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The Earth in the solar system: shape, motions and seasons

The Earth: its place, motions and interior · Part 1 / 3 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

The solar system and the IAU's 2006 definition of a planet, the Earth's shape and size, rotation and revolution, perihelion and aphelion, the tilt of the axis and the seasons, solstices and equinoxes, and how day length changes with latitude.

Physical geography starts with the planet itself: where the Earth sits in space and how it moves. These basics look easy, yet Prelims questions keep returning to them: when the Earth is closest to the Sun, why the seasons are reversed in the two hemispheres, or why in June the Sun does not set for weeks in the far north. This part places the Earth in the solar system and describes its shape and size. It then explains the Earth's two motions, rotation and revolution, and how the tilt of its axis turns them into day and night, the seasons, the solstices and the equinoxes. Part 2 builds on these motions to draw the grid of latitude and longitude and to work out time, and Part 3 goes below the surface to the Earth's layers and its rocks.

The solar system is the Sun together with everything its gravity keeps in orbit: eight planets and their moons, dwarf planets, asteroids, comets and dust. The Sun is a star that shines by nuclear fusion, and it holds almost all of the system's mass. Counting outward, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are small and rocky and are called the inner or terrestrial planets; the outer four are giants built mostly of gases and ices, and a belt of asteroids between Mars and Jupiter separates the two groups. In August 2006 the International Astronomical Union laid down three tests for a planet: it must orbit the Sun, be massive enough for its own gravity to pull it into a nearly round shape, and have cleared other bodies from the neighbourhood of its orbit. Pluto passes the first two but fails the third, so it is now classed as a dwarf planet, like Ceres and Eris. The Moon is the Earth's only natural satellite.

The Earth is the third planet from the Sun and the biggest of the four rocky ones. Its average distance from the Sun is close to 150 million km, a gap that sunlight crosses in a little over eight minutes. The Earth is not a perfect ball. Its spin has made it swell slightly around the equator and flatten at the poles, so the equatorial radius (about 6,378 km) is roughly 21 km longer than the polar radius (about 6,357 km). Geographers call this true, slightly irregular shape a geoid, which simply means 'earth-shaped'. Two features make the Earth fit for life as we know it: its distance from the Sun lets most of its water stay liquid, and its atmosphere, mostly nitrogen and oxygen, screens out harmful radiation and keeps temperatures from swinging wildly between day and night. Because oceans cover about 71 per cent of the surface, the Earth looks blue from space and is often called the blue planet.

The first motion is rotation, the Earth spinning on its axis, an imaginary line joining the North and South Poles. The spin is from west to east, which is why the Sun, the Moon and the stars seem to climb out of the eastern sky and sink in the west: we are the ones moving, carried eastward towards them. One full turn measured against the Sun takes 24 hours and is called a solar day; measured against distant stars, a turn takes about 23 hours 56 minutes, because the Earth also moves a little along its orbit during each day. Rotation gives us day and night. At any moment half the globe faces the Sun and half is in darkness, and the moving boundary between the two halves is called the circle of illumination. Rotation also matters well beyond the daily rhythm: because every place passes under the Sun once a day, it is the basis of time-keeping, and it deflects moving air and water, an effect taken up in the chapter on climate.

The second motion is revolution, the Earth's journey around the Sun along a path called its orbit, which is an ellipse very close to a circle. One trip takes about 365 days and 6 hours. A calendar year has 365 days, so the leftover quarter-days are saved up and added as 29 February every fourth year, giving a leap year of 366 days; to keep the calendar exact, century years are leap years only when divisible by 400, which is why 2000 was a leap year but 1900 was not. Because the orbit is slightly oval, the Earth's distance from the Sun changes during the year. It is nearest, about 147 million km away, in early January, a position called perihelion, and farthest, about 152 million km away, in early July, called aphelion. Many students are surprised that the Northern Hemisphere has its winter when the Earth is closest to the Sun. That one fact proves that the changing distance is not what makes the seasons; the cause lies in the tilt of the axis.

