Rotation, Revolution & Seasons
Day & night, seasons, solstices, equinoxes, leap year, time zones.
Rotation, Revolution & Seasons
Motions of the Earth
What you'll learn
- Earth has two main motions: rotation (spin on axis) and revolution (orbit around Sun).
- Rotation causes day and night.
- Revolution + tilt of Earth's axis causes seasons.
- Solstices and equinoxes.
- The circle of illumination and time zones.
Key concepts
Rotation
| Feature | Detail |
|---|---|
| What | Earth spinning on its own axis (imaginary line through North & South Poles) |
| Direction | West to East (anti-clockwise when viewed from North Pole) |
| Speed | ~1670 km/h at equator |
| Time | 24 hours (1 day) |
| Effect | Day and night; Sun appears to rise in east and set in west |
| Circle of illumination | The boundary dividing day side and night side of Earth |
Time zones: Earth divided into 24 time zones (360° ÷ 24 = 15° per zone). Every 15° east = 1 hour ahead.
Revolution
| Feature | Detail |
|---|---|
| What | Earth orbiting the Sun in an elliptical (slightly oval) path |
| Time | 365¼ days (1 year); leap year adds extra day every 4 years |
| Direction | Anti-clockwise (viewed from above North Pole) |
| Effect | Change of seasons; variation in day length |
Why seasons occur
- Earth's axis is tilted at 23.5° to the plane of its orbit — this tilt does NOT change.
- As Earth revolves, different hemispheres receive more direct sunlight at different times.
| Season (Northern Hemisphere) | Position | Sun's direct rays |
|---|---|---|
| Summer (June 21) | Northern hemisphere tilts towards Sun | Tropic of Cancer (23.5°N) — Summer Solstice |
| Winter (December 22) | Northern hemisphere tilts away | Tropic of Capricorn (23.5°S) — Winter Solstice |
| Spring (March 21) | Neither tilts | Equator (0°) — Spring Equinox |
| Autumn (September 23) | Neither tilts | Equator (0°) — Autumn Equinox |
Note: When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere (and vice versa).
Solstices and Equinoxes
| Event | Date | Day length |
|---|---|---|
| Summer Solstice | ~June 21 | Longest day in Northern Hemisphere |
| Winter Solstice | ~December 22 | Shortest day in Northern Hemisphere |
| Spring / Vernal Equinox | ~March 21 | Day = Night everywhere (12 hours each) |
| Autumn / Autumnal Equinox | ~September 23 | Day = Night everywhere (12 hours each) |
Leap year
- Every revolution = 365 days 5 hours 48 minutes 46 seconds.
- Extra ~6 hours per year → 1 day extra every 4 years → February has 29 days in a leap year.
- Exception: century years (1700, 1800, 1900) are NOT leap years unless divisible by 400 (2000 WAS a leap year).
Quick check
- What is the difference between rotation and revolution of the Earth?
- What causes day and night?
- Why does the Earth experience seasons?
- What is a solstice? Name the two solstices and their approximate dates.
- What is a leap year? Why do we have one every 4 years?
Open the Practice tab for graded questions on Motions of the Earth.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Quick check
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