Revolution of the Earth
While spinning on its axis in a west-east direction, the Earth also moves round the Sun in an anticlockwise direction along its 960 million km long elliptical orbit. This motion round the Sun is called the Earth’s Revolution. The Earth makes a complete journey round the Sun in about 365 days and 6 hours, at an average speed of 29.6 km per second. This period is called a year, and the Revolution is known as the ‘Annual Motion’ of the Earth. Since it is not possible to show 6 hours in our calendar year of 365 days, an extra day is added to February every fourth year which has 366 days. Such a year is called a Leap Year.
Since the orbit of the Earth is elliptical, the Sun is not centrally placed in it. Therefore, as the Earth revolves round the Sun, its distance from the Sun varies. On July 4, the Sun is farthest (152 million km) from the Sun, and on January 3, it is closest (147 million km). The above two positions are called ‘Aphelion’ (In Greek language ‘ap’ means ‘away from’ and ‘Helios’ means Sun) and ‘Perihelion’(‘peri’ means ‘near’), respectively.
Effects of Revolution & Inclined Axis
We are now aware that, the Axis of the Earth is tilted, instead of being vertical. It is inclined at an angle of 23½º to the vertical or 66½º to the plane of the earth’s orbit. Had it been perpendicular, the rays of the Sun would have fallen equally, in the same manner, all over the Earth, the duration of days and nights would have been equal throughout the Earth’s surface and there would be no change of season. However, as the Axis is tilted and always points to the same direction, the northern half of the Earth or the Northern Hemisphere is tilted towards the Sun for half the year and the Southern Hemisphere during the other half.
On December 22, with the Southern Hemisphere tilting towards the Sun, the Sun is overhead the Tropic of Capricorn and all the places in the Southern Hemisphere have their longest day and the shortest night and it is mid-summer in that Hemisphere. The length of day increases with the increase of latitudes south of the Equator. During this time the Sun never sets at the Antarctic Circle and the region beyond it, up to the South Pole.
On June 21st, the Sun is overhead at noon along the Tropic of Cancer, and all the latitudes in the Northern Hemisphere have their longest day of the year. The length of the day increases with the increase of latitude towards the north of the Equator. During this time, the Sun never sets at the Arctic Circle. The region beyond the Arctic Circle is famous for its midnight Sun and is popularly known as the ‘Land of the Midnight Sun’. Hammerfest in Norway experiences continuous daylight from May 13 to July 29.
The days and nights at the Equator are of equal lengths of 12 hours each, throughout the year. This is because, the ‘Shadow Circle’ bisects the Equator every day of the year.
The low latitude areas enjoy a long summer and a short cool season, while the high latitude or Polar areas have a long winter and a short warm season. Throughout the year the Equatorial areas get direct rays of the Sun, while the Polar Regions get the most oblique rays. Thus the former has only one season, summer, while the latter has only winter. It is only the temperate lands, lying in the mid-latitudes (between 30ºN/S and 60ºN/S) where the four seasons - summer, autumn, winter and spring - are distinctly seen.