# Seasons

The Earth orbits the Sun once a year with its axis at an angle to the plane of the orbit. Because of this the northern hemisphere is tilted towards the Sun during part of the orbit and away from it for another part.

It is this tilting that gives us the seasons. In winter we tilt away from the Sun and in Sumner we tilt towards it. It is not the difference in distance between us and the Sun that matters but the angle at which sunlight strikes the ground, the same "amount" of sunshine covering a larger area in winter than in summer. The sunlight is concentrated onto a much smaller area in summer and so it is warmer.

If the axis of the Earth was not tilted compared with the plane of its orbit we would not have any seasons!

The two thick lines in the diagrams show the relative areas that would be covered by the same amount of solar energy, first in mid summer in the northern hemisphere and then in mid- winter.

(The actual areas are proportional to the squares of these two lengths)

This means that the same amount of solar energy is spread over an area roughly eleven times larger in the winter than in the summer at the latitude shown. This is why summers are generally warmer than winters.

The next two pictures show the effects at the surface of the Earth. The first picture shows light coming from the Sun at noon in mid-summer near my house at latitude 51oN in the Northern Hemisphere. At noon the Sun is at its highest in the sky and the angle between the sunlight and the ground is about 62o. The Sun is so far away that all the rays of sunlight are shown parallel.

The pool of light on the ground shows the area covered by solar energy between the two rays A and B.

The next picture shows the situation at noon in mid-winter. The angle that the sunlight makes with the ground is now only 16o. This time the area covered by the same amount of solar energy is much larger.

An accurate calculation for latitude 51oN shows the area to be nearly three times larger in mid-winter than it is in mid-summer.

© Keith Gibbs 2013