Revolution and Seasons

Class 7 Science · Chapter 12: Earth, Moon, and the Sun · NCERT Curiosity section 12.2

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While spinning, the Earth also travels around the Sun: revolution. Its path, the orbit, is nearly a circle when seen from above, and one trip takes about 365 days and 6 hours. Because we face a different part of space at night as the year goes on, different stars rise at sunset in different months. Some communities of the Tapi valley watched for certain star patterns to know the monsoon was coming.

The Earth's axis is tilted, and it keeps this tilt all around the orbit. In June the Northern Hemisphere leans towards the Sun: sunlight falls on a smaller area, so it is more intense, and days last more than 12 hours: summer. In December it leans away: weaker sunlight and shorter days: winter. The Southern Hemisphere has the opposite seasons.

Seasons are not caused by the Earth being nearer the Sun: in fact it is closest in January. The longest day in the north comes around 21 June (summer solstice) and the shortest around 22 December (winter solstice); around 21 March and 23 September day and night are equal (equinoxes). The poles get six months of daylight and six of darkness, while the equator has about 12 hours of each all year, so seasons are mild in the far south of India.

Explore: summer or winter?

1. Sort each one.

Remember

• The Earth revolves around the Sun in about 365¼ days; the night sky changes through the year.
• The tilted axis gives more intense sunlight and longer days to the hemisphere leaning towards the Sun: summer.

Quick check

It is summer in India in June. What season is it in Australia then?

1. winter
2. summer
3. spring
4. the same as India

Think about it

Why are summer days longer than winter days in Delhi?

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.