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The Two Motions of the Earth
The Earth has two main motions:
1. Rotation — spinning on its own axis → causes day and night. 2. Revolution — moving around the Sun → causes a year and the seasons.
Rotation
- The Earth spins on an imaginary line called its axis, which runs from the North Pole to the South Pole.
- It rotates from west to east, the same direction in which the Sun appears to move.
- One rotation takes 24 hours → this gives us day and night.
- The half of the Earth facing the Sun has day; the other half has night.
Because the Earth rotates from west to east, the Sun rises in the east and sets in the west.
Revolution
- The Earth moves around the Sun in a fixed, oval-shaped path called its orbit.
- One revolution takes 365¼ days (one year).
- The extra ¼ day each year adds up: ¼ + ¼ + ¼ + ¼ = 1 whole day every four years → a leap year of 366 days with 29 February.
The Tilt of the Axis and the Seasons
- The Earth's axis is not straight up — it is tilted at an angle of 23½° from the vertical, and this tilt stays fixed in the same direction as the Earth revolves.
- Because of this tilt, the two hemispheres do not receive equal heat all year:
- When the Northern Hemisphere tilts towards the Sun → it is summer there (and winter in the Southern Hemisphere).
- When it tilts away → it is winter there (and summer in the Southern Hemisphere).
Equinoxes and Solstices
| Date | Name | What happens |
|---|---|---|
| 21 March | Equinox | Neither pole is tilted towards the Sun; day and night are equal (about 12 hours each) all over the Earth |
| 23 September | Equinox | Again equal day and night all over the Earth |
| 21 June | Summer Solstice (Northern) | Northern Hemisphere tilted most towards the Sun → longest day in the Northern Hemisphere |
| 22 December | Winter Solstice (Northern) | Northern Hemisphere tilted away → shortest day (longest night) in the Northern Hemisphere |
During an equinox, the whole Earth has roughly equal day and night. During a solstice, one hemisphere has its longest or shortest day.
Key Vocabulary
| Term | Meaning |
|---|---|
| Axis | The imaginary line through the North and South Poles on which the Earth spins |
| Rotation | Spinning of the Earth on its axis (24 hours → day and night) |
| Revolution | Movement of the Earth around the Sun (365¼ days → one year) |
| Orbit | The fixed path along which the Earth revolves around the Sun |
| Leap year | A year of 366 days occurring every four years (29 February added) |
| Equinox | A day on which day and night are equal all over the Earth (21 March, 23 September) |
| Solstice | The time when one hemisphere has its longest or shortest day (21 June, 22 December) |
Exam Tips
- Rotation → day and night; Revolution → year and seasons — never confuse the two motions and their effects.
- The axis is tilted at 23½° — this single fact explains why seasons occur.
- Memorise the four dates: 21 March and 23 September (equinoxes), 21 June and 22 December (solstices).
- A leap year comes every four years and has 366 days with 29 February — a common 1-mark question.
- Remember the ¼ day logic: four ¼-days add up to one extra day every four years.
Frequently Asked Questions
Rotation is the spinning of the Earth on its axis from west to east. It takes 24 hours to complete one rotation, which causes day and night.