04-08-2007 Shooting stars (Article 97)

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Simon Kenny
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Location: Shannon, Co. Clare, Ireland

04-08-2007 Shooting stars (Article 97)

Post by Simon Kenny » Wed Feb 04, 2009 11:15 am

Shooting stars (Article 97)

Most of us will have seen a meteor, often called ‘shooting or falling stars” during our lives, even if we are surrounded by street lamps. These are not actually stars, but very small pieces of dust averaging about the size of a grain of sand or smaller, that is vaporised around 100km high in our atmosphere, at speeds up to 42km/s. At selected times of the year, Earth’s orbit of the Sun encounters a stream of these dust particles, mainly the debris from comet tails, resulting in concentrations of meteors all appearing to come from the same point in space, known as it’s Radiant. There are over 63 meteor showers throughout the year presenting visually observable meteors, but most are quite poor. The Perseids (so called because it’s Radiant lies in the constellation Perseus) are one of the best showers of the year. Given the right circumstances of no moonlight, clear skies and Earth’s intersection point with the dust stream, one can be presented with quite a celestial spectacle.

The shower has a range starting from as early as July 23rd and running right through to August 20th, a consequence of a widely spread dust stream, so enhanced meteor sightings are common during this time of the year. In 2006, the predicted maximum period is on the night of August 12th, morning of 13th, though bright moonlight this year from a moon a few days past Full will drown out the fainter meteors. However, once darkness falls and especially into the wee hours with the moon low in the sky, you should see more shooting stars than normal. Why the wee hours? Well, as the Earth turns on its axis and the sun is at it lowest over the horizon, which occurs at local midnight, you are actually facing in the direction of the Earth’s travel through space. This means your night sky view is heading directly into the shower’s path. Thus when a meteor strikes our atmosphere, you have a better chance to see many more. This shower often provides between 40 to 50 meteors an hour, and we have had some very good activity through-out the 1990s and in recent years. It is best to look up to 45 degrees away from the radiant to better appreciate the meteor’s trail.

The parent of the Perseid shower is Comet Tuttle 1862 III, but earliest records of the shower date back to 811 AD. Many of the Perseid meteors are quite swift, and rather faint, with long trains, the spike shaped tail of vaporised dust behind the meteor, representing its flight path through the atmosphere. Colours tend to be between off white to pale yellow. Perseus, the famed hero of Greek Mythology, stands high in the NE sky at 11.00pm during August, beside the ‘W-shaped’ constellation of Cassiopeia. Both constellations straddle the MilkyWay, so even without meteors, this area is rich in stars.

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