The Equator is an imaginary line around the middle of the Earth. It is halfway between the North and South Poles, and divides the Earth into the Northern and Southern Hemispheres. The Earth is widest at its Equator. The distance around the Earth at the Equator, its circumference, is 40,075 kilometers (24,901 miles). Orbital plane is the plane formed by the orbit. The axis of the Earth is an imaginary line that makes an angle of 66½° with its orbital plane. Latitudes and Longitudes are imaginary lines used to determine the location of a place on earth.
- Parallels of Latitudes are the angular distance of a point on the earth’s surface, measured in degrees from the center of the Earth.
- As the earth is slightly flattened at the poles, the linear distance of a degree of latitude at the pole is a little longer than that at the equator.
- Besides the equator (0°), the north pole (90°N) and the south pole (90°S), there are four important parallels of latitudes–
- Tropic of Cancer (23½° N) in the Northern Hemisphere.
- Tropic of Capricorn (23½° S) in the Southern Hemisphere.
- Arctic Circle at 66½° North of the Equator.
- Antarctic Circle at 66½° South of the Equator.
Latitudinal 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 Zone.
- 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 raise much above the horizon. Therefore, its rays are always slanting. These are, therefore, called Frigid Zone.
- Longitude is the angle east or west of a reference meridian between the two geographical poles to another meridian that passes through an arbitrary point.
- All meridians are halves of great circles, and are not parallel to each other.
- They converge only at the north and south poles. A line passing to the rear of the Royal Observatory, Greenwich (near London in the UK) has been chosen as the international zero-longitude reference line and is known as the Prime Meridian.
- Places to the East are in the Eastern Hemisphere, and places to the West are in the Western Hemisphere.
- The antipodal meridian of Greenwich serves as both 180°W and 180°E. There are 360° of the meridians and the longitude of the prime meridian is 0°.
- Length of all meridians is equal. The distance between two meridians is farthest at the equator and it decreases as we move towards poles and becomes zero at poles.
- They determine local time in relation to G.M.T. or Greenwich Mean Time, which is sometimes referred to as World Time.
Longitude and Time:
- Since the earth makes one complete revolution of 360° in one day or 24 hours, it passes through 15° in one hour or 1° in 4 minutes.
- The earth rotates from west to east, so every 15° we go eastwards, local time is advanced by 1 hour. Conversely, if we go westwards, local time is retarded by 1 hour.
International Date Line (IDL):
- The International Date Line (IDL) is an imaginary line on earth’s surface defining the boundary between one day and the next.
- The International Date Line is located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich, London, UK, the reference point of time zones. It is also known as the line of demarcation.
- A traveler going eastwards gains time from Greenwich until he reaches the meridian 180°E, when he will be 12 hours ahead of G.M.T.
- Similarly, in going westwards, he loses 12 hours when he reaches 180°W. There is thus a total difference of 24 hours or a whole day between the two sides of the 180° meridian.
Circle of Illumination
- The circle of illumination is the circle that divides the day from night on the globe.
- Earth goes around the sun in an elliptical orbit. Note that throughout its orbit, the earth is inclined in the same direction.