Why is winter colder than summer?
The seasons, spring, summer, fall and winter, occur because of the inclination of the earth's axis while the earth moves around the sun...

It is a common fallacy to suppose that it is colder in winter because the sun is then farther from the earth. As a matter of fact the sun is farthest from the earth in summer and closest in winter. In astronomy the point of nearest approach of a body to the sun is called perihelion, and the earth is at perihelion about January 3. Likewise the earth is farthest from the sun early in July. The seasons, spring, summer, fall and winter, occur because of the inclination of the earth's axis while the earth moves around the sun; that is, the seasons are produced mainly by the relative positions of the earth's axis in respect to the sun.

The principal reason for the difference in temperature between summer and winter is the angle at which the rays of the sun strike the earth, coupled with the longer duration of daylight in summer. The imaginary line from pole to pole which is known as the axis and around which the earth rotates is slightly tilted. As the earth moves in its annual orbit the Northern Hemisphere is tilted toward the sun half the year and away from it the other half. It is summer in the Northern Hemisphere when that portion of the earth is tilted toward the sun. The sun, while actually farther away then, is more directly overhead. It is winter in the Northern Hemisphere when that part of the earth is tilted away from the sun. Then the sun, while actually closer to the earth, is lower in the sky and its rays strike the earth at a much greater slant than they do in summer. This means that the rays of the sun must pass through a greater distance of atmosphere in the winter.

In the Southern Hemisphere the seasons are reversed. It is winter there while it is summer in the Northern Hemisphere, because the Southern Hemisphere is inclined away from the sun while the Northern Hemisphere is inclined toward it. There would be no seasons if the earth's axis were perpendicular to the plane of its orbit. Perpetual summer would exist on a belt near the Equator, while perpetual winter would exist on the parts nearer the poles.