Constellations are groupings of stars that form easily recognized and remembered patterns, such as Orion and the Big Dipper. The Big Dipper is actually an asterism, not a constellation, because it is only part of the constellation Ursa Major (the Big Bear). Actually, the stars in the majority of all constellations do not “belong together.” Usually they are at greatly varying distances from Earth and just happen to lie more or less in the same line of sight as seen from our solar system. But in a few cases, the stars of a constellation are actually associated; most of the bright stars of the Big Dipper travel together and form what astronomers call an open cluster.
If you observe a planet, say Mars, for one complete revolution, you will see that it passes successively through 12 constellations. All planets (except Pluto at certain times) can be observed only in these 12 constellations, which form the so-called zodiac, and the Sun also moves through the zodiacal signs, though the Sun's apparent movement is actually caused by the movement of Earth.
Although the constellations are due mainly to the optical accident of line of sight and have no real significance, astronomers have retained them as reference areas. It is much easier to speak of a star in Orion than to give its geometrical position in the sky. During the Astronomical Congress of 1928, it was decided to recognize 88 constellations. A description of their agreed-upon boundaries was published in Cambridge, England, in 1930, under the title Atlas Céleste.
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