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 from space 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, briefly leaving a visible, incandescent trail. A meteor shower may result when the Earth moves through a stream of such particles, which are usually left by a comet.

Normally a small number of "sporadic" meteors can be seen each hour of a moonless night. Sporadics are likely to be seen in any part of the sky. During a shower the number of meteors visible may increase considerably. The meteors will appear to originate from a small area of the sky, called the radiant. The spreading out from the radiant is a perspective effect due to the meteors travelling in parallel lines but as they approach the observer they appear to fan out. The shower is named after the constellation which contains the radiant. In general the meteor trails do not start from the radiant, but a few degrees from it.

The list of meteor showers shows some 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. Those showers with a green background colour are not likely to be affected by Moon-light in the current year, those with a grey background are likely to be more or less washed out by moon light.

Also shown is the right ascension and declination of the radiant, and a nearby bright star. This latter may help give a quick idea of where the radiant is.

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

Notes on the Streams

Centaurids

There are two showers with close radiants, the alpha-Centaurids and the beta-Centaurids. The two radiants, are near to one another and to the Pointers, α and β Cen. Both are active and peak near the beginning of February. The beta-Centaurids tend to be a little brighter than the alpha-Centaurids. Some can be 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.

Viewing is best after local midnight when the Pointers, and the radiant, gain a reasonable altitude. But the radiant is visible all night in NZ.

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.

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. The material from the comet may be spreading along the comet's orbit, so meteors may become visible in years following perihelion, such as 2010. The shower is best observed in the evening once the sky is dark. The radiant gets lower after midnight.

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. This has the advantage that if the Moon is waxing and a few days before full, it will set before the radiant is high enough for observation. The fast and often bright meteors make the wait for radiant-rise worthwhile, with 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.

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. Observation is best from late evening when the radiant, which is within a few degrees of Fomalhaut, has gained sufficient altitude.

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. The radiant transits close to local midnight, so observation is possible for most of the night from mid evening.

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. The radiant is sufficiently high for observation from late evening.

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, because rates may become too high for accurate plotting paricularly from southern hemisphere sites, .

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. The radiant transits close to local midnight near the time of the peak, so observation is possible for most of the night from mid evening.

Orionids

The Orionids is associated with Halley's Comet as are the eta-Aquarids in May. The shower has 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. Both 2006 and 2007 produced unexpectedly strong Orionid rates, with ZHRs better than the normal peak seen on two or three consecutive nights, at best up to 50-70. The 2006 return also produced a good number of bright Orionids. Good reappearances in the 2008 to 2010 are possible.

For mid southern hemisphere latitudes the radiant does not reach a reasonable altitude for observation until about 2 hours before sunrise, despite rising about local midnight.

A weak shower, the epsilon Geminids, is nearly coincident with the Orionids and has 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. This shower could produce some high count rates. Unfortunately, from mid southern latitudes the radiant does not rise until about 2 hours before sunrise, so activity would only be observable while very low.

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. There is little likelihood of another outburst until about 2020, so activity at best will be low. The radiant, which is only a few degrees from Procyon, is sufficiently high for observation by about an hour after local midnight at the latitude of New Zealand.

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 being high enough for observation virtually throughout the hours of darkness.

Geminids

One of the finest annual showers but with a radiant well north of the equator. In the southern hemisphere the radiant is highest about 2 hours after local midnight, but even then is low to the north for the latitude of New Zealand. This results in only a small proportion of the meteors being visible. Even so this is a splendid shower of often bright, medium-speed meteors.

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.