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When the Earth passes through the path of a comet, the meteoroids crash into the Earth’s atmosphere as meteorites and cause a meteor shower as they burn up in the air.
Because the orbits of comets cross that of the Earth, comets are among the most likely bodies to collide with the Earth, and some people believe that collisions in the past may have caused widespread destruction, climate change, and the extinction of many species.
roughly circular, the orbits of comets are very elongated, so that they come very close
to the Earth for a short while and then speed off into space and are lost from
sight for many years. Because they are so small,
they lie immediately behind the Sun; those with orbits inside the Earth’s are in conjunction when they lie both in front of and behind the Sun. Conjunctions also occur between any two other planets and the Earth and also between the Earth, the Moon, and any other planet.
Constellation
The sky is divided up into 88 regions to allow any star to be found quickly. Each of these regions is called a constellation. They are based on shapes described by the Greek astronomer Ptolemy in the 2nd century a.d., with later additions. Examples of constellations include Ursa Major, or Great Bear, in the northern sky (its brightest star is the Pole Star, Polaris) and Crux (the Southern
 Constellation—Constellations are patterns of stars in the night sky invented to help describe space as seen from Earth. The seven brightest stars of the constellation Ursa Major,
for example, make up
the easily seen
Big Dipper or
Plow.
Cross) in the southern sky, which is identified from four bright stars— Alpha, Beta, Gamma, and Delta Crusis. (See also: Andromeda.)
Copernicus
Nicolas Copernicus (1473-1543) was a Polish astronomer and churchman who was the first person to argue against the idea accepted since Ptolemy in the 2nd century a.d. that the Earth was
the center of the Universe. Instead, he put forward the theory that the Earth spins on its axis each day and makes a yearly orbit of the Sun. He also believed that the Universe stretched out forever.
Copernicus set out a revolutionary idea that would eventually become accepted by
all scientists. But at the time, the Copernican system went against the accepted teaching of the church and so was not made publicly available until the day of his death.
1940
1950
Unlike most planets, which have orbits that are
Copernicus
1960
many comets are only detected as they come close to the Earth. About ten new comets are spotted each year; and because each one only returns after many tens of years, this means there must be a large number of them speeding through space.
The comet with the shortest orbit is Encke’s Comet, which returns every 3.3 years. However, those with the longest orbits may only return once in several thousand years. The most famous and brightest comet is Halley’s Comet, which can be seen every 76 years. The most famous recent comet was Shoemaker-Levy 9, which was captured by the world’s telescopes as it crashed into the surface of Jupiter in 1994 and disrupted the Jovian atmosphere.
Conjunction
The time when two bodies in the Solar System are in line. The planets with orbits outside the Earth are in conjunction when
MILKY WAY
Altair
Vega
Deneb
Alderbaran ORION
Polaris
Capella
The Plow or Big Dipper
Arcturus
Betelgeuse
Pollux
Procyon
Regulus
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