The earth has two types of motions, namely rotation and revolution. Rotation is the movement of the earth on its axis. The movement of the earth around the sun in a fixed path or orbit is called Revolution. 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.
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. The circle that divides the day from night on the globe is called the circle of illumination. This circle does not coincide with the axis. The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earthday. This is the daily motion of the earth.
The Earth spins on its axis from West to East (counter-clockwise). It takes the Earth 23 hours, 56 minutes, and 4.09 seconds to complete one full turn. Day and night are produced by the rotation of the Earth. The speed of rotation at any point upon the equator is at the rate of approximately 1,038 miles per hour, decreasing to zero at the poles.
How rotation periods are calculated: The period of rotation is calculated with reference to a star and with reference to the sun. When it is calculated with reference to a star, it is called a sidereal day and when it is calculated with reference to the sun, it is called a solar day.
Solar days and sidereal days: The solar day is a time period of 24 hours, and the duration of a sidereal day is 23 hours 56 minutes. This difference of four minutes between a solar day and a sidereal day is due to the fact that the position of the Earth keeps changing with reference to the sun due to the revolution around it; while with reference to a star at infinity, it will remain unchanged. Thus, a sidereal day is the actual time taken by the planet for a rotation of exactly 360 degrees on its axis.
While the Earth is spinning on its axis, it is revolving around the Sun in a counter- clockwise direction. It takes the Earth one full year to complete one full revolution around the Sun. This path is known as the Earth’s orbit. It is very near a circle. The mean distance of the Earth from the Sun is about 93 milling miles and the distance varies by 3 million miles, forming a slightly oval path. The revolution of the Earth around the Sun traverses a distance of 595 million miles in 365 days, 6 hours, 9 minutes and 9.5 seconds. This means a speed of 18 miles a second (or 66,000 miles per hour) while at the same time rotating once each twenty-four hours. Earth rotates in an elliptical orbit around the Sun The orbit of the Earth around the sun is elliptical and not circular. Due to this, the distance between the Earth and the sun keeps changing.
- When this distance is minimum, the Earth is said to be in perihelion (around January 3).
- When the distance is the maximum, it is said to be in aphelion (around July 4).
Leap Year: It takes 365¼ days (one year) to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for the sake of convenience. Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February. Thus, every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.
Seasons: A year is usually divided into summer, winter, spring and autumn seasons. Seasons change due to the change in the position of the earth around the sun. The Earth’s axis points constantly to the same point (the polar star) in the celestial sphere. As a consequence, the latitude on the surface of the earth at which the sun’s rays fall vertically keeps changing as the earth moves it its orbit around the sun. Due to this the earth attains four critical positions with reference to the sun.
- The Spring equinox: On 21st March, the Earth is so positioned with reference to the sun that the sun’s rays are vertical at the equator and the entire world experiences equal day and night It is called Spring Equinoxes.
- The Autumnal equinox is similar situation occurs on September 23.
- Summer solstice: On 21st of June the sun’s rays are vertical over the Tropic of Cancer as the north pole of the Earth is inclined at its maximum towards the sun. At this time, the north pole experiences a long continuous day and the south pole a long continuous night (ergo, what we know as summer solstice). The northern hemisphere has the summer season at this time and the southern hemisphere experiences winter now. Also, the days are longer than the nights in the northern hemisphere at this time.
- Winter solstice: On December 22, the position of the earth with respect to the sun is such that the south pole is inclined at its maximum towards the sun and the Tropic of Cancer receives the vertical rays of the sun.
This position is called the winter solstice when the sun shines continuously in the south polar region and it is a long continuous night at the north pole. This is the winter season in the northern hemisphere and the summer in the southern hemisphere. During the winter solstice, the days are longer than the nights in the southern hemisphere.
Thus, the variation in the duration of day and night and the change of seasons are due to the earth’s revolution and the inclination of the axis of the earth. Also, the seasons are reversed from the northern to the southern hemisphere.