From the dawn of history until the beginning of the 17th century the knownuniverse consisted of only 8 planets:
1. Earth, 2. Sun, 3. Moon, 4. Mercury, 5. Venus, 6. Mars, 7. Jupiter,8. Saturn and the "fixed" stars. These are the ones that can be seen easily withoutany optical instruments. In Europe, the prevailing view was the Ptolemaic system withthe Earth at the centre and the other bodies revolving around it.
In 1610, Galileo first turned a telescope on the heavens and the universe wasexplored. By the end of the 17th century, 9 new planets had been discovered andCopemmican’s heliocentric theory was widely accepted. The total number of knownplanets had more than doubled to 17. In 18th Century, only 5 new planets (not counting
comets) were discovered, all by William Herschel , bringing the total to 22. Thenumber of planets in the solar system increased dramatically in the 19th century withI the discovery of the asteroids (464 of which were known at by 1899) but only 9 more"major" planets were discovered. The number of major planets rose to 31 (almostdouble the 17th century). In the first three quarters of the 20th century, 13 more majorplanets (and thousands of comets and asteroids) were discovered bringing the total upto 43. In the Space Age 27 more small moons were discovered by the two Voyagerspacecraft.In CCD Age dozens more small moons have been discovered in recentyears with large ground based telescopes and CCD cameras. There are well over 100now known.
Overview:
The solar system consists of the Sun; the nine planets, more than 130 satellitesofthe planets, a large number of small bodies like the comets and asteroids, and theinterplanetary medium. There are probably also many more planetary satellites that, have notyet been discovered.
The inner solar system contains the Sun, Mercury, Venus, Earth and Mars.Themain asteroid belt lies between the orbits of Mars and Jupiter. The planets ofthe outersolar system are Jupiter, Saturn, Uranus, Neptune and Pluto.The first thing to noticeis that the solar system is mostly empty space. The planets are very small comparedto the space between them.
The orbits ofthe planets are eclipses with the Sun at one focus, though all exceptMercury and Pluto are very nearly circular. The orbits ofthe planets are all more orless in the same plane .called the ecliptic and defined by the plane of the Earth's orbit,Theecliptic is inclined only 7 degrees fromtheplaneof the Sun'sequator.Pluto'sorbit
deviates the most from theplane ofthe ecliptic with an inclination ofl7 degrees. Theyall orbit in the same direction, counter-clockwise looking down from above the Sun’s north pole, including Venus, Uranus and Pluto also rotate in that same sense.
One way to help visualize the relative sizes in the solar system is to imaginemodel in which everything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter. The Moon would be about 30 cm from the Earth. The Sun would be 1.5 meters in diameter and 150 meters from the
more...