The seasons

The plane of the Earth's equator forms a 23 degree 27' angle with the ecliptic's plane.
This angle represents the latitude of the tropics :

Tropic of Cancer : 23 degrees 27' lat. North

Tropic of Capricorn : 23 degrees 27' lat. South

Also, the Earth's axis forms a 23 degree 27' angle with a vertical of the ecliptic. it's then easy to emphasize a other angle that determines the latitude of the Arctic circles :

Arctic circle : 66 degrees 33' lat. North

Antarctic circle : 66 degrees 33' lat. South

It's the revolution of the Earth and the inclination of the Earth's axis that results in seasons.
The summer
The summer solstice is on the 21st or 22nd of June when the Sun's rays are perpendicular to the Tropic of Cancer,
which means that it's summer in the Northern hemisphere and winter in the Southern hemisphere.
~ In the Northern hemisphere, Day > night;
~ In the Southern hemisphere, Day < night;
~ at the equator, day = night.
The winter
The winter solstice is the 21st or 22nd of December when the Sun's rays are perpendicular to the tropic of Capricorn
which means that it's winter in Northern hemisphere and summer in the Southern hemisphere.
~ In the Northern hemisphere, day < night;
~ In the Southern hemisphere, day > night;
~ At the equator, day = night
Spring and Autumn
The ecliptic's two plans touch in two points of the Earth's orbit.
These two points correspond to the equinox and mark the beginning of spring and autumn, respectively on the 21st of March and the 23rd of September.
Night is as long as day during the equinox everywhere in the world.

Webmaster: Antoine van Ruymbeke

Last modifications: 3 July'99