Pariksha SaathiLesson 2 of 10 · Physical and World Geography
The Earth's Motions, Seasons, Latitudes, Longitudes and Time Zones
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Shape and size
The Earth is an oblate spheroid, flattened at the poles and bulging at the Equator, a shape also called a geoid. Its equatorial diameter is about 12,756 km, its polar diameter about 12,714 km and its circumference at the Equator about 40,075 km.
Rotation
The Earth spins from west to east, anticlockwise when seen from above the North Pole. One rotation measured against the stars takes 23 hours 56 minutes 4 seconds (the sidereal day); measured against the Sun it gives the 24-hour solar day. Rotation causes day and night, makes the Sun seem to rise in the east, and produces the Coriolis force that deflects winds and ocean currents. Its speed is about 1,670 km per hour at the Equator and zero at the poles.
Revolution and the tilted axis
The Earth orbits the Sun in about 365 days 6 hours, at about 30 km per second. The extra quarter day becomes 29 February in every leap year; a century year is a leap year only if divisible by 400, so 2000 was one and 1900 was not. The Earth is nearest the Sun (perihelion, about 147 million km) around 3 January and farthest (aphelion, about 152 million km) around 4 July, so distance does not cause the seasons. The tilt does: the axis leans 23.5 degrees from the perpendicular (66.5 degrees to the plane of the orbit) and always points towards the Pole Star, so each hemisphere leans towards the Sun for half the year.
Solstices and equinoxes
On 21 June the Sun is overhead at the Tropic of Cancer. This summer solstice brings the longest day of the Northern Hemisphere, 24 hours of daylight north of the Arctic Circle and winter in the south. On 22 December it is overhead at the Tropic of Capricorn, giving the winter solstice in the north and continuous daylight south of the Antarctic Circle. On 21 March (spring equinox) and 23 September (autumnal equinox) it is overhead at the Equator, and day and night are nearly equal everywhere. The Sun's apparent northward journey from 22 December to 21 June is called Uttarayan and the southward one Dakshinayan, and each pole gets about six months of day and six months of night.
Latitudes and heat zones
Latitude is the angular distance of a place north or south of the Equator, from 0 degrees to 90 degrees at the poles, and 1 degree of latitude is about 111 km. Parallels shrink towards the poles, and the Equator is the only parallel that is a great circle. The key parallels are the Tropics of Cancer and Capricorn (23.5 degrees N and S) and the Arctic and Antarctic Circles (66.5 degrees N and S). They mark the Torrid Zone between the tropics, where the noon Sun is overhead at least once a year, the two Temperate Zones, and the two Frigid Zones beyond the polar circles. Among the countries the Equator crosses are Ecuador, Colombia, Brazil, Gabon, both Congos, Uganda, Kenya, Somalia and Indonesia, and it passes through Lake Victoria; Brazil is the only country crossed by both the Equator and the Tropic of Capricorn. In India the Tropic of Cancer passes through 8 states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram.
Longitudes
Longitude is the angular distance east or west of the Prime Meridian (0 degrees), which passes through Greenwich near London and was adopted at the International Meridian Conference in Washington in 1884. Meridians are equal half-circles from pole to pole, 180 degrees east and 180 degrees west, meeting at the 180-degree meridian. The gap between meridians is about 111 km per degree at the Equator and shrinks to nothing at the poles.
Time and time zones
The Earth turns 360 degrees in 24 hours, so 15 degrees of longitude make 1 hour and 1 degree makes 4 minutes. Each country adopts a standard meridian. Indian Standard Time is the local time of 82.5 degrees E, which passes near Mirzapur in Uttar Pradesh, and is 5 hours 30 minutes ahead of Greenwich Mean Time, while China, spanning about 60 degrees of longitude, keeps a single time zone 8 hours ahead of Greenwich. To find a time difference, take the difference in longitude (subtract if both places are on the same side of Greenwich, add if they are on opposite sides) and multiply by 4 minutes; then add the result for a place to the east and subtract it for one to the west. So noon at Greenwich is 4 pm at 60 degrees E and 9 am at 45 degrees W.
International Date Line
Going east, the 180-degree meridian is 12 hours ahead of Greenwich; going west, it is 12 hours behind, so its two sides differ by a full day. The Date Line follows this meridian roughly but zigzags around land, through the Bering Strait and past Fiji and Tonga, so that no country has two dates. A traveller crossing it from west to east (Asia to America) repeats a day; one crossing from east to west skips a day.
Key facts
- Sidereal day: 23 hours 56 minutes 4 seconds; solar day: 24 hours. One revolution: about 365 days 6 hours, so a leap year comes every fourth year (century years only if divisible by 400).
- Perihelion around 3 January (about 147 million km) and aphelion around 4 July (about 152 million km): distance does not cause the seasons, the 23.5-degree tilt does.
- 21 June: Sun overhead at the Tropic of Cancer, longest day in the north. 22 December: overhead at the Tropic of Capricorn. 21 March and 23 September: overhead at the Equator (equinoxes).
- Key parallels: Equator 0, Tropics 23.5 degrees N and S, Arctic and Antarctic Circles 66.5 degrees N and S. The Torrid Zone lies between the tropics, the Frigid Zones beyond the polar circles.
- The Tropic of Cancer crosses 8 Indian states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura, Mizoram.
- Brazil is the only country crossed by both the Equator and the Tropic of Capricorn; the Equator also passes through Lake Victoria.
- Prime Meridian (0 degrees) passes through Greenwich, adopted at the International Meridian Conference, Washington, 1884.
- Time rule: 15 degrees = 1 hour, 1 degree = 4 minutes; east is ahead, west is behind. Noon at Greenwich = 4 pm at 60 degrees E = 9 am at 45 degrees W.
- IST meridian 82.5 degrees E passes near Mirzapur (Uttar Pradesh); IST = GMT + 5 hours 30 minutes. China uses one time zone, 8 hours ahead of GMT.
- Date Line: about 180 degrees, bends around land and through the Bering Strait; crossing west to east repeats a day, crossing east to west skips a day.
Quick revision
Earth an oblate spheroid (geoid), equatorial diameter about 12,756 km; rotates west to east in 23 h 56 min 4 s; revolves in about 365 days 6 hours; perihelion 3 January, aphelion 4 July; axis tilted 23.5 degrees, parallelism causes seasons; solstices 21 June and 22 December, equinoxes 21 March and 23 September; 1 degree of latitude about 111 km; Prime Meridian at Greenwich, 1884; 1 degree of longitude = 4 minutes; IST on 82.5 E near Mirzapur, GMT+5:30; Date Line about 180 degrees.