Why we have seasons

Every year we go through a cycle of seasons, and summer days are longer than winter days. Is that connected to the Earth's journey round the Sun?

The Earth's axis is not upright compared with its orbit: it is tilted, and it keeps the same tilt all the way round the Sun. This tilt, together with the Earth's round shape, gives us the seasons.

In June, the Northern Hemisphere (the half with India) leans towards the Sun and the Southern Hemisphere leans away. Because the Earth's surface is curved, the same bundle of sunlight is spread over a smaller area in the North than in the South, so it heats that area more — like a torch shining straight down makes a small, bright spot, while a slanting torch makes a big, dim one.

On top of that, the Northern Hemisphere gets more than 12 hours of sunlight in June. More intense sunlight, for longer: that is summer. In December it is the other way round, and the North has winter, with shorter days.

The Southern Hemisphere has the opposite season: winter in June, summer in December.

Two common mistakes the book corrects:

• 'In summer our half is closer to the Sun because it leans towards it.'
• 'The Earth's oval orbit puts it nearer the Sun in summer.'

In both cases the difference in distance is tiny and is not the reason for seasons. In fact the Earth is closest to the Sun in January — in the middle of our winter!

Special dates in the Northern Hemisphere:

• around 21 June: longest day, the summer solstice;
• around 22 December: shortest day and longest night, the winter solstice;
• around 21 March and 23 September: day and night both about 12 hours, the spring and autumn equinox.

At the North Pole, the Sun rises around 21 March and stays up for six months, setting around 22 September; the South Pole is the opposite. On the Equator there are always about 12 hours of light and 12 of dark, with little change in strength through the year. That is why seasons are not very noticeable in India's southern states.