HOW DO THE PLANETS MOVE?

TWO MOTIONS

As seen from the earth, all the stars and the other planets appear moving eastwards across the sky. All the planets spin like a top and also move around the sun in the same direction. The following three laws govern the planetary motion.

1. Each planet spins like a top around its axis (an imaginary line through its center). This motion is called rotation. The period of rotation is different in the case of each planet. All the planets (except Pluto) rotate in a tilted position.

2. The planets also revolve round the sun along fixed paths (orbits) that are oval in shape. As a result, each planet is a little closer to the sun at some points on its orbit than at other points on it. This motion is called revolution.

3. The area lit by sunlight on a planet is always the same whether it is tilted towards the sun or tilted away from it.

EFFECTS OF MOTIONS AND THE TILT

1. Rotation of a planet round its axis causes periods of light (day) and darkness (night) on it. This is because only half the surface of each planet faces the sun at one time.

2. The tilt of the axis causes the days and nights to be of unequal duration because the tilt causes the northern or southern half of a planet to remain lit for a much longer span of time when it is tilted towards the sun.

3. The tilt leads to a hot season in the northern or southern half of a planet when it is tilted towards the sun and to a cold season when it is tilted away.

4. But two moderate seasons occur when the tilt has no effect on its distance from the sun. These moderate seasons occur in both the halves of a planet at the same time. Moreover, they occur twice during one round of its revolution once when the hot season changes into the cold season and again when the cold season changes into the hot season.