Inclination of the Earth's axis
Also called: Axial tilt, Obliquity · Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"; Class 11, Ch 11 "World Climate and Climate Change"
Meaning
The Earth's axis is tilted. It makes an angle of 66.5° with the plane of its orbit around the Sun, which is 23.5° from the vertical. The axis always points in the same direction as the Earth moves around the Sun. So during the year, the point where the noon Sun is directly overhead moves between the Tropic of Cancer and the Tropic of Capricorn. This tilt causes the seasons, unequal lengths of day and night, and the change in insolation with latitude.
Example
On about 21 June (the summer solstice), the noon Sun is overhead at the Tropic of Cancer (23.5°N), which passes through central India. The Northern Hemisphere has its longest day, and north of the Arctic Circle there is daylight for all 24 hours.
Don't confuse with
- Earth-Sun distance: the Earth is nearest the Sun at perihelion, about 3 January. That is during the Northern Hemisphere's winter. So distance does not cause the seasons. The tilt does.