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# ROTATION AND REVOLUTION

## Rotation

In a general sense, rotation is defined as the circular motion of an object about an imaginary axis through its middle. This imaginary axis is called the rotation axis, and the geometric plane through which the circular motion is defined is called the rotation plane. The rotation axis is perpendicular to the rotation plane. A three-dimensional object with any shape or dimension can be rotated about an infinite number of rotation axes.

## Rotation in Astronomy

All celestial bodies like planets, stars etc., rotate about their own axes. Due to the rotation of the Earth, a centrifugal acceleration is produced in the reference frame of the Earth, which affects the gravitational force. The effect is most clearly seen at the equator, where all matter weighs a little less than they do at the poles. Another effect is that over the course of millions of years, the Earth has slightly deformed into an oblate spheroid developing a bulge at the equator. Yet another effect the rotation of the Earth has had on the planet is that the rotational movement has caused the axis of rotation to wobble and tilt to the side over time. As of now, the degree of tilt between the rotational plane and the axis of rotation is 23.44 degrees, but this angle will keep on going up with time. This phenomenon is called precession.

## Centrifugal Force

Centrifugal force is the inertial force that acts upon objects that are in a rotational movement. It acts outwardly on an axis parallel to the axis of rotation and passes through the origin of the system. It is given by F=mω2r

where

m = the mass of the body
ω = the angular velocity
r = the distance from the origin (radius)

## Revolution

Revolution is much similar to rotation in that it is also a circular motion of an object or body in space, the difference being that the axis of rotation is outside of the body. The body revolves around the axis of rotation of another object. It is referred to as the orbital revolution, to be more precise. This phenomenon is seen in planetary motions where a planet revolves around a star or when a satellite revolves around its designated planet. The path followed by a celestial body is a curved trajectory influenced by the gravitational forces of both the body in motion and the body it is revolving around. These trajectories are often elliptical orbits, as explained by Kepler’s law of planetary motion. Orbital motions are governed by Newtonian mechanics, which describes gravity as a force following an inverse-square law and Einstein’s general theory of relativity, which explains gravity as a product of the curvature of spacetime.

## Kepler’s law of planetary motion

The orbits of the planets around the sun was described by Johannes Kepler through his three laws of planetary motion. They were derived from the heliocentric theory of Nicolaus Copernicus by replacing the circular orbits with the more accurate elliptical orbits. From his observations and calculations, Kepler established that all planets, including the Earth, had elliptical orbits and also that planets have higher velocities when near the Sun compared with their position anywhere else in their orbit. The three laws are as follows:

1. All planets of our solar system revolve in elliptical orbits around the Sun.
2. In equal lengths of time, a line joining the planet and the Sun will cover equal areas.
3.The square of the time taken by a planet to complete one revolution around the Sun is directly proportional to cube of the mean distance of that planet from the Sun.

## Summary

To understand the differences between rotation and revolution more precisely, let us consider the Earth and discuss it further.

Rotation Revolution
The rotation of Earth is defined about an imaginary axis through the center of the poles. The revolution of planet Earth around the Sun is defined as an elliptical path around the Sun.
The time taken by the Earth to complete one full rotation about its axis is one day (24 hours). The time taken by the Earth to complete one full revolution around the Sun is 365 days (8760 hours).
The rotation of Earth is from the west to the east. The revolution of planet Earth around the Sun is in the counter-clockwise direction.
The Earth’s rotation is responsible for the phenomenon of day and night. The Revolution of the Earth causes seasons.
The position of the Earth remains the same throughout a rotation. The position of the Earth constantly changes during a revolution because it is around an external axis.      Talk to our expert
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