Day, Night and Seasons

Seasons happen because Earth's axis is tilted by about 23.4° while it goes round the Sun. For part of the year the northern half leans toward the Sun and gets long days and a high Sun (summer); six months later it leans away (winter). They are not caused by Earth's distance from the Sun.

Rotation is Earth spinning on its own axis, once every 24 hours, which causes day and night. Revolution is Earth moving round the Sun, once every 365¼ days, which together with the tilt of the axis causes the seasons and gives us a year.

Earth's axis is tilted about 23.4° (often written 23.5° or 23½°) from upright, compared with its path round the Sun. It always points toward the Pole Star, which is why the Pole Star stays still in the sky.

On or near 21 June, the June solstice, the northern half of Earth is tilted most toward the Sun. The noon Sun is at its highest and every place north of the Equator, including all of India, gets its longest day. The shortest day is near 22 December.

Zero Shadow Day is when the Sun is exactly overhead at noon, so upright objects cast almost no shadow. It happens twice a year at places between the Tropic of Cancer and the Tropic of Capricorn. In Chennai and Bengaluru it falls around 24 April and 18 August, and in Mumbai around 15 May and 27 July.

When Earth's northern half is tilted toward the Sun, the southern half is tilted away. So in December, while India has short winter days, Australia has long summer days, and the other way round in June.