The Solar System consists of the Sun, together with the major planets, their natural satellites, the asteroids, and comets, all of which are circling around the Sun in paths called orbits. The word "solar" comes from the Latin word sol, which means "sun".
The Sun contains nearly all the material in the Solar System. It is over 1,000 times as massive as the largest planet, Jupiter, which in turn is more than 300 times as heavy as the Earth. The Sun's gravitational pull keeps the planets and other members of its family in orbit and stops them from traveling off into space. The Sun also provides the heat and light in the Solar System. Large amounts of nuclear energy are generated inside the Sun and radiate out from it. The other members of the Solar System shine only because they reflect sunlight. The Sun is a fairly typical star, not unlike many of those seen in the night sky, but the next nearest star is very much further away from the Sun than any of the planets.
The orbits of the planets, asteroids, and comets around the Sun are elliptical (oval) in shape. The Sun is not at the center of the ellipse, but at a point off-center called a focus (plural: foci). An ellipse has two foci. The Sun is at one of them and there is nothing at the other. The orbits of the nine major planets are in fact very nearly circular. The most elliptical are those of the innermost and outermost planets, Mercury and Pluto. In an elliptical orbit, the distance of the planet from the Sun varies as it travels. Mercury's distance from the Sun ranges between 46 and 70 million kilometers (28.5 and 43.5 million miles), for example.
Viewed from the Earth, the Sun seems not only to rise and set but also to change its position against the background pattern of constellations. The Sun makes one complete "trip" through the constellations in one year. Its path through the sky is called the ecliptic, because eclipses can only occur when the Moon's path crosses it. In fact, the Earth is traveling round the Sun. Because the orbits of the other planets are all in the same plane as that of the Earth, the paths of the planets through the constellations never stray far from the ecliptic. The planets can only be seen against the ring of constellations, through which the ecliptic passes, called the zodiac.
Sometimes, two or more planets may appear to be quite close together in the sky. If they get very close, there is said to be a conjunction. Planets can often be recognized in the night sky because they shine with a steadier light than the twinkling stars. The movements in the Earth's atmosphere cause twinkling. Planets twinkle less because their apparent sizes as seen from Earth, though very small, are greater than those of the stars are. Stars are only points of light, even in the largest telescopes.
© 1988 Encyclopaedia Britannica, Inc.