Text 8. THE A TO Z OF ASTRONOMY

By Professor Chris Kitchin

Here are some simple difinitions of the more puzzling words commonly encountered in astronomy.

Meteorite The fragment of a meteoroid which has survived passage through the Earth's (or other planet's) atmosphere to reach the surface. Small meteorites impact at their terminal velocity, but larger ones may retain some of their cosmic velocity and hit at up to several tens of kilometres per second. In the latter cases an impact crater will be produced and the meteorite will survive only as thousands of tiny fragments. Most meteorites are found to be of rocky composition but about six percent are almost pure nickel-iron. About two percent are formed from mixtures of rock and iron.Two small but very significant sub-groups are the carbonaceous chondrites which contain some simple organic molecules and are thought to pre-date the formation of the Solar System, and the SNC meteorites which may have come from Mars.

Meteoroid A small body independently orbiting the Sun. The meteoroids merge into the asteroids at the larger end, and into the inter-planetary dust at the smaller end of the scale.

Meteor shower A series of meteors lasting from a few hours to several days which have parallel paths through space. Perspective means that the meteor tracks appear to diverge from a point in the sky called the radiant. The position of the radiant is often used to give the shower a name. Thus we have the Leonid (after the constellation Leo) and the Geminid (after Gemini) meteor showers amongst many others. The number of meteors in a shower can range from a few tens to hundreds of thousands per hour. The particles producing the meteors are thought to be debris from a comet which are continuing to follow the comet's orbit. The Leonids, for example, originate from comet Temple-Tuttle.

Metonic cycle The period required for the Moon's phases to repeat themselves on the same days of the month. Its value is 19 years.

Milky Way The faint irregular glowing band which circles the sky. It is a small part of our own galaxy and comprises tens of millions of stars, each too faint to be seen with the naked eye individually, but clearly to be seen in aggregate. It gives its name to our galaxy; so that we are a part of the Milky Way Galaxy. Interstellar dust restricts our view of the galaxy to within a few thousand parsecs of the Sun, and so the Milky Way is just a small part of the whole galaxy.

Monopole An hypothetical magnetic equivalent to the electron. A monopole would be a sub-atomic particle with a single magnetic (north or south) pole. They are predicted to have been produced in huge numbers during the big bang but none has yet been detected. This scarcity of monopoles is one of the arguments supporting an inflationary period during the early stages of the formation of the Universe.

(ASTRONOMY NOW / SEP 1997)

I. Read the definitions and translate them into Russian.

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