Motion of the Earth Geography notes for UPSC Examination

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Geography notes UPSC, Geography optional Subject notes UPSC, Geography notes IAS, Geography optional Subject notes IAS, what is earth, motion of the earth, Solar system, planets in solar system

Motions of the Earth

The earth has two main motions: (i) Rotation and (ii) 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 as you see in the given figure.

Rotation:

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 minutes, 4.09 seconds to be exact. The earth rotates from west to east. The period of rotation is known as the earthday.

Geography notes UPSC, Geography optional Subject notes UPSC, Geography notes IAS, Geography optional Subject notes IAS, what is earth, motion of the earth, Solar system, planets in solar system

 Effects of the Rotation of the Earth

  • Causation of day and night
  • A difference of 1 hour between two meridians which are 15°apart.
  • Deflection of ocean currents and winds.
  • Rise and fall of tides every day

Revolution:

It is earth’s motion in its elliptical orbit around the sun. One revolution is completed in 365 1/4 days, resulting in one extra day every fourth year. The year, consisting of 366 days is called a “leap year” having 29 days in the month of February.

Geography notes UPSC, Geography optional Subject notes UPSC, Geography notes IAS, Geography optional Subject notes IAS, what is earth, motion of the earth, Solar system, planets in solar system

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.

On 21st June, the Northern Hemisphere is tilted towards the sun. The rays of the sun fall directly on the Tropic of Cancer. As a result, these areas receive more heat. The areas near the poles receive less heat as the rays of the sun are slanting. The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months. Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21st June. At this time in the Southern Hemisphere all these conditions are reversed. It is winter season there. The nights are longer than the days. This position of the earth is called the Summer Solstice.

On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23½° S), a larger portion of the Southern Hemisphere gets light. Therefore, it is summer in the Southern Hemisphere with longer days and shorter nights. The reverse happens in the Northern Hemisphere. This position of the earth is called the Winter Solstice.

On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.

On 23rd September, it is autumn season in the Northern Hemisphere and spring season in the Southern Hemisphere. The opposite is the case on 21st March, when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere. Thus, we find that there are days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.

Some terminologies related to revolution are:

  • Perihelion: The position of the earth when it is at its nearest point to the sun. The earth reaches its perihelion on about 3rd January at a distance of about 147 million km from the sun.
  • Aphelion: The position of the earth when it is at its greatest distance from the sun. The earth reaches its aphelion on 4th July when it is at a distance of 152 million km from the sun.
  • Perigee: The point in the orbit of the moon when it is nearest to the earth.
  • Apogee: The point in the orbit of the moon when it is farthest from the earth.

Effects of the Revolution of the Earth

  • Change of seasons.
  • Variation in the lengths of day and night at different times of the year.
  • Shifting of wind belts.
  • Determination of latitudes.

Lattitude and Longitude

Latitude: Latitude of a place on the earth is the angular distance of the place from the equator. 1° of latitude is approximately equal to 111 km.

Parallels of Latitude: They are circles drawn on the globe parallel to the equator. All the places on a parallel of latitude will have the same latitudinal angle.

Important Parallels of Latitude

  1. Equator 0°
  2. Tropic of Cancer 23 ½°N
  3. Tropic of Capricorn 23½°S
  4. Arctic circle 66½°N
  5. Antarctic circle 66½°S



Heat Zones of the Earth

The mid-day sun is exactly overhead at least once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. This area, therefore, receives the maximum heat and is called the Torrid Zone.

The mid-day sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn. The angle of the sun’s rays goes on decreasing towards the poles. As such, the areas bounded by the Tropic of Cancer and the Arctic Circle in the Northern Hemisphere, and the Tropic of Capricorn and the Antarctic Circle in the Southern Hemisphere, have moderate temperatures. These are, therefore, called Temperate Zones.

Areas lying between the Arctic Circle and the North Pole in the Northern Hemisphere and the Antarctic Circle and the South Pole in the Southern Hemisphere, are very cold. It is because here the sun does not rise much above the horizon. Therefore, its rays are always slanting. These are, therefore, called Frigid Zones.

Great Circles: Any circle which divides a globe into hemispheres is a great circle. The equator is a great circle and Greenwich meridian together with meridian 180° make another great circle. The number of great circle is limitless. Great circle can extend in any direction: east to west, north to south, north east to south west, and so on. Great circles are of equal length.

