Meteors.
What is a meteor?A meteor or "shooting star0' is produced when an interplanetary dust particle (meteoroid) enters the Earth's atmosphere and deflagrates, leaving a train of excited and ionized particles along its path. If sufficient light is produced by the subsequent deexcitation, as well as various ionic processes, the meteor may be observed visually. Meteors can also be detected using techniques that rely on the scattering of radio signals by free electrons in their wakes.
The typical visible meteor is about the size of a grain of sand; a meteor the size of a grape produces a very memorable fireball. In general, meteors appear in the upper atmosphere, at an altitude of between 80 and 120 km and disappear between 60 and 80 km above the Earth.
Many meteors are sporadic, but many more are members of showers. The origin of sporadic meteoroids is still uncertain. They are probably very minor streams of low particle density, the remains of minor streams that have long dispersed, as well as solitary particles travelling in isolation. Under ideal conditions, about 10 sporadic meteors can be seen per hour.
Meteor showers
A meteor shower occurs when the Earth intersects a stream of meteoroids that has formed as a result of the disintegration of a larger body (usually a comet but occasionally an asteroid). Thus each shower recurs at the same time each year, and the meteors seem to radiate from a point (the radiant) corresponding to the direction from which the meteoroids approach the Earth. Table 13 lists meteor showers in need of observation.
April Lyrids: This shower normally peaks at around 15-20 per hour, but has shown outbursts traced back to ancient times, and most recently in 1982. The maximum is quite sharp, lasting at most two days above half peak activity.
Orionids: Conditions during 2015 favour the observation of the inbound stream from comet 1P Halley (the eta Aquariids are lost to the Full Moon in May). The Orionids are similar to the eta Aquariids in appearance, yielding fast, often bright meteors with persistent trains, although rates are lower at maximum, seeing that the stream is located at greater distance from Earth's orbit. Nevertheless, rates of around 30 per hour can be expected near maximum. This stream has however also shown a tendency to produce years with unexpected higher activity.
Geminids: This shower is generally the highlight of the meteor year, with high zenithal hourly rates. The Moon will not interfere with observations this year. The activity shows a slower rise to maximum, followed by a sharp drop. Due to mass-sorting of the stream, the brightness of the meteors tends to increase with date. Best opportunities typically occur on the mornings of December 13-15, when it is best observed from about midnight onwards.
Meteorites and impact craters
Most meteors vaporize during their descent, but a few survive and strike the Earth and are called meteorites. Once a meteorite lands on South African territory it belongs to the State (Act 25 of 1999) and is protected by the South African Heritage Resources Agency. Trading in South African meteorites is illegal.
A number of significant meteorite falls have been recorded in southern Africa. The world's largest known meteorite is the Hoba Meteorite in Namibia (see p 110). A meteorite fall consisting of at least 51 individual fragments, with a total mass of 15 tons, occurred some time in the distant past near Gibeon, also in Namibia. Although known since 1836, the first fragments were only collected in 1911. The most recent meteorite fall in southern Africa happened on 2002 July 21 at Thuathe in Lesotho. The event was witnessed as a daytime fireball from Johannesburg and the Free State. At least 500 fragments were recovered with a total mass of about 30 kg. It is estimated that on entry the meteoroid weighed about a ton.
The Vredefort structure (a World Heritage Site) is currently regarded as the oldest and largest clearly visible impact structure on Earth, formed when a gigantic meteorite (larger than Table Mountain) hit the Earth some two billion years ago. The original crater is estimated to have been 250-300 km in diameter. The central uplift is near Parys in the Free State and the outer wrinkle is the Braamfontein Ridge in Johannesburg. Vredefort played a role in the concentration of the Free State and Transvaal gold deposits.
The oldest (3 billion years) known impact structure is in the Manitsoq region of West Greenland and was discovered in 2009.
Craters large and small
Crater Size (km)
Vredefort (Free State, South Africa) 300
Sudbury (Ontario, Canada) 130
Chicxulub (Yucatan, Mexico) 240
Popigai (Siberia) 100
Manicouagan (Quebec, Canada) 100
Acraman (South Australia) 90
Chesapeake Bay (Virginia, USA) 85
Morokweng (Kalahari Desert) 70
Kara (Yugorsky Peninsula, Russia) 65
Beaverhead (Idaho, USA) 60
Table 13. Meteor showers observable in 2015
Shower Date Duration Radiant
max. R.A.
