The Reason for the Seasons
There are two solstices and two equinoxes each year which mark four important points in Earth’s orbit around the Sun. It is the Earth’s position in its orbit and orientation of its tilted axis at these points that defines the seasons. The axis is an imaginary line passing through the Earth from the North Pole to the South Pole. The way the Earth rotates means that certain areas of the globe are tilted towards the Sun while other parts are tilted away from it so there are different levels of sunlight reaching different parts of the Earth.
The Earth’s axial tilt is about 23.4° and it is because of this that the Sun’s rise and set positions vary by up to 23.4° north or south of due east or west throughout the course of a year.
The winter and summer solstices are extreme points when the northern and southern hemispheres receive either minimum or maximum light. Solstices occur when the Sun is furthest from the celestial equator, making the longest day and shortest night in summer, and the shortest day and longest night in winter. The celestial equator is the projection into space of the Earth’s equator and is an imaginary circle dividing the northern and southern hemispheres.
The Sun reaches the summer solstice in Australia around 21 December each year and marks the astronomical beginning of summer for the southern hemisphere. This is when the South Pole is most fully orientated towards the Sun. The Sun reaches winter solstice around 21 June each year and marks the astronomical beginning of winter for the southern hemisphere. This is when the North Pole is most fully orientated towards the Sun.
Wherever you are at the spring and autumn equinoxes (except the North and South Poles) the Sun rises due east and sets due west. These points mark the intersection of your horizon with the celestial equator. As these are the only times in the year that the Sun rises due east and sets due west, viewing the location of the Sun on the horizon with reference to surrounding landmarks is a great way of knowing east and west from your own home.
Earth has seasons because its axis of rotation is tilted
Image credit: Lisa Mitchell
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