Chapter – 3 Motions of the Earth

IAS Exam preparation

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Earth Has Two Motions

  • Our Earth has two types of motions , they are rotation and revolution.

  • Rotation is the movement on its axis and revolution is the orbiting of earth around the Sun on a fixed path*.

  • The axis of the earth which is an imaginary line, makes an angle of 66½° with its orbital plane. The plane formed by the orbit is known as the orbital plane.

Rotation Causes Day & Night

  • Because of spherical shape of earth and its rotations on its axis, half portion facing Sun is bright(day) and the other half is dark(night).

  • The circle that divides the day from night on the globe is called the circle of illumination.

  • Illumination – The circle that divides the day from night on the globe is called the circle of illumination. The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earth day.

  • This circle does not coincide with the axis.

  • The earth takes about 24 hours to complete one rotation around its axis which is known as the earthday.

  • The surface of the earth at the equator moves at a speed of 460 meters per second–or roughly 1,000 miles per hour.

What is Earth Did Not Rotate?

  • The portion of earth facing Sun would always experience day and similarly the other part will always face night.

  • Temperature equilibrium will be lost and hence here will be no life.


  • One revolution around the Sun is the period of one earth year (365¼ days).

  • This ¼ days time is covered in leap year making February.

  • The earth is moves around the sun in an elliptical orbit.

  • This motion of earth is responsible for the change of season. (How?)

  • Our orbital speed around the sun is about 67,000 mph (107,000 km/h).

Weathers and Motion

  • On 21st June, the Northern Hemisphere is tilted towards the Sun. The rays of the Sun fall directly on the Tropic of Cancer. As a result, these areas receive more heat.

  • The longest day and the shortest night at these places occur on 21st June.

  • At this time southern hemisphere has all conditions reversed.

  • This position of earth is called Summer solstice.

  • On 22nd December, the Tropic of Capricorn receives direct rays of the sun. As a result this southern hemisphere gets light.

  • This is summer in the southern hemisphere and winter in the north.

  • This postion of earth is called winter solstice.

  • On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole eart experiences equal days and equal nights. This is called an equinox.

Keplar’s Laws of Motion

  • The orbit of a planet is an ellipse with the Sun at one of the two foci.

  • A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.

  • The square of the orbital period of a planet is directly proportional to the cube of the semi major axis of its orbit.

Everything is Moving

  • Earth spins because of the way it was formed. Our Solar System formed about 4.6 billion years ago when a huge cloud of gas and dust started to collapse under its own gravity. As the cloud collapsed, it started to spin.

  • The Milky Way does not sit still, but is constantly rotating. As such, the arms are moving through space. The sun and the solar system travel with them. The solar system travels at an average speed of 515,000 mph (828,000 km/h).

Understand Spacetime and barycenter

  • If a star has planets, the star orbits around a barycenter that is not at its very center. This causes the star to look like it’s wobbling.

  • As seen from above, a large planet and a star orbit their shared center of mass, or barycenter. … They are hidden by the bright glare of the stars they orbit.

Understand Spacetime

Understand barycenter

Take this example