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Orsides shower in the dawn sky

Get up early in the morning, go outside the city, look up and hope for the best

Photo: Ofer Gabzo

"Leonides" meter. The shower that should be seen early tomorrow morning is of meteors called Ursids

Tomorrow (22/12/00) early in the morning, those who stay awake and raise their heads to the sky may see a strong shower of meteors - so predict astronomers Peter Janiskens from NASA and Asko Litanen. The expected meteor shower is a well-known annual shower of meteors called Ursids, but it usually yields only a few individual meteors per hour. Several times in the past - 1945, 1986 and 1993 are the most recent examples - a shower stronger than usual, of about 100 meteors per hour, was observed, a phenomenon called an "eruption". It is possible, as mentioned, that something similar will happen this year as well.

Meteors are an atmospheric phenomenon that occurs when dust particles enter the Earth's atmosphere. Due to their high speed, several tens of kilometers per second, they burn up in the atmosphere and ionize it. Ionization is the removal of the electrons from their "orbits" around the atom. The ionization of the atmosphere causes it to glow (similar to a neon lamp), and we see a trail of light in the sky. The trail of light is called a meteor, or shooting star, although it has nothing to do with the stars.

The origin of most dust particles is comets - lumps of ice and rock that circle the sun in elliptical orbits. When the comets pass near the Sun, they emit large amounts of dust and ice grains, which continue to move in "swarms", in a path similar to that of the parent comet, for hundreds of years. When the Earth passes through such a dust swarm, one can witness a meteor shower. All the meteors in the swarm move together in parallel orbits, therefore, when they enter the Earth's atmosphere their paths in the sky appear as if they came from a single point. For the meter of the Ursids, this point is in the group of the small cart (Ursa Minor) - hence the name of the meter.

The Orsid swarm that the Earth will pass through tomorrow was ejected from the meteor's parent comet, Comet Tuttle, in 1405. Janiskens and Littanen calculated, using a computer simulation, the trajectory of the dust grains ejected from the comet throughout its history, and showed that they are able to successfully reproduce the dates of "eruptions" The meter of the Ursids in the past. The big test for their model will be tomorrow morning. According to their forecast, the rain is expected to reach its peak at 9:26 Israel time. At this time the sun will be in the sky and it will not be possible to see meteors, but it is possible that a few hours earlier, towards the end of the night, it will be possible to observe them. It is recommended to do the observation in a dark sky as much as possible, outside the city.
{Appeared in Haaretz newspaper, 21/12/2000}

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