Solar System

The Solar System

The term “Solar system” is used to designate the group of celestial objects which travel through space under the gravitational attraction of the Sun. In addition to the Sun, it consists of eight planets with their satellites, several thousand asteroids, as well as comets and countless meteors.

At the centre of this system is the Sun, an enormous mass of glowing gas whose surface temperature is roughly 5,500 degrees Centigrade. In the Sun is concentrated the greatest mass in the system – about 99% – and hence it is dominating body, exerting a controlling influence over the motions of the other members.

Year after year the planets continue to follow their respective paths around the Sun with periods which vary from 88 days for Mercury to 248 years for Pluto, an “ex-planet”, considered today as a big asteroid. Mercury is the nearest to the Sun, and is at a mean distance of 36,000,000 miles, while Pluto is the farthest “planet-asteroid”, with a mean distance of 3,664, 000, 000 miles.

Illustration by Elena

The names of the planets in order of increasing distance from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto used to be classified as a planet, but is considered an asteroid now, with other asteroids situated mainly between Mars and Jupiter. Accompanying six of the planets as they travel through space are 32 satellites or moons. In keeping with its enormous size, Jupiter has the greatest number of moons, namely 12, next comes Saturn with 10, then Uranus with 5, Mars and Neptune with 2 each and lastly the Earth with only one. While the sizes of the satellites are not all known, it is believed that those of Mars are the smallest, having diameters of the order of 5 and 10 miles respectively.

The diameters of the planets vary greatly, from 3,100 miles in the case of Mercury to 88,700 miles for Jupiter. The asteroids are in quite a different category, for their diameters range from less than a mile to 480 miles. By way of contrast the diameter of the Sun is 8644,000 miles.

In the endless journey around the Sun, the Earth encounters numerous small objects which we often see flashing across the sky. These are meteors – sometimes called “falling stars”. It is estimated that many millions of them enter the Earth’s atmosphere daily, but relatively few survive the swift flight through our protective blanket of air to reach the Earth’s surface as meteorites.

Finally a word about comets, those strange objects which in most cases are too faint to be seen except through a telescope. For many of them orbits have been computed and their times of return to the vicinity of the Sun are known and carefully observed. Comets like Halley’s, which are bright enough to excite popular interest, are quite rare, and during the years which elapse between their visits to the Sun’s neighbourhood, they are travelling at great distances from the Sun, often far beyond the orbits of the distant planets.

Mercury: Mean distance from Sun (millions of miles) = 35,98; distance from Earth (millions of miles) = 50 -136; period of revolution = 88 days; period of axial rotation at equator = 59 days; Equatorial Diameter – Miles: 3,100; Mass (Earth=1) = 0,056; Number of satellites = 0; Density (Water = 1) 0,054.

Venus: Mean distance from Sun (millions of miles) = 67; distance from Earth (millions of miles) = 26 -160; period of revolution = 225 days; period of rotation = 244 days; Equatorial Diameter – Miles: 7,700; Mass (Earth=0,817); Number of satellites = 0; Density (Water = 1) = 4,99.

Earth: Mean distance from Sun (millions of miles) = 93; distance from Earth (millions of miles) = 0; period of revolution = 365 days; period of rotation = 23 hours 56 minutes; Equatorial Diam-Miles: 7,927; Mass (Earth=1)=1; Number of satellites = 1; Density (Water = 1) = 5,52.

Mars: Mean distance from Sun (millions of miles) = 142; distance from Earth (millions of miles) = 368 -600; period of revolution = 687 days; period of rotation = 24 hours 37 minutes; Equatorial Diam-Miles: 4,200; Mass (Earth=1)= 0,108; Number of satellites = 2; Density (Water = 1) = 3,94.

Jupiter: Mean distance from Sun (millions of miles) = 483; distance from Earth (millions of miles) = 368-600; period of revolution = 12 years; period of rotation = 9 hours 50 minutes; Equatorial Diam-Miles: 88,700; Mass (Earth=1) =318, 0; Number of satellites = 12; Density (Water = 1) = 1,33.

Saturn: Mean distance from Sun (millions of miles) = 886; distance from Earth (millions of miles) = 784 – 1,028; period of revolution = 29, 5 years; period of rotation = 10 hours 14 minutes; Equatorial Diam-Miles: 75,100; Mass (Earth=1) = 95, 2; Number of satellites = 10 (?); Density (Water = 1) = 0,71.

Uranus: Mean distance from Sun (millions of miles) = 1,782; distance from Earth (millions of miles) = 1,606 – 1,958; period of revolution = 84 years; period of rotation = 10 hours 49 minutes; Equatorial Diam-Miles: 29,200; Mass (Earth=1) = 14, 6; Number of satellites = 5 (?); Density (Water = 1) = 1,70.

Neptune: Mean distance from Sun (millions of miles) = 2,792; distance from Earth (millions of miles) = 2,674 – 2,910; period of revolution = 165 years; period of rotation = 14 hours (?); Equatorial Diam-Miles: 27,700 (?); Mass (Earth=1) = 17,3; Number of satellites = 2 (?); Density (Water = 1) = 2.26.

Pluto (asteroid): Mean distance from Sun (millions of miles) = 3,664; distance from Earth (millions of miles) = 91, 3-94, 4; period of revolution = 248 years; period of rotation = 6, 39 days (?); Equatorial Diam-Miles: 3, 500; Mass (Earth=1) = 0,06; Number of satellites=0; Density (Water = 1) = 5.5.

Moon: Mean distance from Sun (millions of miles) = 93; distance from Earth (millions of miles) = 22, 000 – 253,000; period of revolution = 27 days 7 hours, 7 minutes; period of rotation = 27 days, 7 hours, 7 minutes; Equatorial Diam-Miles: 2,160; Mass (Earth=1) = 0, 012; Number of satellites=0.

Sun: Mean distance from Sun (millions of miles) = 0; distance from Earth (millions of miles) = 91, 3 = 94, 4; period of revolution = n.a.; period of rotation = 25 – 35 days; Equatorial Diam-Miles: 864,000; Mass (Earth=1) = 333,000,0; Number of satellites = 8 planets (+ Pluto) + thousands of asteroids, comets and meteors.

Data taken from the Handbook of the British Astronomical Association.

Additions to the Solar System

Besides controlling the earth and the five planets of antiquity, the sun holds sway over three other planets, Uranus, Neptune, and Pluto, and also over a large number of fly-weight bodies called minor planets or planetoids. Of these additions, Uranus and the minor planet Ceres can be glimpsed with the unaided eye, but only when conditions are particularly favourable. All the rest look like stars even in quite large telescopes, and are therefore not shown in the Planetarium sky. They make themselves known in the real sky by their motions relative to the fixed stars.

Incredibly reach world… Illustration: Megan Jorgensen.