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One of the most beautiful sights of the night sky is to see two or even more planets group together as if they were a cluster of jewels on a ring. Less rare, although still fascinating, is watching a series of several planets form a straight line across the sky. Are groupings such as these merely a matter of chance?
Thousands of years ago, people noticed that the behavior of the objects in the sky that were later to be called planets was different from the other stars. Most stars maintain a fixed position relative to each other. They form patterns – called constellations – which move through the sky every night but keep the same form. The stars forming the constellation Orion rise and set, but Orion doesn’t suddenly drop his outstretched arms.
But five unusually bright stars – Mercury, Venus, Mars, Jupiter and Saturn behaved differently. Within a single night they would remain in a fixed position relative to the other stars. From night to night, their position would gradually change and they would wander from the area of one constellation to another, which is why these particular stars were called planets – the wanderers. During one month, Mars would be seen in Gemini and then after several weeks it would gradually move over to Cancer. Also, you didn’t find the planets everywhere in the sky, but they restricted their movements to a band of constellations consisting of Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer and Leo which are the constellations of the Zodiac. One last thing noticed that while on occasion, a planet would cover a fixed star, a fixed star would never cover a planet.
They explained this by assuming the planets and stars (as well as the Moon and Sun) were fixed on large, concentric, invisible, rotating spheres with the Earth at their center. All the fixed stars were on a common sphere that rotated once in slightly less than 24 hours. Each planet was on a sphere that would rotate at different speeds. Jupiter’s sphere rotated once every 12 years or so while Venus’ would rotate several times a year. This was a fairly good theory, except that Mars would be traveling nicely along across the background of fixed stars and suddenly change direction and loop around before resuming its original direction. To solve this and other problems that arose as more accurate observations were made, more and more spheres were added to the model until thanks to the work of Coprenicus, Brahe Kepler, the Sun was placed in the center and Mars’ strange movements were explained as a result of the relative movements of Mars and Earth while they revolved around the Sun.
Most of the Solar System is restricted to a relatively narrow plane, for reasons that are currently not fully understood. The Solar System is shaped much like an old fashioned phonograph record; most objects of the Solar System stay within the area of a large circular slice of space. The Sun, most of the planets, their moons – in fact almost everything in the Solar System except for the most distant planet Pluto and the comets keeps within this relatively narrow band of space.
The fixed stars that are far beyond the Solar System can be seen anywhere but almost all the objects from within the Solar System stay within the band. Since the Earth is also within this giant phonograph record of space, the Sun, Moon and the planets all travel close to the ecliptic: an imaginary line in the sky that runs along the constellations of the Zodiac. Since the planets travel along the ecliptic they frequently form a straight line across the sky and often bunch up together which would rarely happen if they were traveling randomly across the entire sky. Several times a year, the Moon will cover (occultate) a planet because they travel in the same plane. (The Moon will also occultate fixed stars, but only those that are close to the ecliptic.)