Question

Please write all the 3 laws of Kepler in simple words

Answer 1

Answer:

There are actually three, Kepler’s laws that is, of planetary motion: 1) every planet’s orbit is an ellipse with the Sun at a focus; 2) a line joining the Sun and a planet sweeps out equal areas in equal times; and 3) the square of a planet’s orbital period is proportional to the cube of the semi-major axis of its orbit. As it’s the third which is most often used, Kepler’s law usually means Kepler’s third law (of planetary motion).

Answer 2

★ Concept

Our solar system consists of a sun which is stationary at the centre of the universe and eight planets which revolve around the sun in separate orbitals. The names of these planets are : Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. There are certain celestial bodies which revolve around the planets. These are called 'satellites'. For e.g moon revolves around the Earth, hence moon is a satellite of the Earth. Similarly, Mars has two satellites, Jupiter has sixteen satellites, Saturn has nineteen satellites and so on.

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$\underline{\underline{ \sf { \red {Kepler's \: laws}}}}$

Kepler's found important regularities in the motion of the planets. These regularities are known as 'Kepler's three laws of planetary motion '. These laws are as follows :

(1) Laws of Orbits : All planets around the sun in elliptical orbits having the sun at one focus of the orbit.

(2) Laws of Areas : A line joining any planet to the sun sweeps out equal areas in equal times, that is, the areal velocity of the planet remains constant.

(3) Laws of Periods : The square of the period of revolution of any planet around the sun is directly proportional to the cube of its mean distance from the sun.

If the period of a planet around the sun is 'T' and the mean radius of its orbit is 'r', then

$\sf { \red{{T}^{2} \propto {r}^{3}}} \\ \sf \: or \qquad \: {T}^{2} = k{r}^{3}$

where, k is constant. Thus, larger the distance of a planet from the sun, larger will be its period of revolution around the sun.

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Note

Earlier, the number of planets of Solar System was considered as nine. In conference of International Astronomical Association on 24-08-2006, Pluto was considered a 'Dwarf Planet '. Hence, the number of planets has been reduced to eight.

Moreover, the constant 'K' in Kepler's third law is same for all planets and has a value 2.97 x 10ˉ¹⁹s²mˉ³. If the orbit is an ellipse, then r may be replaced by semi major axis a of the ellipse.