If the Earth's axis were perpendicular to the ecliptic, as in the drawings here, the Sun's position in the sky would be halfway between the celestial poles, and its daily path, seen from any point on Earth, would stay exactly the same, day after day.

  Actually, the axis of rotation is not perpendicular to the plane of the orbit. Instead, it points towards the celestial pole, near the direction of the pole star, and makes an angle of about 23. 5 degrees with the perpendicular to the ecliptic. That makes life a lot more interesting--we get longer days in summer, longer nights in winter.

Summer and Winter

  But because of the tilted axis, as each point on Earth is carried on its daily trip around the rotating Earth, the part of the trip spent in daylight (unshaded part of the drawing) and in the shadow (shaded) are usually not equal. North of the equator, day is longer than night, and when we get close enough to the north pole, there is no night at all. The Sun is then always above the horizon and it just makes a 360-degree circuit around it. That part of Earth enjoys summer.

  A mirror-image situation exists south of the equator. Nights are longer than days, and the further one gets from the equator, the larger is the imbalance--until one gets so close to the pole that the sun never rises. That is the famous polar night, with 24 hours of darkness each day. In that half of the Earth, it is winter time.

  Half a year later, the Earth is on the other side of the Sun, that is, the Sun's position in the above drawing should be on the right, and the shaded part of the Earth should now be on the left (light and dark portions in the drawing switch places). The Earth's axis however has not moved, it is still pointed to the same patch of sky, near the star Polaris. Now the south pole is bathed in constant sunshine and the north one is dark. Summer and winter have switched hemispheres.

  A big difference between summer and winter is thus the length of the days: note that on the equator that length does not change, and hence Spring, Summer, Fall, and Winter do not exist there (depending on weather patterns, however, there may exist a "wet season" and a "dry season"). In addition (as the drawing makes clear), the Sun's rays hit the summer hemisphere more vertically than the winter one. That, too, helps heat the ground, as explained further in section #4, "The Angle of the Sun's Rays."

  At equinox, the situation is as in the first drawing, and night and day are equal (that is where the word "equinox" comes from)

Some interesting facts

  If June 21 is the day when we receive the most sunshine, why is it regarded as the beginning of summer and not its peak? And similarly, why is December 21, the day of least sunshine, the beginning of winter and not mid-winter day?

  Blame the oceans, which heat up and cool down only slowly. By June 21 they are still cool from the winter time, and that delays the peak heat. Similarly, in December the water still holds warmth from the summer, and the coldest days are still ahead.