KEPLER’S LAWS OF PLANETARY MOTION

Kepler’s laws are laws describing the motions of the planets in the solar system. They were derived by the German astronomer Johannes Kepler, whose analysis of the observations of the 16th-century Danish astronomer Tycho Brahe enabled him to announce his first two laws in the year 1609 and a third law nearly a decade later, in 1618. Kepler himself never numbered these laws or specially distinguished them from his other discoveries.

Kepler's three laws of planetary motion can be stated as follows: (1) All planets move about the Sun in elliptical orbits, having the Sun as one of the foci. (2) A radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time. (3) The squares of the sidereal periods (of revolution) of the planets are directly proportional to the cubes of their mean distances from the Sun. Knowledge of these laws, especially the second (the law of areas), proved crucial to Isaac Newton in 1684-85, when he formulated his famous law of gravitation between the Earth and the Moon and between the Sun and the planets. Newton showed that the motion of bodies subject to central gravitational force need not always follow the elliptical orbits specified by the first law of Kepler but can take paths defined by other, open conic curves; the motion can be in parabolic or hyperbolic orbits, depending on the total energy of the body. Thus, an object of sufficient energy--e.g., a comet--can enter the solar system and leave again without returning. From Kepler's second law, it may be observed further that the angular momentum of any planet about an axis through the Sun and perpendicular to the orbital plane is also unchanging.

The usefulness of Kepler's laws extends to the motions of natural and artificial satellites as well as to unpowered spacecraft in orbit in stellar systems or near planets. As formulated by Kepler, the laws do not, of course, take into account the gravitational interactions of the various planets on each other. The general problem of accurately predicting the motions of more than two bodies under their mutual attractions is quite complicated; analytical solutions of the three-body problem are unobtainable except for some special cases. It may be noted that Kepler's laws apply not only to gravitational but also to all other inverse-square-law forces and, if due allowance is made for relativistic and quantum effects, to the electromagnetic forces within the atom.

Exercise 1. Complete the following sentences adding information from the text.

1. Kepler’s laws are laws describing … .

2. They were derived … .

3. Kepler's three laws of planetary motion can be stated as follows: … .

4. Knowledge of these laws, especially the second (the law of areas) …

5. The usefulness of Kepler's laws … .

6. Kepler's laws apply …… but also to…..

Exercise 2. Formulate the main idea of the text.

Exercise 3. Give a short summary of each paragraph by choosing the key sentence.

Exercise 4. Speak on the problem described in the text. Make use of the following prompts:

a) The main idea of the text is …

The text runs (is) about …