The angle of tilt A spherical Earth Summer, autumn, winter and spring

The four seasons of the year, as we know them in Southern Australia, are the product of three factors which between them determine the amount of sunlight which strikes the Earth at any time.

These factors are:

1. The angle of tilt of the Earth's rotation with respect to its orbit around the Sun.
2. The spherical shape of the Earth.
3. The orbit of the Earth around the Sun.

The angle of tilt
The Earth is tilted at an angle of 23.5°, through its poles, to the perpendicular of the Earth's orbit around the Sun. As the Earth moves along its orbit around the Sun, the tilt of the Earth affects the length of day and night, and thus the number of hours of sunlight. The spherical shape of the Earth also affects the angle at which the sunlight strikes the ground.

Sun heights for Melbourne

The warmth we associate with summer is due to long hours of sunlight coming from the Sun high in the sky warming the Earth's surface.

The cold of winter conversely is due to the Sun being lower in the sky with fewer hours of sunlight to warm the Earth.

If the Earth was not tilted then there would be no seasons, only very hot and humid conditions at the equator, with conditions getting cooler as one moved away from the equator towards the poles. Think how this has affected evolution of life on our planet - many of the cycles, great migrations, and habits of both animals and plants are very much dependent on the ebb and flow of the seasons.

A spherical Earth
Over a year, those parts of the Earth where the Sun is directly overhead move from the Tropic of Capricorn in the south, over the equator to the Tropic of Cancer in the north, and then back again to the Tropic of Capricorn in one complete orbit of the Earth around the Sun.

South of the Tropic of Capricorn, the Sun will always be seen to the north, while north of the Tropic of Cancer, the Sun is always seen to the south.

The two tropic lines are defined as the furthest north and south of the equator where the Sun appears directly overhead at some time of the year. They are 23.5° from the equator, the same as the angle of the tilt of the Earth.

The seasons

Summer, autumn, winter and spring
In the Southern Hemisphere, the four seasons - summer, autumn, winter and spring - are regarded as beginning on the two solstice and equinox. Summer starts on the longest day of the year - summer solstice, December 22 and autumn begins on March 21 - the vernal equinox (12 hour day and night). The winter solstice on June 21 heralds winter and the autumnal equinox on September 22 starts spring. Note:- the names of the equinox come from the Northern Hemisphere season changes, but we use them all the same in the south!

In the Northern Hemisphere, summer and winter are reversed, as are spring and autumn.

The seasonless Earth

Seasons in the Southern Hemisphere

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