Earth: Rotation, Revolution, Inclination and Seasons
The Earth has two main motions - rotation on its own axis and revolution around the Sun. Together with the tilt of the axis, these motions produce day and night, the seasons, the year and the deflection of winds and currents. Examinations ask this topic through the period of rotation and revolution, the effects of each motion, the value of the axial tilt, and the dates and positions of solstices and equinoxes. The concepts are easy to visualise, so a clear diagram and a small table of dates make the chapter scoring.
Rotation and its effects
Rotation is the spinning of the Earth on its own axis. The Earth rotates from west to east (anti-clockwise when seen from above the North Pole). One rotation takes about 23 hours 56 minutes 4 seconds, which is the sidereal day, while the normal solar day is taken as 24 hours. The axis is an imaginary line joining the North Pole and the South Pole, and it is inclined at 23.5 degrees to the vertical. The effects of rotation are day and night, the apparent movement of the Sun and stars from east to west, a difference in local time between longitudes, the Coriolis force that deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, and the occurrence of high and low tides. The Foucault pendulum is the classic experiment that proves the Earth rotates.
Revolution and its effects
Revolution is the movement of the Earth around the Sun in an elliptical orbit. One revolution takes about 365 days and 6 hours, which we count as one year. The extra six hours every year are adjusted by adding one day to February every fourth year, giving a leap year of 366 days. Because the orbit is elliptical, the Earth is closest to the Sun in early January at about 147 million kilometres, a position called perihelion, and farthest in early July at about 152 million kilometres, called aphelion. Revolution, combined with the tilt of the axis, causes the seasons and the changing length of day and night.
Axial inclination and the seasons
The axis of the Earth is inclined at about 23.5 degrees from the vertical, or 66.5 degrees from the plane of the orbit. This tilt always points in the same direction, a property called axial parallelism. As the Earth revolves, first the Northern Hemisphere and then the Southern Hemisphere leans towards the Sun. The hemisphere leaning towards the Sun receives more direct sunlight for a longer time and has summer, while the other hemisphere has winter. It is important to note that seasons are caused by the tilt and revolution, not by the distance from the Sun; in fact the Earth is nearer the Sun during the Northern Hemisphere winter.
Solstices and equinoxes
On 21 June, the North Pole leans towards the Sun and the Sun is directly overhead at the Tropic of Cancer (23.5 degrees North). This is the summer solstice in the Northern Hemisphere, giving the longest day and shortest night there, and the winter solstice in the Southern Hemisphere. On 22 December, the Sun is directly overhead at the Tropic of Capricorn (23.5 degrees South), which is the winter solstice in the Northern Hemisphere. On 21 March and 23 September, the Sun is directly overhead at the Equator; these are the equinoxes, when day and night are almost equal everywhere. During summer near the poles, the Sun does not set for weeks, a phenomenon called the midnight sun, and during winter it does not rise, called the polar night.
Quick revision
Remember that rotation causes day and night and takes about 24 hours, while revolution causes the year and the seasons and takes about 365 days and 6 hours. The axis is tilted at 23.5 degrees, and this tilt is the real cause of the seasons. The Sun is overhead at the Tropic of Cancer on 21 June (summer solstice), at the Tropic of Capricorn on 22 December (winter solstice), and at the Equator on 21 March and 23 September (equinoxes). Perihelion is in January and aphelion in July, and the Coriolis force deflects winds to the right in the north and to the left in the south.
How to prepare this topic
- Separate the two motions clearly: rotation equals a day and revolution equals a year, and learn the effects of each.
- Draw the Earth with its 23.5 degree tilt and mark the four important positions for the solstices and equinoxes.
- Memorise the four dates: 21 June, 22 December, 21 March and 23 September, with the latitude where the Sun is overhead.
- Note the leap year rule and the difference between perihelion and aphelion in the elliptical orbit.
- Remember that seasons are caused by the tilt and revolution and not by the changing distance from the Sun.
Common exam traps
- Seasons are caused by the axial tilt with revolution, not by the distance between the Earth and the Sun.
- Rotation takes about 23 hours 56 minutes (sidereal day), while the civil day is taken as 24 hours.
- 21 June is the summer solstice in the Northern Hemisphere but the winter solstice in the Southern Hemisphere.
- The Earth is closest to the Sun (perihelion) in January, which is winter in the Northern Hemisphere.
- The Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Frequently Asked Questions
What is the difference between rotation and revolution?
Rotation is the spinning of the Earth on its own axis, takes about 23 hours 56 minutes and causes day and night. Revolution is the movement of the Earth around the Sun in an elliptical orbit, takes about 365 days and 6 hours and, along with the axial tilt, causes the seasons and the year.
What is the tilt of the Earth's axis and why does it cause seasons?
The Earth's axis is tilted at about 23.5 degrees from the vertical, or 66.5 degrees from the orbital plane, and always points in the same direction. As the Earth revolves, each hemisphere alternately leans towards the Sun and away from it, receiving more or less direct sunlight, which produces summer and winter. Seasons are caused by this tilt and revolution, not by distance from the Sun.
What are solstices and equinoxes and when do they occur?
Solstices occur when the Sun is overhead at a tropic: 21 June at the Tropic of Cancer (summer solstice in the Northern Hemisphere) and 22 December at the Tropic of Capricorn (winter solstice there). Equinoxes occur when the Sun is overhead at the Equator on 21 March and 23 September, when day and night are almost equal all over the world.
What are perihelion and aphelion?
Because the Earth's orbit is elliptical, the Earth is nearest to the Sun in early January at about 147 million kilometres, which is called perihelion, and farthest in early July at about 152 million kilometres, called aphelion. Perihelion happens during the Northern Hemisphere winter, showing that distance does not cause the seasons.
What is the Coriolis force and what is the midnight sun?
The Coriolis force is the deflection caused by the Earth's rotation; it turns winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The midnight sun is a summer phenomenon near the poles in which the Sun remains visible even at midnight for several weeks, while in winter the opposite, the polar night, occurs.