Monday, December 13, 2010

Geminids rich and mysterious Starwatch

The Geminids rich and mysterious Starwatch :Tonight sees the peak of perhaps the most puzzling, meteor shower of the year. The Geminids shower, which began almost a week ago and continues until Thursday, should be most spectacular before dawn tomorrow when up to 120 meteors per hour might be counted under ideal skies.

Geminids are slow (35 km per second) and often bright as they trace parallel paths into the Earth's upper atmosphere. Perspective means that they appear to diverge from a radiant in Gemini, hence the shower's name. That point, close to the star Castor and plotted on our chart of the constellation, climbs from near the NE horizon at nightfall to pass some 70° high in the S at 02:00 before sinking through the W before dawn. Meteors, though, rain down in all parts of the sky – it is just their paths that point back to the radiant.

Three factors make the morning hours more favourable for Geminids spotting. The first is moonlight which swamps the fainter meteors. The Moon lies close to the bright planet Jupiter this evening and does not set until after midnight. The second factor is the radiant's altitude; when the radiant is highest in the sky we see more meteors because we are facing more "head-on" into the stream of Geminid meteoroids as they orbit the Sun. And thirdly, the Earth is expected to plough through the densest part of the stream tomorrow morning.

Meteoroids in that stream take about 524 days to plunge from the asteroid belt, beyond the orbit of Mars, to inside the orbit of Mercury, and back again. They follow the same eccentric orbit as the asteroid Phaethon, 5km wide and at the root of the mystery. Most meteor showers occur when the Earth encounters the dust left along the orbits of comets. Phaethon, though, is not a comet and nor does it appear to be an extinct comet nucleus; indeed its spectrum links it to the large asteroid Pallas, 544km wide.

Were Phaethon and other so-called Palladian asteroids blasted from Pallas in some ancient collision? And where does the dust in Phaethon's orbit, our Geminid meteoroids, come from? One theory, backed up last year by observations of Phaethon as it passed through perihelion only 21 million km from the Sun, is that the Sun's intense heat can cause Phaethon's rocks to shatter, with the fragments able to escape Phaethon's feeble gravitational pull to replenish the Geminids stream.

Incidentally, the star cluster M35, near the right edge of our chart, lies only 4° E (left) of the Moon's position during the total lunar eclipse before dawn on the 21st. The Moon begins to enter the central dark umbra of the Earth's shadow at 06:33 and is fully within the umbra by 07:41 as it dips towards our NW horizon.Articel From www.guardian.co.uk ..

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