The Earth – Facts at a Glance

• Earth is the only known planet with an existence of life.
• It is a blue planet because ⅔ of its surface is covered with water.
• It is the third planet from the sun and the densest planet in the solar system.
• It is the largest of the four terrestrial planets and the fifth-largest planet overall.
• Earth has a slightly flattened shape, called a geoid, due to its poles being slightly flattened.
• It revolves around the sun at an average distance of 149 million kilometers, or 1 astronomical unit.
• Scientists estimate that Earth is around 4.5 billion years old.
• Earth takes 365.25 days, or one Earth year, to complete one revolution around the sun.
• Only 3% of Earth’s water is fresh and 97% is salty.
• 71% of Earth’s surface is covered by water and 29% is covered by land.
• Earth’s atmosphere is divided into 6 layers: the troposphere, stratosphere, mesosphere, thermosphere, exosphere, and ionosphere.
• Earth has one satellite, the Moon, and also has temporal artificial satellite

Size and Shape of the Earth

• Earth is not a perfect circle but an oblate spheroid.
• Its shape is called “geoid,” meaning “an Earth-like shape”.
• It is bulged at the equator and flattened at the poles due to centrifugal force.
• The Earth’s equator moves faster than the poles due to the Earth’s rotation. This resultant centrifugal force causes the matter to be pulled towards the equator and bulges the Earth outwards.
• Geodesy is the science that studies the shape and size of the Earth.
• Earth’s circumference and diameter differ because of its oblate spheroid shape.
• The poles of Earth are slightly flattened, causing a bulge at the equator and larger circumference and diameter there.
• The equatorial bulge is measured at 26.5 miles (42.72 km) and is caused by the Earth’s rotation and gravity.
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Motions of the Earth

• Motion is the process of moving or changing of position.
• The Earth is constantly in motion, revolving around the Sun and rotating on its axis. These motions account for phenomena such as night and day, changing seasons, and different climates.
• The Earth has two movements: rotation and revolution
• Rotation:
  • It is a circular movement around a center or point of rotation.
  • The Earth rotates on its axis from west to east (counter-clockwise) in 23 hours, 56 minutes, and 4.09 seconds for a sidereal day, and 24 hours for a mean solar day.
  • The Earth’s axis has a 23.45° tilt.
  • The Earth’s rotation speed is 1,674.4 km/h or 1,040.4 miles per hour at the equator.
• Revolution:
  • The Earth revolves around the Sun in a counter-clockwise direction, completing one full revolution in one year.
  • Its orbit around the Sun is slightly oval, with a mean distance of 93 million miles and varying by 3 million miles.
  • The Earth’s revolution around the Sun covers 595 million miles in 365 days, 6 hours, 9 minutes, and 9.5 seconds, at a speed of 18 miles per second or 66,000 miles per hour.
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Leap year

• It takes 365.25 days for Earth to orbit the Sun, known as a solar year.
• In a regular year, there are 365 days in a calendar from January to December.
• Therefore, approximately every 4 years, February has 29 days instead of 28, resulting in a leap year with 366 days.

Solstice and Equinoxs

• The Earth’s orbit around the sun is elliptical, not circular, so the distance between the Earth and sun changes.
• When this distance is minimum, the Earth is said to be in perihelion (around January 3) and when it is the maximum, it is said to be in aphelion (around July 4).
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• The Earth’s axis always points to the same star in the sky, but the latitude where the sun’s rays fall vertically changes as the Earth moves in its orbit. Due to this, the earth attains four critical positions with reference to the sun.
• Solstice:
  • It refers to a day with either the longest day or the shortest.
  • There are two solstices in a year:
    • Winter solstice on December 22
    • Summer solstice on June 21
• Equinoxes:
  • It refers to a day with an equal duration of day and night.
  • There are two equinoxes in a year:
    • Spring equinox on March 20
    • Autumnal equinox on September 22

Equinox	Solstice
sun is nearest to the equatorial plane	sun is farthest from the equatorial plane
equal lengths of day and night	long nights and days
occurs at the start of the spring and fall	occurs during the summer and the winter
occurs on March 20 (spring equinox) and on September 22 (autumnal equinox)	occurs on June 21(Summer Solstice) and on Dec 22 (Winter Solstice)

Latitudes and Longitudes

Latitudes and Longitudes are imaginary lines used to determine the location of a place on earth. 

Latitudes

• The parallels of latitudes run from the Equator to the North Pole (90 degrees North) and South Pole (90 degrees South).
• There are 89 latitude lines in each hemisphere, for a total of 179 lines, with an interval of 1 degree.
• They are east-west circles that connect all points on Earth.
• The distance between two parallel latitudes is 111km.
• All latitudes are parallel to the Equator.
• Major Latitudes:
  • Equator:
    • Equator is the imaginary line that divides the earth into two hemispheres.
    • The northern hemisphere and the Southern hemisphere.
    • It is the longest line of latitude.
    • The Equator covers 40075km out of which 78.8% covers the water area while 21.3% covers the surface area.
  • Arctic Circle:
    • The Arctic Circle is the northernmost circle of five parallel latitudes.
    • It is located 66 and 1/2 degrees north of the Equator.
    • The Arctic Circle is not always in the same position, it is drifting towards the North at a rate of 14.5 m (48 ft) per year.
    • It is 16000 km long and covers 4% of the Earth’s surface.
  • Antarctic Circle:
    • Antarctic Circle is the Southernmost circle which is currently at 66 and a 1/2 degrees south of the Equator.
    • Like the Arctic Circle, the Antarctic Circle is also shifting southward about 14.5 m (48 ft)/year.
    • The Antarctic Circle is also 16000km long which covers 4% of the Earth’s surface towards the south.
  • Tropic of Cancer:
    • It is also known as Northern Tropic and the Sun is directly overhead at this position in June.
    • The Tropic of Cancer is located at 23 and a 1/2 degrees in the Northern Hemisphere.
    • The position of the tropic of cancer is fluctuating because of the longitudinal alignment. But the distance between the tropic of cancer and the Antarctic Circle remains the same as they move at the same speed.
    • The length of the Tropic of Cancer is 36,788 km.
  • Tropic of Capricon:
    • It is the Southern Tropic which is currently at 23 and a 1/2 degree south.
    • The length of the Tropic of Capricorn is the same as the Tropic of Cancer.
    • The Southern Tropic covers 3% of the total world’s population.
    • The sun is overhead once a year in December and therefore experiences the maximum heat.
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Geographical zones of the Earth

