| Meteors |
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Meteors What are they? Meteor showers recur on nearly the same date of every year, although it might take a day or two to reach maximum intensity and a day or two to fall off. A meteor shower occurs when the Earth, carried by its orbit around the Sun is ploughing through a cloud of meteoric debris. All meteors that belong to a shower will appear to come from the same point in the sky – the radiant. The shower is named after the nearest bright star to the radiant. The brightest meteors are called fireballs and a very large fireball can be as bright as the Moon or even the Sun. Typical meteorite trails are usually somewhere in the vicinity of 100 kilometres up. At this altitude, the atmospheric pressure is 0.00003% of that at the surface of the Earth. Imagine you are holding a piece of a comet stationary above the Earth, and then let it go. It speeds up as it falls. By the time it reaches the Earth’s upper atmosphere, it will be moving at escape velocity – about 11 km/sec. Ordinarily a comet or meteor will have some velocity relative to the Earth before it is attracted by the Earth’s gravity and will therefore hit at a higher speed. A comet on a highly eccentric orbit, moving in a retrograde direction so it collides with the dawn hemisphere, can be travelling as fast as 72 km/sec. When a meteor enters the Earth’s atmosphere, it is heated to incandescence by friction with the thin air at an altitude of around a hundred kilometres. Big particles the size of your fist, or larger, streak through the Earth’s atmosphere, heat up by friction with the air, and char, melt, or burn off a thin crust. This crust can protect the interior. Very small particles are able to radiate their heat away because they have such a large area for their mass and so do not melt. They simply slow down at a hundred kilometres altitude. Gently, they fall for years. Particles of intermediate size are too small to survive the charring of even a thin crust, and too large to float down gently. They burn up entirely during entry. These are the meteors. Typical visual meteors are millimetre sized. A fireball as bright as the brightest star typically weighs less than 100g; but a fireball temporarily as bright as the Sun would weigh a hundred thousand tons. Possible New Meteor ShowerContributed 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
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