froa one another, as the following descriptions and tables ahoir. The period at rotation ia the tine it takes the planet to turn onoe Itself; It la the length of the planet's day. The period of revolution ( BpameSHe ) ie the planet's year the time it takes to go onoe around the Sun. By axis we aeon the central line around wbioh a planet rotates. Ita tilt ie the angle at which it lies to a line drawn at right angeles to the direction of the SUB. MERCURY. It ia the smallest planet, and also the nearest to the Sun. It has a surface oovered with craters. It turns so slowly that Mercury day - froa one sunrise to the next - lasts two third of a mercury year. VBKUS comes nearer to Earth than. any other planet but little is known of its surface because it is always hidden beneyth clouds. A picture from a Russian space probe shows the surface to be stony. Venus spins ( BpaiuaTI>oa ) clockwise, in the opposite direction to the outer planets. MARS ia the beet-known planet apart ( He CTBTaH ) from Earth, as space probes have mapped the surface and landed there. Their pictures show craters and huge volcanoes, 'the ground being stony and red in colour. There are white ice-caps at the poles. JUPITER is the largest planet of the Solar System. Buna-ing around it are wide bands of different oolous, probably clouds, aaiong which can always be seen the Great Red Spot ( IMTHO ), probably a continuous storm in the atmosphere. Surface temperature at cloud level -150 C, SATURN with ita many rings is the most beautiful sight in the Solar System. The rings are composed of particles and atones, possibly ice-covered, floating in belts only a few kilometres thick. They may be the remains of a moon that broke up. URANUS was the first planet to be discovered with a telescope, although it can be made out without one. It is greenish ( seJieHOBaruB ) in colour, and probably has a surface of ice and frozen gasea beneath swirling clouds. In 1977 it was discovered to have a ring system. NEPTUNE has a blue tinge ( OKpacM ) when seen in a tele scope but no (Barking can be observed. It is probably like Uranus. PLUTO is usually the most distant planet, but part of ita orbit passes within that of Neptune. For this reason Neptune will in fact be the most distant planet until about 2000. Pluto is a smallplanet and little is known about it, though it ia thought to h^ve once been a moon of Neptune.