• The temperature zones are also identified based on the latitudes drawn from the Equator to the Poles. The difference between the above five parallels of Latitudes relates to the changes in Climate.
• Frigid zones:
  • There are two frigid zones: North Frigid Zone and South Frigid Zone.
  • North Frigid Zone is between North Pole (90-degree) and the Arctic Circle.
  • South Frigid Zone is between South Pole (90-degree) and the Antarctic Circle.
  • These zones are the coldest regions on Earth.
  • Summers in these zones last for 2-3 months with 24 hours of sunlight.
  • Sun rays are always slanting in these zones causing the region to stay cool.
• Temperate zones:
  • Two temperate zones exist: North Zone and South Zone.
  • North Zone lies between the Arctic Circle and the Tropic of Cancer.
  • South Zone lies between the Antarctic Circle and the Tropic of Capricorn.
  • The temperate zones experience mild weather because the sun rays never fall directly.
  • These zones experience all four seasons: summer, spring, autumn, and winter.
• Torrid zones:
  • The Torrid Zone lies between Tropic of Cancer and Tropic of Capricorn and is called Tropics.
  • In this zone the sun passes directly overhead seasonally.
  • Hence it experiences the maximum heat once in a year.

Longitudes

• Longitude is the east-west measurement of the prime meridian.
• There are 180 vertical east longitudes of the Prime Meridian and 180 vertical west longitudes of the Prime Meridian.
• Prime Meridian:
  • It is the line that passes through the Greenwich British Royal Observatory and is considered 0 degrees.
  • It serves as the base for the east and west directions, with 180 degrees east and 180 degrees west. Hence, it is the base of world time.
  • It divides the Earth into two halves: the Eastern and Western hemispheres.
• Eastern Hemisphere:
  • It includes Africa, Asia, Europe, Australia, and islands of Oceania.
  • The landmass of Eastern Hemisphere is larger than Western part, hence it accounts for 80% of the human population.
  • It is also referred to as the Oriental Hemisphere
• Western Hemisphere:
  • It covers North and South America and some parts of Africa, Europe, Antarctica, and also Asia.
  • The center of the Western Hemisphere is at the Pacific Ocean whose nearest land is Genovesa Island.
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Longitude and Time

• Earth completes one rotation in 24 hours.
• It means, the sun takes 12 hours to cross Eastern and Western hemispheres.
• Sun crosses 15 degrees of Longitude per hour in every 4 minutes.
• To maintain uniformity in the time range in all the territorial limits of the country, the central meridian is regarded as the Standard Meridian whose local time is considered as the standard time for the whole country.
• There are 24 total time zones in the world.
• Time is measured on Earth due to movement of Earth, moon and planets.
• International Date Line (IDL) passes through 180-degree longitude. Hence, difference between 0-degree longitude and IDL is 12 hours.
• Calculation of time:
  • The earth rotates 1-degree in 4 minutes.
  • When it is noon at Greenwich, time at 15-degree east of Greenwich will be (15*4=60 minutes) which is 1 hour ahead of Greenwich Time.
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Standard Time and Time Zones

• If each town were to keep its own meridian time, there would be much difference in local time between one town and the other. This would become very impractical and inconvenient. 
• To avoid difficulties, a system of standard time is observed by all countries.
• Most countries adopt the central meridian of their country as the standard time.
• In India, the longitude of 82° 30′ E is treated as the standard meridian and is known as Indian Standard Time (IST). It is 5 hours 30 mins, ahead of Greenwich Mean Time. (+5:30 GMT)
• Earth is divided into 24 time zones, each covering 15° of longitude.
• Countries with a large east-west stretch may adopt several time zones for practical purposes.
• For example, Canada and USA have 5 time zones (Atlantic, Eastern, Central, Mountain, and Pacific Time Zones). Russia has 9 time zones.

International Date Line

• A traveler going east gains time from Greenwich and reaches 12 hours ahead of GMT at the meridian 180°E.
• Similarly, a traveler going west loses 12 hours when he reaches 180°W.
• There is a total difference of 24 hours or a day between the two sides of the 180° meridian.
• This line is called the International Date Line where the date changes by 1 day when crossed.
• The International Date Line curves from the normal 180° meridian in the mid-Pacific to prevent confusion in some island groups.
• Some island groups follow Asiatic or New Zealand standard time, others follow American date and time.
• The line appears zig-zag because it is drawn over waterbodies and not on land, since the time difference on either side is 24 hours.