Rotation: Earth's spin
Rotation is Earth spinning on its axis, an imaginary line through the North and South Poles.
- Direction: west to east (anticlockwise seen from above the North Pole). So the Sun seems to rise in the east and set in the west.
- Time for one turn: about 24 hours (23 h 56 min measured against the stars).
- Speed at the equator: about 1,670 km/h; it gets slower towards the poles and is zero at the poles.
The line between the lit half and the dark half is the circle of illumination.
Effects of rotation
- Day and night follow each other.
- Time: 360° ÷ 24 h = 15° per hour, or 4 minutes per 1° of longitude. Going east, local time is ahead. This is the basis of time zones and of the International Date Line at about 180°.
- Coriolis effect: moving air and water are deflected – to the right in the Northern Hemisphere, to the left in the Southern Hemisphere. This shapes winds and ocean currents.
- Tides rise and fall twice a day as Earth turns under the Moon's pull.
- Earth is slightly flattened at the poles because of its spin.
Revolution and the ecliptic
Revolution is Earth's journey round the Sun.
- One orbit takes about 365¼ days. We add one day every four years: a leap year (29 February).
- The orbit is a slightly stretched circle (ellipse). Earth is nearest the Sun (perihelion, about 147 million km) in early January and farthest (aphelion, about 152 million km) in early July. This small change does not cause seasons.
- Direction: the same as rotation (anticlockwise seen from above the North Pole).
- The plane of the orbit is the ecliptic. Earth's axis makes 66.5° with this plane, i.e. it is tilted 23.5° from the perpendicular. The equator and the ecliptic cross at an angle of 23.5°.
Seasons, solstices and equinoxes
Because the tilted axis keeps pointing the same way (towards the Pole Star), first one hemisphere and then the other leans towards the Sun. The hemisphere leaning in gets higher, more direct sunlight and longer days – summer.
- ~21 June – June solstice: Sun overhead at the Tropic of Cancer (23.5°N). Longest day in the north; 24-hour daylight inside the Arctic Circle.
- ~22 December – December solstice: Sun overhead at the Tropic of Capricorn (23.5°S). Longest day in the south.
- ~21 March and ~23 September – equinoxes: Sun overhead at the equator; day ≈ night (12 h) everywhere.
At the equator, day length stays near 12 hours all year; the farther from the equator, the bigger the change. Seasons in the two hemispheres are opposite.
Key formulas and definitions
- Rotation: 1 turn ≈ 24 h, west → east
- 360° ÷ 24 h = 15° per hour = 1° per 4 minutes
- Revolution: 1 orbit ≈ 365¼ days → leap year every 4 years
- Axis tilt = 23.5° from the perpendicular (66.5° to the ecliptic plane)
- Overhead Sun: 23.5°N (≈21 Jun) · 0° (≈21 Mar, ≈23 Sep) · 23.5°S (≈22 Dec)
Worked examples
1. Place A is at 0° longitude and place B at 75°E. When it is 12:00 noon at A, what is the local time at B?
Difference = 75°. Time = 75 × 4 min = 300 min = 5 h. B is east, so ahead: 5:00 p.m.
2. Two cities differ by 3 hours in local time. How many degrees of longitude apart are they?
3 h × 15° = 45° of longitude.
3. Why does a city at 60°N have 18-hour days in June but 6-hour days in December?
In June the northern hemisphere is tilted towards the Sun, so most of the 60°N circle is in daylight. In December it is tilted away, so most of it is in darkness.
4. On which date is the noon Sun overhead at the Tropic of Capricorn, and what season is it in Australia?
About 22 December (the December solstice). It is summer in Australia.
5. Earth is closest to the Sun in January. Why is January cold in Europe?
Seasons depend on tilt, not distance. In January the northern hemisphere leans away from the Sun, so sunlight is low and days are short.
6. What is Earth's speed of rotation at the equator, given the circumference is about 40,000 km?
Speed = 40,000 km ÷ 24 h ≈ 1,670 km/h.
Common mistakes
- Thinking seasons happen because Earth is nearer the Sun in summer. They are caused by the tilt.
- Mixing up rotation (spin, 1 day) and revolution (orbit, 1 year).
- Saying Earth rotates east to west. It rotates west to east; that is why the Sun rises in the east.
- Thinking the axis wobbles to face the Sun each season. It keeps pointing the same way in space.