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Motions of the Earth and their effects

Study Material > Geography
  • The Earth has two motions:
    1. Rotation around its own axis and
    2. Revolution around the sun
  • Earth’s rotation: The Earth rotates around its Axis from west to east in anticlockwise direction and completes one rotation in 23 hours 56 minutes and 4 seconds. The axis of the Earth is an imaginary line passing through the centre of the earth and joining north and south poles. It is tilted at an angle of about 66.5 degree to the plane of the ecliptic ,i.e., at an angle of about 23.5 degree from the line perpendicular to that plane. The velocity of rotation decreases from equator to poles. It is highest at the equator and zero at poles.
  • Effects of Earth’s rotation:
    1. Formation of day and night
    2. Concept of time
    3. Deflection of winds and ocean and ocean currents
    4. Spheroidal shape of the earth
    5. Knowledge of directions
    6. Regular intervals of tides
    7. Weight of a body is less at the equator than at the poles due to Greater centrifugal force at the equator caused by the earth's rotation and slightly less gravitational pull than poles because of bulging out of earth at equator.
  • Earth’s revolution: The earth revolves around the Sun in counterclockwise direction along fixed path called orbit. Earth’s orbit around the sun in a elliptical orbit. Its average velocity is around 18.5 miles per second or 29.6 kms per second. It completes on revolution around the Sun in 365 days 5 hours and 48 minutes and 45.68 seconds approx 365.25 days. Earth-Sun relationships are described by a tropical year, which is defined as the period of time (365.25 average solar days) from one vernal equinox to another. To balance the tropical year with the calendar year, a whole day (February 29) is added every fourth year (leap year). The average distance between Earth and the Sun is approximately 93 million miles (150 million km).
  • Since, the Earth’s orbit is an ellipse rather than a circle, the Earth is closest to the Sun on about 3 January a distance of 91.5 million miles (147 million km). This position in space is called perihelion, which comes from the Greek peri, meaning around or near, and helios, meaning the sun. The earth is farthes from the Sun on about July 4, at Aphelion, with a distance of 94.5 million miles (152 million Kms).
  • Earth’s inclination:The plane in which the earth’s orbit lies is known as the palne of the ecliptic. The plane of the earth’s equator is inclined as an angle of 23.5 degree with the plane of the ecliptic. The earth’s axis makes an angle of 66.5 degree with the plane of the ecliptic, and is tilted at 23.5 degree from a line perpendicular to that plane. Through out the revolution of the earth around the sun, its axis remains tilted in the same direction and continues to point to the same spot in heavens called Polaris or polestar. This is known as parallelism of the axis.
  • Effects of the earth’s revolution around the Sun:
    1. Change of seasons
    2. Variations in the lengths of day and night at different latitudes and different times of the year
    3. Difference in altitudes of sun at noon at different times of the year
  • Following factors are responsible for the change of season:
    1. Revolution of the Earth around the sun on its orbit.
    2. Inclination of the earth’s axis at 66.5 to the plane of earth’s orbit.
    3. Rotation of the earth
    4. Constant orientation of the earth’s axis towards Polaris (Pole star).
  • During the course of its revolution around the sun, the earth passes through different phases and occupies different positions. This four positions, results in four different seasons. Two of them are known as solstices and other two are called equinoxes.

  • Solstice: Solstice is the time when the sun’s rays are vertical either at the Tropic of Cancer or at the Tropic of Capricorn. Solstice occurs twice in a year, once when the sun’s rays are verticle over the Tropic of Cancer and the other when the sun’s rays are verticle over the Tropic of Capricorn.
  • Summer Solstice: On June 21 or 22, the earth's Axis leans at maximum angle of 23.5 degree towards the sun and the sun's vertical rays are at the Tropic of Cancer. The northern hemisphere has maximum leaning towards the sun and receives more insulation than the southern hemisphere. This situation is called summer solstice because this is the time of summer season in the Northern hemisphere. Duration of day increases towards the north and decreases towards the South. Between the Arctic Circle and the north pole, the day light lasts for 24 hours because the sun's rays crossover to the north pole and the earth's turning on its Axis through 360° fails to bring any part of this area into the zone of darkness. The duration of day is 14 hours on the Tropic of Cancer, 12 hours on the equator, 10 hours on the Tropic of Capricorn and zero hours on the Antarctic circle. The sun rises in North East and sets in North West.

  • Winter Solstice: On December 21 or 22 to the opposite end of its orbit and the southern hemisphere has maximum inclination towards the sun. The sun’s vertical rays are at the Tropic of Capricorn. This is the time of winter season in the Northern hemisphere and this position is known as Winter solstice. The duration of day light increases towards south and decreases towards the north. Between in Antarctic Circle and the south pole, the daylight lasts for 24 hours because the sun's rays crossover to South Pole and the earth’s turning on its Axis through 360 degree fails to bring any part of this area into the zone of darkness. The duration of Daylight is 14 hours on the Tropic of Capricorn, 12 hours on the equator, 10 hours on the Tropic of Cancer and zero on the Arctic Circle. The sun rises in the south east direction and sets in south west direction.

  • Equinoxes: On March 20 or 21 and September 22 or 23 neither of the two hemispheres is tilted towards or away from the sun and the sun's vertical noon rays strike the earth at the equator. The duration of day and night is exactly the same, being 12 hours, at all the parallels. This explains the word equinox, from Latin acquus, equal and nox, night. This is the reason that both the equinoxes are treated jointly. However, they are named differently. The Equinox of March 21 or 22 is called the vernal or spring Equinox and that of September 22 or 23 is known as autumnal equinox.

  • Variation in the length of day: Length of day changes with season. Although this change is negligibly small near the equator it is considerably high near the poles. During summer solstice circle of illumination divides equator into two equal halves as a result, at equator there is equal duration of day and night ,i.e., of 24 hours. However as sun’s rays are vertical at tropic of cancer, northern hemisphere gets more solar insolation then southern hemisphere as a result larger part of northern hemisphere gets more solar light during day. Circle of illumination cuts Arctic Circle in such a way that sun never sets on northern pole where as at the same time sun never rises above horizon at southern pole. Similarly during winter solstice reverse happens.
  • Watch this video for more info.


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