Earth's Motions and Seasons
NCERT Class 6 - The Earth Our Habitat, Chapter 3 • NCERT Class 11 - Fundamentals of Physical Geography
Earth's Motions and Seasons
The earth has two motions - rotation on its own axis, which gives day and night, and revolution around the Sun, which with the tilt of the axis gives the seasons. Without the tilt there would be no seasons at all.
Rotation
- The earth rotates on its axis from west to east, completing one rotation in about 24 hours, called an earth day.
- The axis is tilted at an angle of about 23 degrees 30 minutes from the perpendicular to the orbital plane.
- The circle that divides the lit half from the dark half is the circle of illumination; it does not pass through the poles because of the tilt.
- Rotation causes day and night, the apparent movement of the Sun across the sky, the deflection of winds and currents by the Coriolis force, and the daily rhythm of tides.
- If the earth did not rotate, one half would face permanent day and unbearable heat, and the other permanent night and extreme cold.
Revolution and the tilt
- The earth revolves around the Sun in an elliptical orbit, taking about 365 days and 6 hours, which is why an extra day is added every fourth year in a leap year of 366 days.
- The axis stays tilted in the same direction throughout the orbit, so different parts of the earth receive direct sunlight at different times of the year - this is the cause of seasons.
- The earth is nearest the Sun, at perihelion, in early January, and farthest, at aphelion, in early July. Distance is therefore not the cause of seasons.
Solstices and equinoxes
| Date | Position | What happens |
|---|---|---|
| 21 June | Sun vertical over the Tropic of Cancer | Summer solstice - longest day in the northern hemisphere |
| 22 December | Sun vertical over the Tropic of Capricorn | Winter solstice - shortest day in the northern hemisphere |
| 21 March | Sun vertical over the Equator | Spring equinox - day and night equal everywhere |
| 23 September | Sun vertical over the Equator | Autumn equinox - day and night equal everywhere |
- At the summer solstice the northern hemisphere is tilted towards the Sun, gets longer days, and experiences summer, while the southern hemisphere has winter.
- On the equinoxes neither pole is tilted towards the Sun, so day and night are of equal length across the world.
- Areas beyond the Arctic and Antarctic Circles experience continuous daylight for part of the year and continuous darkness for another part.
हिंदी संकेत: ऋतुएं सूर्य से दूरी के कारण नहीं होतीं - वे अक्ष के झुकाव के कारण होती हैं। पृथ्वी जनवरी में सूर्य के सबसे निकट होती है, जब उत्तरी गोलार्ध में सर्दी रहती है।
Exam me kaise aata hai
Factual questions ask the dates of solstices and equinoxes and the tilt of the axis. Match-the-following links date with the tropic over which the Sun is vertical. Statement-based questions test whether distance from the Sun causes seasons - it does not.
UPSC / State PSC ke liye
The reasoning to remember is that seasons depend on the angle at which sunlight strikes and on the length of daylight, both of which follow from the tilt. Use the perihelion and aphelion facts as the decisive evidence against the distance explanation.
Yahan confuse hote hain
✗ Seasons are caused by the earth's changing distance from the Sun | ✓ They are caused by the tilt of the axis
✗ The earth rotates from east to west | ✓ It rotates from west to east, which is why the Sun appears to rise in the east
✗ The longest day in the northern hemisphere is 22 December | ✓ That is the shortest day; the longest is 21 June
Ek nazar me
- Rotation is on the axis from west to east and takes about 24 hours.
- Revolution around the Sun takes about 365 days and 6 hours; a leap year has 366 days.
- The axis is tilted about 23 degrees 30 minutes and stays tilted in the same direction.
- 21 June - summer solstice, Sun over the Tropic of Cancer; 22 December - winter solstice.
- 21 March and 23 September are equinoxes, with day and night equal.
- The earth is nearest the Sun in January and farthest in July, so distance does not cause seasons.