Earth's Two Motions: Rotation and Revolution
As a planet of the sun, the Earth revolves around the sun. Besides this, it also rotates on its own axis. Thus the Earth has two motions: the rotation and the revolution.
The Earth rotates on its axis from west to east. The axis is an imaginary line passing through the northern and the southern poles. Earth's rotation is completed in about 24 hours — this is called the daily motion of the earth. This motion is responsible for the occurrence of day and night.
- One rotation is completed when a given heavenly body crosses the observer's meridian two times in succession.
- 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.
- As a result of the apparent motion of the sun, the appearance of the night sky as seen from the Earth changes from day to day. The stars rise every day, four minutes earlier than the preceding day. Also, every night, one degree of celestial sphere is added to the eastern sky while one degree is lost from the western sky.
Rotation of the Earth
Revolution of the Earth
The movement of the Earth around the sun in its orbit is called revolution. This movement of the earth is also from west to east. The period of revolution is one year (365 1/4 days).
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).
The average of the maximum and minimum distances is called the mean distance and this distance of the sun from the earth is 150 million kilometers.
The apparent annual track of the sun through the fixed stars in the celestial sphere is called the ecliptic, and an imaginary plane passing through this plane and extending outward through all points is called the plane of ecliptic. This plane is imagined to be horizontal. The axis of the Earth (rotational or polar axis) makes an angle of 66 1/2 to the plane of ecliptic.
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 equinoxes: 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.
- The autumnal equinox: A 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.
The Midnight Sun
At the time of the summer solstice, there is continuous day at the north pole as the circle of illumination passes through the Arctic Circle beyond the pole. As a result, at the places situated beyond the Arctic Circle, the sun will be visible continuously even when it is night at the same longitude in the lower latitudes. This phenomenon is called the midnight sun. A comparable situation occurs in the Antarctic Circle during the winter solstice when the circle of illumination passes through this circle beyond the south pole.