Seasons come from the tilt of the axis. The Earth's axis leans about 23½° away from a line drawn perpendicular to the plane of its orbit; put the other way, it makes an angle of about 66½° with that plane. Just as important, the axis keeps pointing in the same direction in space all year round, towards a spot close to the Pole Star. So as the Earth travels around the Sun, for part of the year the Northern Hemisphere is tipped towards the Sun and for the opposite part it is tipped away. The half tipped towards the Sun gets rays that strike more directly, concentrating heat on a smaller patch of ground, and it also gets longer days, so it has summer. The other half gets slanting rays spread thinly over a larger area and shorter days, so it has winter. If the axis stood upright, the Sun would always be overhead at the equator, there would be hardly any seasons, and day and night would be close to equal everywhere throughout the year.

Four dates mark the yearly cycle. Around 21 June the Sun's rays fall vertically on the Tropic of Cancer. This is the summer solstice of the Northern Hemisphere: it has its longest day, and everywhere north of the Arctic Circle the Sun does not set at all, while the region south of the Antarctic Circle stays dark. Around 22 December the vertical rays fall on the Tropic of Capricorn; this is the winter solstice of the north and midsummer in the Southern Hemisphere, which is why Christmas comes in summer in Australia. Around 21 March and 23 September the vertical rays fall on the equator, neither pole leans towards the Sun, and day and night are roughly equal all over the world; each of these dates is an equinox, a word that means 'equal night'. Between June and December the overhead Sun moves south, and from December to June it moves back north. It is never directly overhead anywhere outside the belt between the two tropics.

The tilt also explains why the length of the day changes far more in some places than in others. On the equator, day and night stay close to 12 hours each all through the year. Moving towards either pole, summer days grow longer and winter days shorter, so the yearly swing in day length widens with distance from the equator. Beyond the polar circles the swing becomes extreme. Around the June solstice the Sun stays above the horizon through the night north of the Arctic Circle, which is why northern Norway and Finland are called lands of the midnight sun; around the December solstice the same areas have days when the Sun does not rise at all. At the poles themselves, roughly six months of daylight are followed by roughly six months of darkness. Taken together, the ideas of this part form one chain: rotation gives day and night; revolution with a tilted axis that keeps its direction gives the seasons, the solstices and equinoxes, and the changing length of days. Part 2 uses the same motions to fix the special parallels and to measure time.

Exam traps

• The Earth is nearest the Sun (perihelion) in early January, during the northern winter, and farthest (aphelion) in early July. Distance does not cause the seasons; the tilt of the axis does.
• The axis is tilted about 23½° from the perpendicular to the orbital plane, which is the same as an angle of about 66½° with the plane itself. Questions swap the two figures.
• The Earth rotates from west to east; the Sun only appears to move from east to west.
• On 21 June the vertical rays fall on the Tropic of Cancer, not the equator; the equator gets them at the equinoxes (21 March and 23 September).
• A solar day (24 hours) is slightly longer than one turn measured against the stars (about 23 hours 56 minutes), because the Earth also moves along its orbit each day.
• The Earth bulges at the equator: its equatorial radius (about 6,378 km) is longer than its polar radius (about 6,357 km). Its true shape is called a geoid.
• Day and night are about equal everywhere only at the equinoxes; on the equator they stay close to 12 hours each all year.
• Pluto has been classed as a dwarf planet since 2006 because it has not cleared its orbit; having or not having moons is not part of the definition.

Recap

• Eight planets; four inner rocky and four outer giants; Pluto a dwarf planet under the IAU's 2006 definition.
• The Earth is a geoid: equatorial radius about 6,378 km, polar about 6,357 km; about 150 million km from the Sun.
• Rotation: west to east in 24 hours (solar day); gives day and night and the basis of time.
• Revolution: about 365¼ days; leap year; perihelion early January, aphelion early July.
• Tilt of 23½° (66½° with the orbital plane), always pointing the same way, causes the seasons.
• Solstices about 21 June and 22 December; equinoxes about 21 March and 23 September.
• On the equator day and night stay near 12 hours; the yearly swing in day length grows towards the poles.
• Beyond the polar circles: the midnight sun around the June solstice and sunless days around December; about six months of light and six of dark at the poles.

Practice questions

Sources

• NASA Science: Earth facts, the Sun, planetary fact sheets, and seasons and solstices explainers (National Aeronautics and Space Administration, US government) — https://science.nasa.gov/earth/facts/
• International Astronomical Union, Resolutions B5 and B6 of the XXVI General Assembly, Prague, August 2006 (definition of a planet; Pluto) — https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf