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.
