🔄 PART 1: Rotation of the Earth
Rotation is the spinning of the Earth on its own axis (an imaginary line joining the North
and South Poles). The Earth completes one full rotation in approximately 24 hours (23 hours
56 minutes 4 seconds = 1 sidereal day; ~24 hours = 1 solar day).
Direction of Rotation
- The Earth rotates from west to east (anticlockwise when viewed from above the North
Pole).
- This is why the Sun appears to rise in the east and set in the west.
- Speed of rotation at the Equator: approximately 1,670 km/h (=1,037 mph). Speed
decreases towards the poles (poles = 0 km/h — just rotating in place).
Effects of Rotation
| Effect |
Explanation |
| Day and Night |
As Earth rotates, only one half faces the Sun (day) while the other half is in shadow (night).
Dividing line = circle of illumination. |
| Direction of Sun rise/set |
Sun appears to rise in the East and set in the West due to Earth's west-to-east rotation. |
| Coriolis Effect |
Earth's rotation causes moving objects (winds, ocean currents, projectiles) to deflect:
to the right in the Northern Hemisphere, to the left in the
Southern Hemisphere. Causes cyclones, ocean gyres, trade winds to curve. |
| Flattening at poles |
Centrifugal force due to rotation causes Earth to bulge at equator and flatten at poles → oblate
spheroid shape. |
| Tides |
Rotation affects sea levels along with Moon's gravity. |
🌍 PART 2: Revolution of the Earth
Revolution is the movement of the Earth around the Sun in its orbit. The Earth completes
one revolution in approximately 365 days 6 hours (365.25 days) — the basis of 1
solar year. Every 4 years, the extra 6 hours × 4 = 24 hours → this extra day is added as
Feb 29 in a Leap Year.
- Earth's orbit is elliptical (not perfectly circular) — so Earth is closer to the Sun at
Perihelion (~147 million km, around January 3) and farther at Aphelion
(~152 million km, around July 4).
- Earth's axis is tilted at 23.5° from the perpendicular to its orbital plane — this tilt
is constant in direction (always pointing towards Polaris/Pole Star) and is the primary cause of
seasons.
🌞 PART 3: Seasons — Summer and Winter Solstice, Equinoxes
| Event |
Date |
What Happens |
Effects |
| Summer Solstice (Northern Hemisphere) |
~June 21 |
Earth's North Pole is tilted towards Sun. Sun is directly overhead at Tropic of Cancer (23.5°N).
|
Longest day in NH (about 17 hrs at 50°N); shortest night; Arctic Circle has 24-hr daylight.
Southern Hemisphere = Winter Solstice (shortest day). |
| Winter Solstice (Northern Hemisphere) |
~December 22 |
Earth's South Pole is tilted towards Sun. Sun is directly overhead at Tropic of Capricorn
(23.5°S). |
Shortest day in NH; longest night; Antarctic Circle has 24-hr daylight. Southern Hemisphere =
Summer Solstice (longest day). |
| Spring Equinox (Vernal Equinox) |
~March 21 |
Neither pole is tilted towards Sun. Sun is directly overhead at Equator. |
Day = Night = 12 hours everywhere on Earth. Spring begins in NH; Autumn begins in SH. |
| Autumn Equinox (Autumnal Equinox) |
~September 23 |
Neither pole is tilted towards Sun. Sun is directly overhead at Equator. |
Day = Night = 12 hours everywhere on Earth. Autumn begins in NH; Spring begins in SH. |
Why Seasons Occur
- Seasons are caused by the 23.5° tilt of Earth's axis combined with its revolution
around the Sun. (NOT because Earth is closer/farther from the Sun — actually, Earth is closer
to the Sun in January, during NH winter!)
- When the NH is tilted towards the Sun: Sun's rays fall more directly (high angle → more heat per unit
area); days are longer. → Summer in NH.
- When the NH is tilted away from the Sun: rays are more slanted (low angle → less heat per unit area);
days are shorter. → Winter in NH.
Effects of Revolution
| Effect |
Explanation |
| Seasons |
Due to axial tilt + revolution — Earth has 4 seasons in temperate regions |
| Change in length of day/night |
Day length varies throughout the year (longer days around summer solstice, shorter around winter
solstice) |
| Variation in altitude of Sun |
Sun's angle in the sky changes throughout the year at any location outside the tropics |
| Occurrence of year |
One revolution = 1 year (365.25 days) |
📝 Quick Revision – Key Facts
| Topic |
Key Fact |
| Rotation direction |
West to East (anticlockwise from North Pole) |
| Rotation period |
24 hours (approx.) |
| Revolution period |
365.25 days (1 solar year) |
| Earth's axial tilt |
23.5° (23°30') from perpendicular to orbital plane |
| Summer Solstice (NH) |
June 21; Sun overhead at Tropic of Cancer; longest day NH |
| Winter Solstice (NH) |
December 22; Sun overhead at Tropic of Capricorn; shortest day NH |
| Equinox |
March 21 and September 23; day = night = 12 hrs everywhere |
| Coriolis Effect |
NH → deflection to right; SH → deflection to left |
| Perihelion |
January 3 (~147 million km — Earth closest to Sun) |
| Aphelion |
July 4 (~152 million km — Earth farthest from Sun) |
⚠️ Common Misconception
Seasons are NOT caused by Earth being closer to or farther from the Sun. They are caused by the
23.5° axial tilt. In fact, Earth is closest to the Sun in January (Northern Hemisphere
winter)! The tilt changes the angle of sunlight and the length of daylight — these are the true causes of
seasons.
📌 Chapter Summary
- Rotation: W→E; 24 hrs; causes day/night, Coriolis Effect, Earth's shape. Speed: ~1,670 km/h at
equator.
- Revolution: 365.25 days; orbit is elliptical. Leap year = every 4 years (extra 6 hrs × 4 = 1 day).
- Axial tilt: 23.5° — the PRIMARY cause of seasons. Tilt is constant in direction (points toward
Polaris).
- Solstices: June 21 (NH summer; Sun overhead Tropic of Cancer); Dec 22 (NH winter; Sun overhead
Tropic of Capricorn).
- Equinoxes: March 21 and September 23; Sun overhead Equator; day = night = 12 hours everywhere.