Seasons – Rotation and Revolution

 Key Concepts

• Rotation
• Revolution

Introduction

The basic movements of the Earth help scientists to understand the movements of other celestial bodies in the solar system and universe. The movements of Earth are responsible for the changes in weather and formation of seasons. 

Rotation of Earth: 

We know that Earth spins on an imaginary line called the axis, which runs through the center of the Earth from the North pole to the South pole. 

Rotation is the spinning of Earth on its axis. About every 24 hours, Earth makes 1 rotation. Depending on the time of year, during each rotation, all locations on Earth get some amount of Sunlight and some amount of darkness. As Earth rotates from west to east, in the morning, the Sun seems to rise in the east, and then the Sun seems to cross the sky and set in the west. This happens as a result of Earth’s rotation. 

At one time, people thought that the Sun circles the Earth every day. But only the Sun seems to move because we are observing the Sun from a spinning Earth. The Sun looks to rise in the east and move west, and by midday it will reach its highest point. All happens as a result of Earth’s rotation. This path can be followed by observing the change in shadows of objects at various times of day. One of the evidence for the rotation of the Earth comes from the swinging of a simple pendulum back and forth. French scientist Jean Foucault in 1850, hung a heavy ball to a long wire, and he placed a sand on the floor in two semicircles. The device was attached to the ball to trace the path of the pendulum across the sand. He allowed the ball to swing by pulling it back and forth undisturbed. He observed that at the end of each swing, the pendulum cut a little groove from the previous cut. Pendulums in back-and-forth movements were rotating slowly. By this activity, Foucault deduced that the Earth is rotating under the ball. Today, satellites are used to directly observe the Earth’s rotation from space. 

What makes a year? 

Throughout the year, the seasons change in phases. We always notice this in things like the fall and rise in normal temperature, the blooming of plants or the migration of animals. Can you guess what causes the change in seasons? It does not happen due to the distance between the Sun and the Earth. Actually, the Earth is very close to the Sun in the month of January. This is winter in the northern hemisphere. The tilt of the Earth’s axis of rotation causes the seasons. Earth’s axis is tilted at an angle of 23.5 degrees, and in space it always points in the same direction. The northern end of the Earth’s axis points towards Polaris. Polaris is also known as the North Star since it appears above Earth’s axis in the north. Now, let us see how the tilt of the Earth causes the change in season. 

Earth takes 365.24 days to revolve around the Sun. One complete round around the Sun is called a revolution.  In the northern hemisphere, during summer, the hemisphere is tilted towards the Sun. The Sunrays hit that part of the Earth at higher angle, hence the Sun rays hit that side of the Earth very badly and causes summer season in northern hemisphere. After six months, when the southern hemisphere is tilted on the side of the Sun, sunlight hits the northern hemisphere at lower angles, hence less Sun rays hit that area. So it is summer season in the southern hemisphere and winter season in the northern hemisphere. 

Equinoxes 

On equinox day, fall and spring season begins. An equinox is the time when Sun seems to cross the celestial equator. In the sky, the celestial equator is an imaginary line that is directly overhead from the equator on the Earth. During the period of equinox, the Sun rays directly hit the Earth at 90-degree angle along the equator. On the day of equinox, the period of day and night are roughly the same everywhere on the Earth. Equinox means equal nights. 

The autumn equinox occurs on 22^nd or 23^rd September of every year and that is the beginning of the fall season in the Northern hemisphere. The spring equinox occurs on 21^st or 22^nd March of every year and that is the beginning of spring season in the Northern hemisphere. 

Summer solstices 

The summer and winter seasons begin on day called solstices. Solstices is the point where the Sun is as far as possible to the north of the equator or as far as possible to the south of the equator. Every year on 21^st or 22^nd June the north poles tilt towards the Sun is at its highest. On this day, Sun rays strike the Earth at a 90-degree angle along the Tropic of Cancer which is present at 23.5 degrees north latitude. This is known as the summer solstices, and it is the starting of summer season in the northern hemisphere. In summer solstices, the northern hemisphere has longer daylight. If we are far from the north of the equator, we get a longer daytime. North of arctic circle is located at 66.5-degree north latitude, during summer solstice, there is 24 hours of daylight. In the south of the Antarctic circle, there is 24-hours darkness at that time. 

Winter solstices 

By December, the north pole is tilted very far away from the Sun. On December 21^st or 22^nd, the Sunrays hit the Earth at 90-degree angle along the Tropic of Capricorn located at 23.5-degree south latitude. This day is called the winter solstice. It is the starting of winter season in the northern hemisphere. During the winter solstice, the northern hemisphere has the less daylight hours. 

Summary

• Earth spins on an imaginary line called the axis, which runs through the center of the Earth from the North pole to the South pole.
• The movements of Earth are responsible for the changes in weather and formation of seasons.
• Earth takes 365.24 days to revolve around the Sun.
• One complete round around the Sun is called a revolution.
• An equinox is the time when the Sun seems to cross the celestial equator. In the sky, the celestial equator is an imaginary line that is directly overhead from the equator on the Earth.
• Solstices is the point where the Sun is as far as possible to the north of the equator or as far as possible to the south of the equator