Earth's Motions and Seasons
~10 min read
- Rotation: Earth spins on its axis once every 24 hours → day and night. Direction: west to east.
- Revolution: Earth orbits the Sun once every 365.25 days → a year. Combined with axial tilt (23.5°) → seasons.
- Key dates: 21 June (summer solstice in N hemisphere), 21–22 December (winter solstice in N), 21 March & 23 September (equinoxes — equal day and night everywhere).
Earth has two simultaneous motions: rotation (about its own axis) and revolution (around the Sun). The first gives day and night; the second, combined with the Earth's 23.5° axial tilt, gives the seasons. PYQs on this topic test the exact dates of solstices and equinoxes, the direction of rotation, and the cause of seasonal variation.
The Two Motions of Earth
| Motion | About | Period | Effect |
|---|---|---|---|
| Rotation | Earth's own axis (pole to pole) | 23 h 56 min ≈ 24 h | Day and night; deflection of winds (Coriolis); flattening at poles |
| Revolution | Around the Sun | 365.25 days (≈ 365 d 6 h) | The year; combined with tilt, the seasons |
Rotation — Day and Night
Earth rotates from west to east (counter-clockwise viewed from above the North Pole). One rotation relative to the distant stars (sidereal day) takes 23 hours 56 minutes; relative to the Sun (solar day), 24 hours. Surface speed at the equator is about 1,670 km/h; at the poles it is zero.
Consequences of rotation
- Day and night cycle every 24 hours.
- Coriolis effect — winds and currents deflect right in the Northern Hemisphere, left in the Southern.
- Time zones — 24 zones, each 15° of longitude wide, each one hour apart.
- Equatorial bulge — Earth flattens at poles due to rotation (oblate spheroid).
Revolution — The Year
Earth orbits the Sun in an elliptical path. The orbit is nearly circular but not exactly — Earth is closer to the Sun at one point (perihelion, ~147 million km in early January) and farthest at another (aphelion, ~152 million km in early July). The extra ~6 hours per year is why we have a leap year every 4 years (adding 24 hours = 1 day to February).
Axial Tilt and Seasons
Earth's rotation axis is tilted at 23.5° to the perpendicular of its orbital plane. This tilt — not Earth's distance from the Sun — is the reason for seasons. As Earth revolves, different hemispheres lean toward or away from the Sun across the year.
Seasons are NOT caused by Earth's distance from the Sun. In fact, Northern Hemisphere summer (June) occurs when Earth is at aphelion — farthest from the Sun. Seasons are caused entirely by the 23.5° axial tilt.
Solstices and Equinoxes
| Event | Date | Northern Hemisphere | Southern Hemisphere |
|---|---|---|---|
| Summer solstice | 21 June | Longest day, Sun overhead at Tropic of Cancer (23.5° N) | Shortest day; winter |
| Autumnal equinox | 23 September | Day = night (12 h each) | Day = night |
| Winter solstice | 21–22 December | Shortest day, Sun overhead at Tropic of Capricorn (23.5° S) | Longest day; summer |
| Vernal (spring) equinox | 21 March | Day = night | Day = night |
Perihelion and Aphelion
Perihelion (closest to Sun) — around 3 January, distance ~147 million km. Aphelion (farthest from Sun) — around 4 July, distance ~152 million km. The 5-million-km difference is small (~3%) and not the cause of seasons.
NDA PYQ Examples
Q (NDA pattern): The cause of seasons on Earth is —
(a) Distance from Sun varies (b) Earth's elliptical orbit (c) Axial tilt of 23.5° (d) Variation in solar activity
Answer: (c) Axial tilt of 23.5°.
Q (NDA pattern): On 21 June, the Sun is overhead at:
(a) Equator (b) Tropic of Cancer (c) Tropic of Capricorn (d) Arctic Circle
Answer: (b) Tropic of Cancer (23.5° N). This is the summer solstice in the Northern Hemisphere.
Q (NDA pattern): Earth rotates from —
(a) North to South (b) East to West (c) West to East (d) Counter-clockwise as seen from South Pole
Answer: (c) West to East. (This is why the Sun appears to rise in the east.)
Drill Earth's Motions
NDA-pattern items on rotation, revolution, axial tilt, solstices and equinoxes.
Start Free Mock TestFAQs
Why is a year 365.25 days?
Earth completes one revolution around the Sun in 365 days and ~6 hours. To absorb the extra 6 hours, we add one day to February every 4 years — the leap year.
Why are summer days longer than winter days?
Because of the 23.5° axial tilt. In summer, your hemisphere is tilted toward the Sun, so the Sun's path across the sky is longer and higher. In winter, your hemisphere tilts away, so the path is shorter and lower.
What is the difference between solstice and equinox?
Solstice = the days when the Sun reaches its furthest point north (June 21) or south (Dec 21–22) of the equator. Equinox = the days when the Sun crosses the equator (March 21, Sept 23) — day and night are equal everywhere.
Does Earth's distance from the Sun affect seasons?
Almost not at all. In fact, Northern Hemisphere summer occurs when Earth is farthest from the Sun (aphelion, 4 July). Seasons are caused by axial tilt, not distance.
What is the Coriolis effect?
Earth's rotation deflects winds and ocean currents — to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This shapes global wind belts (trade winds, westerlies) and cyclone rotation.