As time goes by, spring turns to summer, summer turns to autumn, autumn turns to winter and eventually winter becomes spring again. The years roll by and the Earth keeps spinning around the Sun.
Why, though, do these different seasons of the year occur? The simple answer is that it's due to the Earth being tilted at an angle of about 23.5° and to its rotation around the Sun.
Proving this at Home
The following experiments should give you a better understanding. You will need:
A spherical piece of fruit. An apple with the stalk still on is particularly good, although an orange or lemon will also do the job1.
A large spacious darkened room.
A lamp or torch in the middle of it.
Imagine your piece of fruit to be the Earth, while the light is your Sun. The stem or core of the fruit represents the Earth's core and the North and South Poles.
Experiment One: Understanding Night and Day
Holding your fruit level with the lamp, notice how the side nearest the lamp is illuminated, while the side further away is in darkness. Try spinning the 'Earth' around its core. Picking a particular point on the surface will allow you to track a town through night and day.
Experiment Two: Earth with No Tilt
Holding the 'Earth' upright with the stem on the top, spin the 'Earth' again. Notice that the whole side of the 'Earth' facing the 'Sun' has sunlight from the 'North Pole' to the 'South Pole'. Again, if you pick a point anywhere on the surface, you will notice that it always receives light at some time during the rotation.
Experiment Three: Tilted Earth
Repeat the experiment as before, but this time tilt the pole of your sphere by about 45° away from or towards the light source. Notice now how despite spinning, the area around one of the poles has sunlight all the time, while the other stays in darkness.
Experiment Four: Creating the Seasons
Now, keeping the 'Earth' tilted at the same angle and in the same direction move to the opposite side of your 'Sun'. Spinning it again you will find that the pole that was previously in darkness now has sunlight all the time. Walking around the 'Sun' you will find how this process of change has occurred and if you can imagine yourself as being in a 'town', you will find that the Sun moves higher or lower in the sky.
Geography of the Earth
There are a number of imaginary lines that circle the Earth perpendicular to its axis.
The Equator runs around the centre of the Earth splitting it equally north and south.
The Tropic of Cancer encircles the Earth 23.5° north of the Equator. During the seasons, this is the furthest point north that the sun can appear to travel.
The Tropic of Capricorn encircles the Earth 23.5° south of the Equator. It is the furthest point that the sun appears to travel south.
The Arctic Circle is 23.5° from the North Pole. When the North Pole is pointing away from the sun, all areas inside this circle stay in perpetual darkness.
The Antarctic circle is 23.5° from the South Pole. Again it represents an area that the sun never reaches during certain times of the year.
There are four key dates in the year. These occur on 21 March, June, September and December.
21 March is the Vernal Equinox and represents the beginning of spring in the northern hemisphere. It is the point at which the Sun crosses the Equator as it travels northwards. At this time day and night are of equal length, hence equinox - 'equal night'.
21 June is the Summer Solstice or Midsummer's Day in the northern hemisphere, when the Sun has reached its further point north of the Equator (Tropic of Cancer). As more sunlight reaches the northern hemisphere the days last longer than the nights. The day after, the sun begins to travel south and the days start to shorten.
21 September is the Autumnal Equinox when the sun crosses the Equator going south.
21 December is the Winter Solstice and is shortest day in the northern hemisphere; it is the middle of summer in the southern hemisphere. The Sun has reached its furthest point of travel south - the Tropic of Capricorn.
The seasons are, of course, reversed in the southern hemisphere - autumn in March, winter in June, spring in September and summer in December.
Precession of the Equinox
In the experiment, you kept the Earth's pole pointing in the same direction. To all intents and purposes this is correct. However, it does wobble ever so slightly. This wobble takes 25,765 years for the Earth's pole to trace a full circle. This phenomenon is known as the Precession of the Equinox.
In terms of the seasons it has no real effect. In fact, about the only effect is that Christmas Day (25 December) no longer falls on the Winter Solstice as it used to when it was first initiated.
Secondly, it serves as a good argument against astrology, as the sign of Aries which begins on around 24 March is supposed to be as the Sun enters the constellation of Aries 'the Ram'. However, precession has led to 24 March now being the date when the Sun enters the constellation of Pisces.