Meteosat view of Earth's day–night boundary on the December solstice, with more of the Southern Hemisphere in daylight.

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Model scale

Earth enlarged. fixed axial direction. T compares actual sizes and distances.

Declination 13.5° · London 14.3 h daylight · Sydney 10.8 h · geometric horizon. Gold: tropics · blue: polar circles. Tilt 23.4°; distance 1.000 AU. Set tilt to zero to compare 12-hour days throughout the orbit.

Why seasons happen

Explain how axial tilt changes daylight and solar angle, then predict which hemisphere experiences summer at a chosen point in the year.

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Why it happens

Earth’s tilted axis changes the angle of sunlight and the length of the day as Earth travels around the Sun. When one hemisphere tilts towards the Sun, the other experiences winter.

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Meteosat-9 SEVIRI, 21 December 2010. The tilted day–night boundary gives the Southern Hemisphere longer days. Axial tilt, rather than a change in Earth–Sun distance, controls this seasonal illumination.

Try a comparison

Set tilt to 23.44° and pause at the June direction, then December. Compare London with Sydney and trace the fixed axis direction.

Reading the result

The adjustable orbit is an experiment for isolating tilt and distance; its calendar alignment is not Earth’s actual orbit.

Film

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Why we have seasons — Khan Academy.

A common misunderstanding

Summer happens because Earth is closer to the Sun.

Earth’s axial tilt changes both the angle of sunlight and the length of daylight. The hemisphere tilted towards the Sun has summer while the other has winter. Earth is closest to the Sun in early January, so orbital distance cannot explain their opposite seasons.

Cases

Exam language

Axial tilt

The angle between a body's rotation axis and the perpendicular to its orbital plane. Earth's tilt is about 23.44°, and its axis keeps nearly the same direction over one orbit.

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Meteosat-9 observed this day–night boundary on 21 December 2010. Earth's tilted axis puts the subsolar point south of the equator and gives southern latitudes longer days. Declination is the Sun's angle north or south of the equatorial plane; the tilt of the boundary is observational evidence, not an angle scale.

Terminator

The boundary between the illuminated and dark hemispheres of a planet or moon. Earth's rotation carries places across it; changing solar declination alters its seasonal orientation.

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Meteosat-9 SEVIRI, 20 March 2011. The terminator passes close to both poles around the equinox; refraction and the Sun's finite disc mean observed daylight is not exactly twelve hours everywhere.

Solstice

The time when the Sun reaches its greatest northerly or southerly declination. It corresponds to the longest or shortest daylight period of the year outside the tropics. Opposite hemispheres experience opposite seasonal solstices.

Equinox

The instant when the Sun crosses Earth's equatorial plane. Daylight is close to twelve hours around this date, but the Sun's disc, refraction and latitude affect observed day length.

Declination

The angular position of the Sun north or south of the celestial equator, equivalent to the latitude of the subsolar point. It varies between about 23.44° north and south through the year and helps determine noon altitude and daylight duration.

Perihelion

The point in an orbit where a body is closest to the Sun. Earth reaches perihelion in early January. The changing distance affects incoming solar energy, while Earth’s axial tilt explains the opposite seasons of the two hemispheres.

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Hubble observed Comet ISON on 2 November 2013, before its closest approach to the Sun. Perihelion names that closest orbital point; the coloured filters show coma and tail structure, not a map of the orbit.

Revolution

The movement of one body around another along an orbit. Earth completes a revolution around the Sun in approximately one year. Rotation is the separate movement around its own axis, which produces the daily cycle of daylight and darkness.

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SDO recorded Venus crossing the Sun's disc in June 2012. A transit is an observable alignment of orbiting bodies. Revolution means motion around another body; this time sequence is not a scale drawing of the orbits.

Year0.40Axial tilt23.4°Orbit stretch0.00