Longitude: The longitude shows the distance of a point east or west of the Prime Meridian which is at 0° and passes through Greenwich, near London. For each degree of longitude there is a difference of four minutes in time.

Longitude and Time: The best means of measuring time is by the movement of the earth and the moon. The sun regularly rises and sets every day, and naturally, it is the best time-keeper throughout the world.

When the Prime Meridian has the sun at the highest point in the sky, all the places along this meridian will have mid-day or noon. As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich time and those to the west will be behind it. The rate of difference can be calculated as follows. The earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes. Thus, when it is 12 noon at Greenwich, the time at 15° east of Greenwich will be 15 × 4 = 60 minutes, i.e., 1 hour ahead of Greenwich time, which means 1 p.m. But at 15° west of Greenwich, the time will be behind Greenwich time by one hour, i.e., it will be 11.00 a.m. Similarly, at 180°, it will be midnight when it is 12 noon at Greenwich.

Greenwich Mean Time: The local time at Greenwich or any place on the Prime Meridian. All meridians to the east of Greenwich meridian have sunrise before that meridian. Local times along these meridians are therefore ahead of G.M.T. Meridians to the west of Greenwich meridian have sunrise after this meridian and therefore their local times are behind G.M.T.

Standard Time: A particular meridian of longitude passing through a country is chosen as the reference meridian. The local time along this meridian, calculated with respect to Greenwich Mean Time in terms of its longitude is taken as the Standard Time for that country.

Geography notes UPSC, Geography optional Subject notes UPSC, Geography notes IAS, Geography optional Subject notes IAS, what is earth, motion of the earth, Solar system, planets in solar system

Why do we have standard time?

The local times of places which are on different meridians are bound to differ. For example, it will be difficult to prepare a time-table for trains which cross several longitudes. In India, for instance, there will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam. It is, therefore, necessary to adopt the local time of some central meridian of a country as the standard time for the country.

Indian Standard Time: Time along 82 ½° E meridians, calculated with respect to G.M.T. India, for being a large country, is unusual in having a single time zone all over the country. It is 5½ hours ahead of G.M.T.

International Date Line: An imaginary zigzag line on the globe, approximately along the 180° meridian of longitude. When a person crosses this line from East to West, he gains one day and when he crosses from West to East, he loses one day.



Solar Day: It is the time interval between successive crossings of the sun across the meridian of the celestial sphere of any fixed place in the same direction. This is equal to 24 hours.

Sidereal Day: The period of rotation of the earth about its axis. This is calculated with respect to any fixed star. It is 4 minutes less than 24 hours.

Solar Year (Tropical year): It is the average interval between successive returns of the sun in its apparent motion along the ecliptic to a fixed position on the celestial sphere of any fixed place. This is equal to 365.24 mean solar days.

Sidereal Year: The period of revolution of the earth around the sun. It is calculated with reference to any fixed star. It is approximately equal to 365.26 days.

To account for 1/4 of a day in a year, the leap year system is adopted in the Gregorian calendar. To account for the excess of 11 minutes in a year, the centurial year is considered a leap year only when it is divisible by 400.

Earth in Figures

  1. Age 4,550 million years
  2. Mass 5.976 × 1024 kg.
  3. Mean density 5.518 kg/litres.
  4. Total Surface Area 510,000,000 km2.
  5. Land Area 29.2% of the total surface area.
  6. Water Area 70.8% of the total surface area.
  7. Highest point (Mt. Everest) 8,848 m
  8. Lowest point (Dead Sea) 397 m.
  9. Greatest Ocean Depth 11,033 m (Mariana Trench)
  10. Mean Equatorial Diameter 12,756 km.
  11. Equatorial circumference 40,076 km.

Theories of Origin of Earth

  • Buffon-Hypothesis: Based on sun-comet collision.
  • Kant-Gaseous Mass Theory: Based on Newton’s law of gravitation.
  • Chamberlain-Moulton: Planetesimal Hypothesis.
  • Jeans & Jeffery: Tidal Hypothesis: Based on sun-giant star attraction.
  • Alfven: Electromagnetic Hypothesis.
  • Russell and Littleton: Binary Star Hypothesis.
  • Ross-Gun-Fission Hypothesis: Rotational and Tidal hypothesis.
  • Hoyle: Super Nova Hypothesis.
  • Big Bang Theory: Latest idea.

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