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.