[alpha] Crucids Jan 19 Jan 06-Jan 28 12h48m
[alpha] Centaurids Feb 07 Jan 28-Feb 21 14 00
[gamma] Normids Mar 13 Feb 25-Mar 22 16 36
[delta] Pavonids Apr 06 Mar 11-Apr 16 20 32
April Lyrids Apr 22 Apr 16 -Apr 25 18 05
[pi] Puppids Apr 23 Apr 15 -Apr 28 07 20
[eta] Aquariids May 05 Apr 21-May 12 22 24
[theta] Ophiuchids Jun 13 Jun 08-Jun 16 17 48
June Lyrids Jun 16 Jun 11-Jun 21 18 32
July Phoenicids Jul 13 Jul 10-Jul 16 02 08
Piscis Australids Jul 28 Jul 19 -Aug 17 22 40
S. 8 Aquariids Jul 29 Jul 21-Aug 29 22 36
[alpha] Capricornids Jul 30 Jul 15 -Aug 25 20 28
Orionids Oct 21 Oct 02-Nov 07 06 20
Southern Taurids Nov 05 Oct 01- Nov 25 03 20
Northern Taurids Nov 12 Oct 01- Nov 25 04 00
Leonids Nov 17 Nov 12-Nov 21 10 08
[alpha] Monocerotids Nov 21 Nov 15-Nov 25 07 48
Dec. Phoenicids Dec 06 Dec 03-Dec 09 01 12
Geminids Dec 14 Dec 04-Dec 16 07 28
Puppid-Velids Dec 29 Dec 05-Jan 07 09 56
Shower ZHR Vel. Watch
Dec Start End
[alpha] Crucids -63[degrees] <5 50 00:00 03:30
[alpha] Centaurids -59 5 60 22:00 03:30
[gamma] Normids -51 8 56 00:00 04:30
[delta] Pavonids -63 5 59 02:00 04:30
April Lyrids +34 15 49 02:00 05:00
[pi] Puppids -45 <5 18 19:00 23:00
[eta] Aquariids -02 60 65 04:00 05:30
[theta] Ophiuchids -20 5 27 20:00 05:30
June Lyrids +35 5 31 23:30 02:00
July Phoenicids -48 <5 47 23:00 05:00
Piscis Australids -30 5 35 21:30 05:00
S. 8 Aquariids -16 25 42 22:00 05:00
[alpha] Capricornids -10 5 25 20:00 04:00
Orionids +16 30 68 00:00 04:00
Southern Taurids +14 10 29 21:30 03:30
Northern Taurids +23 5 31 21:30 03:30
Leonids +22 5-10 70 03:00 04:00
[alpha] Monocerotids +01 5-50 65 23:00 04:00
Dec. Phoenicids -53 5 22 20:30 02:00
Geminids +33 50 36 23:30 03:00
Puppid-Velids -51 5 40 22:30 03:30
Shower Observing
prospect
[alpha] Crucids New Moon
[alpha] Centaurids Unfavourable
[gamma] Normids Poor
[delta] Pavonids Unfavourable
April Lyrids Favourable
[pi] Puppids Good
[eta] Aquariids Unfavourable
[theta] Ophiuchids Favourable
June Lyrids New Moon
July Phoenicids Favourable
Piscis Australids Poor
S. 8 Aquariids Unfavourable
[alpha] Capricornids Full Moon
Orionids Good
Southern Taurids Good
Northern Taurids New Moon
Leonids Good
[alpha] Monocerotids Good
Dec. Phoenicids Favourable
Geminids Favourable
Puppid-Velids Poor
Key: Date of maximum: Meteor shower maxima may vary
one or two days either side of the predicted date.
Prospective observers should observe on a range of
dates centred on the listed date. Radiant: The radiant of
most showers drifts slightly eastward each night.
The position given is for night of maximum. ZHR: zenithal
hourly rate; expected maximum rate under observing
conditions when stars of 6.5-mag. can be seen and with
the radiant at the zenith. Rates under poorer conditions
and when the radiant is low will consequently be lower.
Vel: velocity (km x [s.sup.-1]). Watch: Recommended
watch times. Observing prospect: Showers listed as
"Favourable" have the best prospects for observation.
Those listed as "Good" may be observed with slight
hindrance from the Moon.
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| Publication: | Sky Guide Africa South |
|---|---|
| Article Type: | Calendar |
| Geographic Code: | 60AFR |
| Date: | Jan 1, 2015 |
| Words: | 1391 |
| Previous Article: | Comets. |
| Next Article: | Stars and constellations. |
| Topics: | |