NCERT Summary: Motions of the Earth | Geography (Prelims) by Valor Academy - UPSC PDF Download

MOTIONS OF THE EARTH

• The earth has two types of motion, namely rotation, and revolution.
• Rotation is the movement of the earth on its axis. An axis is an invisible line around which an object rotates or spins.
• The ancient Indian astronomer Aryabhata had stated that the earth is round and rotates on its own axis.
• The earth takes about 24 hours to complete one rotation around its axis.
• The axis of the earth is not 90 degrees, it is tilted (23.5 degrees) and the angle of the earth’s axis is 66.5 degrees.
  

What caused the tilt?

• When Earth was young, it is thought that something big hit Earth and knocked it off-kilter. So instead of rotating with its axis straight up and down, it leans over a bit.
• By the way, that big thing that hit Earth is called Theia. It also blasted a big hole in the surface. That big hit sent a huge amount of dust and rubble into orbit. Most scientists think that that rubble, in time, became our Moon.

DAY and Night

• The earth receives light from the sun. Due to the spherical shape of the earth, only half of it gets light from the sun at a time. The portion facing the sun experiences day while the other half away from the sun experiences night.
  

Day and Night

• The movement of the earth around the sun in a fixed path or orbit is called Revolution.
• Earth takes 365 ¼ days to revolve around the sun in its orbital plane.
• The orbital plane of a planet is the geometric plane in which it revolves around the sun.
  

Orbital plane

• We consider a year as having 365 days. Six hours saved every year are added to make one extra day every four years (leap year).
• The orbit of the earth around the sun is not in a perfect circle. Earth is going around the sun in an elliptical orbit.
• When an object moves around another object in an oval shaped path, it is known to be revolving in an elliptical orbit. All planets move in elliptical orbits around the sun. The Moon also moves around the earth in an elliptical orbit.

➢ Seasons

• A year is usually divided into summer, winter, spring, and autumn seasons.

➢ What Causes the Seasons?

• Earth's tilted axis and its revolution around the sun causes the seasons.
• As Earth orbits the sun, its tilted axis always points in the same direction.
  

• Throughout the year, different parts of Earth receive the Sun's most direct rays. So, when the North Pole tilts toward the Sun, it's summer in the Northern Hemisphere. And when the South Pole tilts toward the Sun, its winter in the Northern Hemisphere.
• Sometimes it is the North Pole tilted toward the sun and sometimes it is the South Pole tilted toward the sun.
• It is summer in June in the Northern Hemisphere because the sun's rays hit that part of Earth more directly than at any other time of the year. It is summer in December in the Southern Hemisphere because that is when it is the South Pole turn to be tilted toward the sun.

➢ Solstices and Equinoxes 

Revolution of the earth and seasons

➢ Summer Solstice

• On 21^st 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 shortest night in the North Hemisphere occur on June 21st. At this time, in the southern hemisphere, all these conditions are reversed. The nights are longer than the days. This position of the earth is called the Summer Solstice.

➢ Winter Solstice

• On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards the sun.  As a result, these areas receive more heat. The longest day and shortest night Southern Hemisphere occur on December 22nd. The reverse happens in the Northern Hemisphere. This position of the earth is called Winter Solstice.
• When it is summer in Northern Hemisphere, places beyond Arctic Circle (66½° N) experience continuous daylight for about six months and, places beyond the Antarctica circle (66½° S) in the Southern Hemisphere experiences continuous light for six months and vice versa.

➢ Equinoxes

• On 21^st March and September 23^rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun. So the whole earth experiences an equal length of days and nights. This is called Equinox.