Solstice and equinox
The earth has two main motions :
1. Rotation 2. Revolution
The Axis of the earth , which is an imaginary line makes an angle of 66 1/2 ° with its orbital plane.The plane which is 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 lights from the sun at a time. The portion facing the sun experience day while other half away from the Sun experiences night. The Circle of Illumination is a circle that divides the day from night on the globe. The circle does not coincide with the axis .
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The earth rotates around its axis . The axis is an imaginary line passing through the centre of the earth. The earth completes one rotation in 23 hours , 56 mins , 4.09 seconds to be exact .The earth rotates from west to east . This period of rotation is called as the Earth day.
Rotation of earth
There are several effects of the rotation of the earth.
- Days and nights are caused by the rotation of earth.
- A difference of 1 hour between two meridians which are 15 degree apart.
- Deflection of the oceans currents and winds.
- Rise and fall of tides everyday.
It is the motion of Earth around the Sun in its elliptical orbit. In 365 1/4 days, one revolution is completed, resulting in one extra day every fourth year. The 366-day year is called a leap year with 29 days in February month. Summer, winter, spring and autumn seasons are usually divided into one year. Seasons change as the earth’s position around the sun changes.Seasons changes due to the change in position of the earth around the sun .
Revolution of earth
There are various effects of the revolution of the earth :
- Variation in the lengths of day and night at different times of the year.
The Northern Hemisphere is tilted towards the Sun on June 21. The Sun’s rays fall directly on tropic of Cancer. Consequently, more heat is given to these areas. The areas close to the poles get less heat as the sun’s rays slant. The northern pole is inclined towards the sun and the locations beyond the Arctic Circle experience continuous daylight for about six months. Since a large portion of the Northern Hemisphere gets light from the sun, it is summer in the northern regions of the equator. The longest day and the shortest night is on 21 June at these regions. All of these conditions are being reversed in the Southern Hemisphere at this time. There’s winter season. The nights are longer than the days. This position of earth is called Summer Solstice.
The Tropic of Capricorn receives direct rays from the Sun on December 22
as the South Pole tilts toward it. As the rays of the Sun fall vertically on the Tropic of Capricorn (23 1/2 ° S), a larger portion of the Southern Hemisphere becomes lighter. Therefore, it’s summer with longer days and shorter nights in the Southern Hemisphere. In the Northern Hemisphere, the reverse happens. This Earth’s position is called the Winter Solstice
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Direct sun rays fall on the equator on 21 March and 23 September. None of the poles are tilted toward the sun at this position, so the entire earth experiences equal days and nights. This is known as equinox.
On September 23, it’s autumn season in the Northern Hemisphere and spring season in the Southern Hemisphere. The opposite is the case on March 21, when it’s spring in the Northern Hemisphere and autumn in the Southern Hemisphere. Thus changing in days and nights are due to rotation of earth and changes in seasons are due to the revolution of the Earth respectively.
Theories of Origin of Earth
- Buffon – Hypothesis :This Hypothesis is Based on Sun – Comet collision
- Kant – Gaseous Mass Theory : Based on Newton ‘s law of gravitation
- Chamberlain – Moulton : planetesimal Hypothesis
- Jeans and Jefferry : Tidal Hypothesis based on sun giant star attractions
- Alfven : Electromagnetic Hypothesis
- Russel and Littleton : Binary star Hypothesis
- Ross – Gun Fission Hypothesis : Rotational and tidal Hypothesis
- F. Hoyle : Supernova Hypothesis
- Big Bang Theory : latest idea
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