We have all been taught that the seasons are caused by the tilt of the Earth's axis of rotation - the 23.4° offset of the axis from a direction perpendicular to the Earth's orbital plane. The direction of the rotational axis stays nearly fixed in space, even as the Earth revolves around the Sun once each year. As a result, when the Earth is at a certain place in its orbit, the northern hemisphere is tilted toward the Sun and experiences summer. Six months later, when the Earth is on the opposite side of the Sun, the northern hemisphere is tilted away from the Sun and experiences winter. The seasons are, of course, reversed for the southern hemisphere.
The solstices mark the two dates during the year on which the Earth's position in its orbit is such that its axis is most directly tilted either toward or away from the Sun. These are the dates when the days are longest for the hemisphere tilted toward the Sun (where it is summer) and shortest for the opposite hemisphere (where it is winter).
However, there is a complication. The Earth's orbit is very close to being a perfect circle, but not quite. It is somewhat elliptical, which means that the distance between the Earth and the Sun varies over the course of the year. This effect is too weak to cause the seasons, but it might have some influence over their severity. The remainder of this page explains this possibilty.
The Earth reaches perihelion - the point in its orbit closest to the Sun - in early January, only about two weeks after the December solstice. Thus winter begins in the northern hemisphere at about the time that the Earth is nearest the Sun. Is this important? Is there a reason why the times of solstice and perihelion are so close? It turns out that the proximity of the two dates is a coincidence of the particular century we live in. The date of perihelion does not remain fixed, but, over very long periods of time, slowly regresses (moves later) within the year. There is some evidence that this long-term change in the date of perihelion influences the Earth's climate.
The whole explanation
