The Solar System.
The solar system comprises the Sun and eight planets (1) which are: ( in order of distance of our star) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Planets turn on their axis (rotation) and round the Sun (revolution) drawing, as say the first law of Kepler, ellipse of which the Sun occupies one of the two focuses. In fact, the word planet derives from the Greek language and means "errant" because, in the sky at night, planets seem to wander among stars.
The ancients built many cosmologies but their contents were mythological. It was the Greek Talete who investigated the nature of the universe trying to explain some phenomena; for example, he understood the mechanism of eclipses. There were also people who affirmed that the Earth and the luminaries were spherical and that was not the Earth in the center of the cosmos but Zeus's fire, but that was due to beliefs of Pythagoreans. However, for many centuries, people shared the system built by Tolomeo (I-II century A.D.), in it the Earth was in the center while the Sun and planets (Mercury, Venus, Mars, Jupiter and Saturn) turned round the Earth drawing circular orbits; exactly planets turned according to an epicycle where center was equidistant from the Earth. Names of days of the week depend to that system too. Copernicus (XV-XVI century A.D.) used it too but made a fundamental change: he put the Sun in the center, not the Earth. The theory of epicycles continued to be shared because the use of circular orbits gave inexplicable errors in ephemeredes. It was necessary waiting for the work of Kepler (XVI-XVII century A.D.) to get a correct representation of orbits that really are elliptical and not circular. With the progress of technology many other plugs have been added: in 1610, Galileo Galilei in "Sidereus nuncius" wrote about satellites of Jupiter and sunspots; in 1781, Herschel discovered Uranus; in 1846, Galls discovered Neptune; in 1930 Tombaugh discovered Pluto (1). However researches are not completed and every day we learn a bit more.
The Numbers of the Solar System.
Mercury and Venus are also called "internal planets" because their orbits are within that of the Earth. Venus and Uranus have a retrograde rotation. Density (kg/dm3) of Jupiter, Saturn, Uranus and Neptune is about one fifth of other planets ones because their structure is in large part of gaseous nature. See also the Ephemerides page.
(1) With the IAU resolution dated 26 August 2006, which gave the "definition of planet", Pluto was not consider properly a planet any longer.
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Paul Davies, Il cosmo intelligente, Milano 2000
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Joachim Hermann, Atlante di astronomia, Milano 1990