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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.

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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:Nov 20, 2014
Words:1393
Previous Article:Comets.
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