Kepler’s laws of Planetary Motion
Johannes Kepler was a 16th century astronomer who established three laws which govern the
motion of planets around the sun. These are known as Kepler’s laws of planetary motion.
1. The Law of Orbits: All planets move in elliptical orbits, with the sun at one of the
foci.
2. The Law of Areas: A line that connects a planet to the sun sweeps out equal areas in
equal times.
3. The Law of Periods: The square of the period of any planet is proportional to the
cube of the semimajor axis of its orbit.
Kepler's laws were derived for orbits around the sun, but they apply to satellite orbits as well.
The detailed description is given bellow,
1. Kepler’s first law:
The planets move in elliptical orbits around the sun, with the sun at one of the two foci of the
elliptical orbit. This means that the orbit or path of a planet around the sun is an ellipse i.e. an
oval-shaped and not an exact circle. An elliptical path has two foci and the sun is at one of the
two foci of the elliptical path. This law is important for us as it helps us discover if other stars
have planets.
Figure-1
, 2. Kepler’s second law:
Each planet revolves around the sun in such a way that the line joining the planet to the sun
sweeps over equal areas in equal intervals of time. We know that a planet moves around the
sun in an elliptical orbit with sun at one of its focus. Now, since the line joining the planet
and the sun sweeps over equal areas in equal intervals of time, it means that a planet moves
faster when it is closer to the sun and moves slowly when it is farther from the sun.
Figure-2
In the above figure a planet P is moving in an elliptical orbit around the sun. When the planet
is nearer to the sun at position A, it travels faster and sweeps over an area ABC in time t. On
the other hand, when the same planet is farther from the sun at position X, then it moves
slowly but sweeps over an equal area XYC in the same time t. Thus the Kepler's second law
states that a planet does not move with constant speed around the sun. The speed is greater
when the planet is nearer the sun and less when the planet is farther away from the sun. A
planet could move around the sun with constant speed only if its orbit were a true circle and
not an ellipse.
3. Kepler’s third law:
The cube of the mean distance of a planet from the sun is directly proportional to the square
of time it takes to move around the sun. With the help of Kepler’s third law of planetary
motion we can show how long does it takes to reach Mars, how long would it take for a
spacecraft from earth to reach the Sun.
Johannes Kepler was a 16th century astronomer who established three laws which govern the
motion of planets around the sun. These are known as Kepler’s laws of planetary motion.
1. The Law of Orbits: All planets move in elliptical orbits, with the sun at one of the
foci.
2. The Law of Areas: A line that connects a planet to the sun sweeps out equal areas in
equal times.
3. The Law of Periods: The square of the period of any planet is proportional to the
cube of the semimajor axis of its orbit.
Kepler's laws were derived for orbits around the sun, but they apply to satellite orbits as well.
The detailed description is given bellow,
1. Kepler’s first law:
The planets move in elliptical orbits around the sun, with the sun at one of the two foci of the
elliptical orbit. This means that the orbit or path of a planet around the sun is an ellipse i.e. an
oval-shaped and not an exact circle. An elliptical path has two foci and the sun is at one of the
two foci of the elliptical path. This law is important for us as it helps us discover if other stars
have planets.
Figure-1
, 2. Kepler’s second law:
Each planet revolves around the sun in such a way that the line joining the planet to the sun
sweeps over equal areas in equal intervals of time. We know that a planet moves around the
sun in an elliptical orbit with sun at one of its focus. Now, since the line joining the planet
and the sun sweeps over equal areas in equal intervals of time, it means that a planet moves
faster when it is closer to the sun and moves slowly when it is farther from the sun.
Figure-2
In the above figure a planet P is moving in an elliptical orbit around the sun. When the planet
is nearer to the sun at position A, it travels faster and sweeps over an area ABC in time t. On
the other hand, when the same planet is farther from the sun at position X, then it moves
slowly but sweeps over an equal area XYC in the same time t. Thus the Kepler's second law
states that a planet does not move with constant speed around the sun. The speed is greater
when the planet is nearer the sun and less when the planet is farther away from the sun. A
planet could move around the sun with constant speed only if its orbit were a true circle and
not an ellipse.
3. Kepler’s third law:
The cube of the mean distance of a planet from the sun is directly proportional to the square
of time it takes to move around the sun. With the help of Kepler’s third law of planetary
motion we can show how long does it takes to reach Mars, how long would it take for a
spacecraft from earth to reach the Sun.
