In astronomy, Keplerβs laws of planetary motion are three scientific laws describing the motion of planets around the sun.
- Kepler first law – The law of orbits
- Keplerβs second law – The law of equal areas
- Keplerβs third law – The law of periods
Table of Content:
Introduction toΒ Keplerβs Laws
Motion is always relative. Based on the energy of the particle under motion, the motions are classified into two types:
- Bounded Motion
- Unbounded Motion
In bounded motion, the particle has negative total energy (E<0) and has two or more extreme points where the total energy is always equal to the potential energy of the particle i.e the kinetic energy of the particle becomes zero.
For eccentricity 0β€ e <1, E<0 implies the body has bounded motion. A circular orbit has eccentricity e = 0 and elliptical orbit has eccentricity e < 1.
In unbounded motion, the particle has positive total energy (E>0) and has a single extreme point where the total energy is always equal to the potential energy of the particle i.e the kinetic energy of the particle becomes zero.
For eccentricity e β₯ 1, E > 0 implies the body has unbounded motion. Parabolic orbit has eccentricity e = 1 and Hyperbolic path has eccentricity e>1.
β Also Read:Β
Keplerβs laws of planetary motion can be stated as follows:
Kepler First law – The Law of Orbits
According to Keplerβs first law,” All the planets revolve around the sun in elliptical orbits having the sun at one of the foci”.Β The point at which the planet is close to the sun is known as perihelion and the point at which the planet is farther from the sun is known as aphelion.
It is the characteristics of an ellipse that the sum of the distances of any planet from two foci is constant. The elliptical orbit of a planet is responsible for the occurrence of seasons.

Kepler First Law – The Law of Orbits
Keplerβs Second Law – The Law of Equal Areas
Kepler’s second law states ” The radius vector drawn from the sun to the planet sweeps out equal areas in equal intervals of time”
As the orbit is not circular, the planetβs kinetic energy is not constant in its path. It has more kinetic energy near perihelion and less kinetic energy near aphelion implies more speed at perihelion and less speed (vmin) at aphelion. If r is the distance of planet from sun, at perihelion (rmin) and at aphelion (rmax), then,
rminΒ + rmaxΒ = 2a Γ (length of major axis of an ellipse) . . . . . . . (1)

Keplerβs Second Law – The law of Equal Areas
Using the law of conservation of angular momentum the law can be verified.Β At any point of time, the angular momentum can be given as, L = mr2Ο.
Now consider a small area ΞA described in a small time intervalΒ Ξt and the covered angle isΒ ΞΞΈ. Let the radius of curvature of the path be r, then the length of the arc covered = r ΞΞΈ.
ΞA = 1/2[r.(r.ΞΞΈ)]= 1/2r2ΞΞΈ
Therefore, ΞA/Ξt = [ 1/2r2]ΞΞΈ/dt
β
Now, by conservation of angular momentum, L is a constant
Thus, dA/dt = constant
The area swept in equal interval of time is a constant.
Keplerβs second law can also be stated as “The areal velocity of a planet revolving around the sun in elliptical orbit remains constant which implies the angular momentum of a planet remains constant”. As the angular momentum is constant all planetary motions are planar motions, which is a direct consequence of central force.
β Check:Β Acceleration due to Gravity
Keplerβs Third Law – The Law of Periods
According to Keplerβs law of periods,” The square of the time period of revolution of a planet around the sun in an elliptical orbit is directly proportional to the cube of its semi-major axis”.
T2 β a3
Shorter the orbit of the planet around the sun, shorter the time taken to complete one revolution. Using the equations of Newtonβs law of gravitation and laws of motion, Keplerβs third law takes a more general form:
P2Β =Β 4Ο2Β /[G(M1+Β M2)] ΓΒ a3
where M1Β and M2Β are the masses of the two orbiting objects in solar masses.
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Frequently Asked Questions on Keplerβs law
What does Keplerβs first law explain?
According to Keplerβs first law, all the planets revolve around the Sun in elliptical orbits with the Sun as one of the foci.
What does Keplerβs second explain?
According to Keplerβs second law, the speed at which the planets move in space continuously changes. The second law helps to explain that when the planets are closer to the Sun, it will travel faster.
What is Keplerβs third law?
Keplerβs third law, also called the law of periods, states that the square of the orbital period is proportional to the cube of its mean distance R.
Why are the orbits of the planets not circular?
For the orbits to be circular, it requires the planets to travel with a certain velocity, which is extremely unlikely. If there is any change in the velocity of the planet the orbit will be elliptical.
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