in particular, rates may become too high for accurate plotting.

Piscids

The Piscids can be observed from either hemisphere throughout September. There is some doubt as to when the Piscid peak occurs, or if there is only one.

Orionids

The Orionids have several maxima apart from the main one. For example a strong peak on October 17-18 has been observed in Europe in 1993 and 1998. Observations are possible from midnight on.

A weak shower, the epsilon Geminids, is nearly coincident with the Orionids and have a peak October 18.

More on observing the Orionids

Leonids

The Leonids derive from comet 55P/Tempel-Tuttle last at perihelion in February 1998. A meteor storm was seen in 1999 in the region of the Canary Islands. From the Southern hemisphere the radiant rises after midnight.

More on observing the Leonids

alpha-Monocerotids

In an outburst in 1995, this shower produced a rate of about 420 per hour - for 5 minutes! The whole outburst lasted about 30 minutes.

Phoenicids

The Phoenicid return in 1956 was impressive with about 100 per hour. Activity is otherwise very uncertain. The radiant is well placed for southern hemisphere observers.

Geminids

One of the finest annual showers but with a radiant well north of the equator. In the southern hemipshere the radiant appears around local midnight, even so this is a splendid shower of often bright, medium-speed meteors. In 2008 the peak occurs 2 days after full Moon so will be badly affected by it

More on observing the Geminids

Possible New Meteor Shower

Contributed by Dennis Goodman

As reported by Stephen J O'Meara in the September 2002 issue of Sky and Telescope, a new meteor shower may have been spotted in Taurus. Observations made by Stephen in September 2001, and by French astronomers in 1996 give some indication to this.

This 'shower' is believed to peak around 14-15 September. The radiant point is between the Hyades and the Pleiades in the constellation Taurus, near a 4.3 magnitude star.

The radiant is rather low for NZ observers, but nevertheless it will be worth watching to see if any meteors radiate from that point. I suggest watching from about 4 am on both September 14 and September 15 until dawn gets too advanced. Don't expect to see huge numbers - more likely just a few per hour, if any. This is not a confirmed radiant, so we are simply interested to see if there is any activity.

If recording observations, please note the start and end time of observing run, weather conditions and limiting magnitude (brightness of faintest stars visible). Count the number of meteors you can trace to come from the radiant point in Taurus. If they radiate from anywhere else, don't count them as a shower meteor.

Please send any observations to Dennis Goodman, P O Box 2214, Christchurch who will then on-send them to Stephen O'Meara and the American Meteor Society. Or you can send observations by email to Dennis Goodman.