Earth, Solar System and Universe

IAS Notes October 08, 2022

 

 Unit - 1 (Earth, Solar system and Universe)


What is Geography? 
  • (Geo - Earth & Graphy - Description) i.e., Geography means "Description of the Earth". The term Geography was coined by a Greek philosopher Erastothenes who is known as the 'Father of Geography'.
  • It also includes the study of places and relationship between people and their environment.
Shape of the Earth
Geocentric System - As per this system, Earth is considered to be the centre of the universe.
Heliocentric System - As per this system, Sun is considered to be the centre of the universe.

Newton proposed that the Earth is not a perfect sphere but instead it is flatten at the poles and bulged at the equator. He explained that the bulging and flattening of the Earth are due to the continuous spinning of the Earth. This shape of the Earth is called Geoid or Oblate Spheroid. 
Evidence for Earth's spherical shape -
  1. Circular navigation by Megallan in 1519.
  2. Circular horizon and change in shape of circular horizon with the change in altitude - Horizon appears circular when viewed from a ship (in open ocean) and horizon widens with increase in altitude.
    Geography, Geomorphology
    Circular horizon at different altitude
  3. Ships visibility - Ships in open ocean become visible gradually from top to bottom.
    Geography

  4. Bedford level experiment - It proves that the Earth is spherical in shape not flat by observing the posts of equal lengths placed at equal distances and when it is viewed the middle one appears to be taller.
    Geography

  5. Sunrise and sunset at different time at different locations on the Earth.
  6. Lunar eclipse during which the circular shadow of the Earth is visible.
  7. The spherical shape of other planetary bodies.
Latitudes and Longitudes
  • Latitudes - It is the angular distance of a point on the Earth's surface measured in degree from the centre of the Earth towards north and south of the equator.
  • The parallels of latitudes are the imaginary lines on the Earth connecting the places with same latitude.
  • They are always parallel to the equator and also parallel to one another.
  • The length of the latitude decreases from equator to the poles.
  • The distance between two latitudes always remains same (111 KM).
    Latitude

  • Total no. of parallels of  latitude - 89 in Northern hemisphere and 89 in Southern hemisphere and 1 Equator, i.e., 179 latitudes (North poles and south poles are point).
  • Important latitudes - 
    Latitudes

  • Longitude - It is the angular distance of a place east or west of Prime Meridian which is considered as zero degree longitude.
  • The meridians of longitude are semicircles running from pole to pole with same latitude.
  • The meridians are not parallel to each other.
  • The distance between two meridians decreases as we move from equator to the poles.
  • The maximum distance between two meridians is 111 KM at Equator where as minimum distance between two meridians is 0 at Poles.
  • The meridian of Longitude which passes through the Greenwich observatory near London has been adopted as Prime Meridian. Therefore, it is also known as Greenwich Meridian.
    Longitude

  • Total no. of meridians of longitude - 360

  • Great circles - A circle with largest possible circumference that can be drawn on a globe is called as great circle. The circumference of great circle is always equal to the circumference of the Earth.
  • Great circles cut the globe/Earth into two equal halves.
  • No. of great circles that can be drawn on the Earth - Infinite
  • Significance -Great circles are used in navigation to find the shortest distance between any two locations.

Rotation & Revolution
  • Rotation - The movement of the Earth on its own axis is called rotation.
  • The axis of rotation is the imaginary line passing through poles and the centre of the Earth around which the Earth rotates.
  • The direction of rotation of the Earth is counter clockwise (or from West to East) when viewed from the North pole.
  • Rotation causes day and night.

  • Orbital Plane - It is the plane in which the Earth orbits around the Sun.
    Rotation
  • Angle of tilt of rotation - 

    23½ degrees

  • Angle between Earth's axis of rotation and orbital plane -

    66½ degrees


  • Period of Rotation -
  1. Solar Day - Time taken by the Earth to rotate on its axis so that sun appears in the same position in the sky. It is equal to 24 hours.
  2. Sidereal Day - Time taken for the Earth to rotate on its axis so that a distant star appears in the same position in the sky, i.e., amounting to a complete 360 degree rotation. It is equal to 23 hours 56 minutes.
  • Linear speed of rotation - It is maximum along the equator and reduces from equator towards the pole. This is the reason why satellites are launched closer to the equator so that the initial thrust due to the Earth's rotation is maximum. Eg. - Kourou (French Guiana)
  • Circle of illumination - It is an imaginary line that separates day and night.
  • Revolution - The movement of the Earth around the Sun is called revolution of the Earth. The Earth revolves around the Sun in an eliptical path called as orbit.
    Revolution
  • The period of revolution of the Earth is 365 days 5 hours 48 minutes. This extra 5 hours and 48 minutes are accommodated in leap year.
  • The speed of revolution is not uniform which increases as Earth comes closer to the Sun.
  • Aphelion - The point in the orbit of an object/Earth where it is farthest from the Sun.
  • Perihelion - The point in the orbit of the Earth where it is nearest to the Sun.

