- Earth, the third planet from the sun, is the fifth largest planet in the solar system; only the gas giant Jupiter, Saturn, Uranus, and Neptune are bigger. Earth is the largest of the terrestrial planets of the inner solar system, bigger than Mercury, Venus, and Mars.
- The earth is not a sphere. It is slightly flattened at the North and South poles and bulged in the). middle (The planet's rotation causes it to bulge at the equator). The radius of Earth at the equator is 3,963 miles (6,378 km) and its polar radius is 3,950 miles (6,356 km) — a difference of 13 miles (22 km).
- It is difficult to identify the location of a point on earth. We need a certain point of reference and lines to find out the locations of places on earth. Latitudes and Longitudes are used for this purpose.
Latitudes and Longitudes
- Latitudes are imaginary lines around the globe running in an east-west direction.
- The Equator is an imaginary line running on the globe that divides it into two equal parts. The northern half of the earth is known as the Northern Hemisphere and the Southern half is known as the Southern Hemisphere. They are both equal halves.
- The Equator represents Zero degree latitude. All the parallel lines from the equator to the poles are called parallels of latitudes.
- The parallel of latitude is an imaginary line around the Earth that is parallel to the equator.
- Latitudes are measured in degrees. The distance between each degree of latitude is about 69 miles (110 kilometers).
- The 90 degrees north latitude from the equator marks the North Pole and 90 degrees south latitude marks the South Pole.
- All parallels north of the equator are called ‘North latitudes’ and indicated by the letter ‘N’, and all parallels south of the equator are called ’South latitudes’ and indicated by ‘S’.
- Think about having imaginary horizontal "hula hoop" around the earth, with the biggest hoop around the equator, and then progressively smaller ones stacked above and below it to reach the North and South Poles.
Try yourself:In which directions the earth spins around its imaginary axis?
- Earth's axis is an imaginary line going through the centre of Earth from the North Pole to the South Pole.
- Earth spins around this line from west to east.
Important parallels of latitudes
Besides the equator (0 degrees), the North pole (90 degrees N), and the South pole (90 degrees S).
There are four important parallels of latitudes:
(i) Tropic of Cancer (23½° N) in the Northern Hemisphere
(ii) Tropic of Capricorn (23½° S) in the Southern Hemisphere
(iii) Arctic Circle at 66½° north of the equator
(iv) Antarctic Circle at 66½° south of the equator
Heat zones of the earth
- Torrid Zone: 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. It, therefore, receives the maximum heat. This zone is called the Torrid Zone.
- Temperate Zones: 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. They have moderate temperatures. This zone is known as the Temperate Zone.
- Frigid 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. This region is known as Frigid Zones.
Try yourself:The area lying between which of the following latitudes/poles is very cold?
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.
- Longitudes are imaginary lines running on the globe in a north- south direction.
- These imaginary lines running in a north-south directions are also called ‘meridians’.
- Longitudes are measured in ’degrees’. Each degree is further divided into minutes and minutes into seconds.
- They are semi-circles and the distance between them decreases steadily polewards until it becomes zero at the poles, where all the meridians meet.
- Unlike latitudes, all meridians are of equal length.
- The meridian which passes through Greenwich, where the British Royal Observatory is located is considered as the Prime Meridian. It is the 0° longitude and from it, we count 180° eastward as well as 180° westward. The Prime Meridian and 180° meridian divide the earth into two halves, the Eastern Hemisphere and the Western Hemisphere
- 180 degrees East and 180 degrees West meridians are on the same line.
Longitude and Time
- The best means of measuring time is by the movement of the earth, the moon, and the planets. The sun regularly rises and sets everyday.
- When the Prime Meridian of Greenwich 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.
- It can be calculated this way- The earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes. Thus, when it is noon at Greenwich, the time at 15° east of Greenwich will be 15 × 4 = 60 minutes, i.e., 1 hour ahead of Greenwich Time, But at 15° west of Greenwich, the time will be behind Greenwich Time by one hour.
➢ Why do we have Standard Time?
- The local time of places which are on different meridians is bound to differ.
- In India, the longitude of 82½° E (82° 30’E) is treated as the standard meridian. The local time at this meridian is taken as the standard time for the whole country. It is known as the Indian Standard Time (IST).
Indian Standard meridian
- India is located east of Greenwich at 82° 30’E. India’s time is 5 hours 30 minutes ahead of GMT (Greenwich Mean Time). So if it is 2:00 pm noon in London, it will be 7:30 pm in India.
Try yourself:Which of the following divides Earth into the Eastern Hemisphere and the Western Hemisphere?
The prime meridian divides Earth into the Eastern Hemisphere and the Western Hemisphere. The prime meridian is at 0° (0 degrees) longitude. The longitude of a place is followed by the letter E for the east and W for the west.