Chapter 3 Motions Of The Earth

Rotation

Our planet Earth exhibits two primary types of motion: rotation and revolution.

Rotation is the movement of the Earth on its axis. The Earth's axis is an imaginary line that passes through the center of the Earth and intersects the surface at the North and South Poles. This axis is tilted, making an angle of $66\frac{1}{2}^\circ$ with the plane of the Earth's orbit around the sun. The plane formed by the Earth's orbit is known as the orbital plane.

The Earth completes one full rotation on its axis in approximately 24 hours. This period of rotation is called an Earthday, and it represents the Earth's daily motion.

Ancient Indian astronomer Aryabhata recognized that the Earth is spherical and rotates on its own axis.

Day And Night

As the Earth rotates, only the portion of the Earth facing the sun receives light and experiences day. The other half, which is turned away from the sun, remains in darkness and experiences night.

The imaginary circle that separates the illuminated portion (day) from the dark portion (night) on the globe is called the circle of illumination. It's important to note that this circle does not align perfectly with the Earth's axis due to the tilt (Figure 3.2).

If the Earth were not to rotate, the side continuously facing the sun would experience perpetual daylight and intense heat, making it uninhabitable. The side permanently facing away from the sun would remain in constant darkness and freezing cold, also making life impossible. The rotation of the Earth is therefore essential for the cycle of day and night and for maintaining habitable temperatures across the planet.

Revolution

The second type of Earth's motion is revolution, which is the movement of the Earth around the sun in its fixed path or orbit. The Earth's orbit around the sun is not perfectly circular; it is elliptical (Figure 3.3).

The Earth takes approximately $365\frac{1}{4}$ days (365 days and 6 hours) to complete one full revolution around the sun. For convenience, we count a year as 365 days. The extra 6 hours saved each year are accumulated over four years to form one additional day (6 hours/year × 4 years = 24 hours or 1 day).

This surplus day is added to the month of February every fourth year, making February 29 days long instead of the usual 28. A year with 366 days is known as a leap year.

Seasons

The revolution of the Earth around the sun, combined with the inclination (tilt) of the Earth's axis in a fixed direction throughout its orbit, is the reason for the change in seasons (Figure 3.3). A year is typically divided into four seasons: summer, winter, spring, and autumn. Seasons change as the Earth's position relative to the sun varies during its revolution.

Summer Solstice

Around June 21st, the Earth reaches a position in its orbit where the Northern Hemisphere is tilted towards the sun. In this position, the sun's rays fall directly on the Tropic of Cancer ($23\frac{1}{2}^\circ$ N). As a result, areas in the Northern Hemisphere receive more direct sunlight and heat. The areas near the poles receive less heat because the sun's rays are slanting.

Due to the tilt, the North Pole is inclined towards the sun, and places beyond the Arctic Circle experience continuous daylight for approximately six months. This period corresponds to summer in the regions north of the equator, characterized by the longest day and the shortest night of the year, occurring around June 21st.

Simultaneously, in the Southern Hemisphere, the conditions are reversed. This hemisphere is tilted away from the sun, experiencing winter with shorter days and longer nights. This position of the Earth is referred to as the Summer Solstice (for the Northern Hemisphere).

Winter Solstice

Around December 22nd, the Earth is in a position where the Southern Hemisphere is tilted towards the sun. The sun's direct rays fall vertically on the Tropic of Capricorn ($23\frac{1}{2}^\circ$ S). Consequently, a larger portion of the Southern Hemisphere is illuminated and receives more heat.

This time corresponds to summer in the Southern Hemisphere, with longer days and shorter nights. The Reverse occurs in the Northern Hemisphere, which is tilted away from the sun and experiences winter with shorter days and longer nights. This position of the Earth is called the Winter Solstice (for the Southern Hemisphere). This is why Christmas, celebrated in Australia (Southern Hemisphere), falls during their summer season.

Equinoxes

There are two positions in the Earth's orbit when direct rays of the sun fall on the equator. This occurs around March 21st and September 23rd. At these positions, neither the North Pole nor the South Pole is tilted towards the sun.

As a result, the entire Earth experiences equal days and equal nights. These two positions are known as equinoxes.

The equinox around September 23rd marks the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. The equinox around March 21st marks the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

In summary, the rotation of the Earth causes day and night, while the revolution of the Earth around the sun, combined with the constant tilt of its axis, causes the changes in seasons.