What are the seasons?
The seasons are a result of the 23.5 degree inclination of Earth's rotational axis in relation to the plane around which it orbits the Sun. This tilt means that throughout Earth's orbit around the sun (our calendar year) certain areas of the globe are tilted towards the Sun, while other areas are tilted away from it.
This creates a difference in the amount of solar radiation (or sunlight) that reaches different parts of the Earth and thus creates the global cycle of fluctuations that we know as the seasons.
This can be seen in the diagram where the northern hemisphere is leaning towards the Sun, while the southern hemisphere is leaning away - this is summer in the northern hemisphere and winter in the southern hemisphere.
This point represents the summer solstice in the northern hemisphere (20/21 June) when every point north of the arctic circle faces the sun for a full 24 hours and is the longest day for the northern hemisphere.
The exact opposite is true for the winter solstice (21/22 December) when every point north of the arctic circle is in total darkness for a full 24 hours and the northern hemisphere experiences its shortest day.
In between the two solstices we experience the equinoxes which mark the beginning of spring and autumn. At equinox, the plane of Earth's equator passes the exact centre of the sun. This means that the Earth is neither tilted towards or away from the Sun.
When this occurs on 20/21 March in the northern hemisphere it marks the point at which the northern hemisphere begins to tilt towards the Sun and consequently the beginning of the astronomical spring. Similarly on 22/23 September, the equinox occurs again this time marking the point at which the northern hemisphere begins to tilt away from the Sun and consequently the beginning of the astronomical autumn.
Another notable attribute of equinox is that the night and the day are of roughly equal length. The word 'equinox' is derived from the Latin aequus (meaning 'equal') and nox (meaning 'night').
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