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Question 2 of 5 · Gravitation
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The three laws of planetary motion were given by:

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Answer: Option B — Johannes Kepler

Detailed Solution

Johannes Kepler (1571-1630) was a German astronomer and mathematician. He worked as an assistant to the Danish astronomer Tycho Brahe, who had made very accurate observations of the positions of the planets with the naked eye. After Brahe's death in 1601, Kepler used his data, especially on Mars, to work out the laws that describe how planets move around the Sun.

Kepler's three laws of planetary motion are:

First law (Law of Orbits, 1609): every planet moves around the Sun in an elliptical orbit, with the Sun at one of the two foci of the ellipse. This replaced the old belief that planets move in perfect circles.

Second law (Law of Areas, 1609): the line joining a planet to the Sun sweeps out equal areas in equal intervals of time. This means a planet moves faster when it is closer to the Sun (at perihelion) and slower when it is farther away (at aphelion). It follows from the conservation of angular momentum.

Third law (Law of Periods, 1619): the square of the time period of revolution of a planet (T²) is directly proportional to the cube of the semi-major axis of its orbit (a³), that is, T² ∝ a³. This means planets farther from the Sun take much longer to complete one revolution.

The first two laws were published in "Astronomia Nova" (1609) and the third in "Harmonices Mundi" (1619).

Kepler's laws described how planets move but did not explain why. In 1687, Isaac Newton, in his book "Principia Mathematica", explained them using his Universal Law of Gravitation and laws of motion. Kepler's laws also apply to the motion of moons around planets and of artificial satellites around the Earth. The Earth is closest to the Sun (perihelion) around 3 January and farthest (aphelion) around 4 July.

Why not the other options?

  • ✗Isaac Newton gave the Universal Law of Gravitation and the three laws of motion.
  • ✗Galileo Galilei used the telescope to discover the four large moons of Jupiter (1610).
  • ✗Nicolaus Copernicus proposed the heliocentric (Sun-centred) model in 1543.

Quick tip

Remember: Kepler's laws = orbits are ellipses (1st), equal areas in equal times (2nd), T² ∝ a³ (3rd); explained by Newton's law of gravitation (1687).

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Gravitation — Preparation

This question tests your understanding of Gravitation, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.

This topic covers Newton's law of universal gravitation, the acceleration due to gravity and how it varies, mass and weight, escape velocity, orbital velocity, satellites, Kepler's laws and weightlessness. Common questions ask how the value of g changes with height, depth and latitude (it is greatest at the poles and zero at the centre of the Earth), the Earth's escape velocity (about 11.2 km/s), and why astronauts feel weightless. Remember that mass stays the same everywhere, while weight changes with gravity. Practise the gravitation questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.

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