Seasons
  • Factors affecting seasons -
  1. Rotation
  2. Tilt in the axis
  3. Revolution
Case - I (When there is no Rotation, no Revolution and no tilt in the axis, i.e., the Earth is still)
Seasons
  • One part of the Earth is getting continuous sunlight and other part remains dark.
  • Part getting continuous sunlight becomes too hot and the other part becomes too cold.
Case - II (When the Earth is only rotating, no revolution and no tilt)
Rotation
  • Equal hours of day and night (12 hours each) experienced at all places.
  • Sunlight will fall perpendicularly on the Equator and thus absorbs maximum amount of heat where as the poles receive slanting (approximately at 0 degree) and thus absorbs minimum mount of heat.
Case - III (When the Earth is rotating on its axis with tilt, no revolution)
Rotation
  • At the Equator- Equal day and night 
  • In the Northern Hemisphere - The length of the day increases as we move towards pole and beyond a certain point (i.e., beyond 66½ degree N), it is always day and thus experiences more heat (Summer).
  • In the Southern Hemisphere - The length of the day decreases as we move towards the pole and beyond a certain point (i.e., beyond 66½ degree S), it is always night and thus experiences cold (Winter).
Case - IV (Rotation with tilt in axis and revolution)

Revolution
  • Seasons are observed on the Earth because of the Earth's revolution around the Sun in a tilted axis of rotation and the angle of inclination is always constant.
  • This brings the variation in the length of day and night across different latitudes which results in seasons.
  • The region receiving more sunlight for longer duration with the angle of incidence of sunlight being more vertical experiences higher temperature which we call it as Summer.
  • The region receiving lesser sunlight for lesser duration with more slanting angle of incidence of sunlight would experiences lesser temperature which we call it Winter.
Summer Solstice (21 June) - The Sun rays are falling exactly perpendicular at Tropic of Cancer. The length of the day increases from the Equator towards the North pole and vice-versa.

Winter Solstice (22 December) - The Sun rays are falling exactly perpendicular on the Tropic of Capricorn. The length of the day increases from the Equator towards the South pole and vice-versa.

Equinox - The Sun rays are falling vertically along the Equator. All the latitudes experiences equal duration of day and night.
  • Spring Equinox - 21 March (Vernal Equinox)
  • Autumn Equinox - 23 September
Solstice and Equinox


 Position of overhead Sun -
  • At Equator - 2 times (21st March & 23rd September)
  • At Tropic of Cancer - 21st June
  • At Tropic of Capricorn - 22nd December
Beyond Tropic of Cancer and Tropic of Capricorn, there is no overhead Sun.

Arctic Circle & Antarctic Circes - Maximum extent up to which 24 hours of sunlight would reach.

Polar Day - Poles to Arctic and Antarctic Circle where the Sun doesn't set even during midnight is called Polar Day.

The position of 24 hours of day or 24 hours of night would be experienced from poles to Arctic and Antarctic Circle.

And the poles would experience 6 months of day and 6 months of night.

Note - At equator, day time and night time is always equal, i.e., 12 hours each.


PYQ
1. On 21st June, the -
a) doesn't set below the horizon at Arctic Circle
b) doesn't set below the horizon at Antarctic Circle
c) shines vertically overhead at noon on equator
d) shines vertically overhead at Tropic of Cancer

Ans
a) correct answer
b) wrong because on 21st June, it would experience 24 hours of night
c) wrong because it happens on Equinox
d) wrong because on 21st June, Sun is vertically overhead at Tropic of Cancer not at Tropic of Capricorn

2. Variation of length of day time and night time from seasons to seasons are due to -
a) Earth's rotation on its axis
b) The Earth's rotation around the sun in an elliptical manner
c) Latitudinal position of a place
d) Revolution of the earth on tilted axis

Ans - d


Time Zones and International Date Line

Time Zones - They are the regions of globe which observes uniform standard time. Entire globe is divided into 24 time zones with Greenwich Meridian as standard reference.

From Greenwich Meridian, the time increases towards the east and decreases towards the west.

1 degree of longitude is equivalent to a time difference of 4 minutes.

India uses 82.5 degree East meridian as standard reference for its time zone which is equivalent to 5 hours 30 minutes ahead from Greenwich Meridian or GMT.

France has maximum number of time zones (12) followed by Russia (11) and USA(11)


International date line (IDL) - It is an imaginary line of demarcation on the Earth running from North to South pole and demarcates the change of calendar day.

International date line


It was agreed in 1884 that 180 degree E/W longitude would represent IDL.

It is not a straight line, it follows a zig-zag pattern to accommodate the islands of Pacific Ocean. The IDL bends and goes zig-zag at Bering Strait between Siberia and Alaska, Fiji island, Tonga island, etc.

If a person crosses IDL from east to west, he loses a day and he has to add a date in his calendar whereas if a person crosses IDL from west to east, he gains a day and has to subtract a date from his calendar.

Daylight saving time (DST) - It is the practice of shifting time by 1 hours ahead by forwarding the time in clock during the beginning of the Summer and bringing it backward at the end of the Summer and the 1 hours back during the beginning of the Winter and bringing it back at the end of the Winter.

This practice will help in aligning the day to day activities along with the daylight and save energy consumption.

DST is not beneficial for the countries of tropical regions as the variation in length of day and night is not that much as that in temperate region.

Leap Second - Earlier GMT was used as reference time, but now UTC (Universal coordinated time) is used, which is based on International atomic time (TAI) and Astronomical time (UT1).

TAI is based on the vibrations of atoms or molecular system and it is very accurate. UT1 is based on the Earth's rotation.

Once in a while, Earth's rotation slows down; so to accommodate this slowing down the atomic clock is slowed down for 1 second so that the Earth catches up. To slow down the atomic clock we need to subtract 1 second to it. That extra second is called Leap Second.

Next Article - Basics of Cosmology

Notes on other subjects


Optional Notes
Note - This is my Vision IAS Notes (Vision IAS Class Notes) and Ashutosh Pandey Sir's Public Administration Class notes. I've also added some of the information on my own. 

Hope! It will help you to achieve your dream of getting selected in Civil Services Examination 👍
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