We call an imaginary line running through the Earth from pole to pole, that Earth's axis. We say that the Earth spins on its axis, one spin being 1 day. But the Earth's axis is not perpendicular to the plane of the solar system. Instead, it is tilted 23.5 degrees. It is this tilt that provides us with seasonal changes. The best way to understand it is to understand the effects as we see them from Earth.
Observers in North America notice that the Sun is higher in the sky in summer than in winter. These changes in position result from an apparent motion of the Sun north and south of the celestial equator (a projection of the Earth's equator into space.) This apparent motion is the result of that 23.5 degree axial tilt.
On the Autumn and Spring Equinoxes the Sun crosses the celestial equator. One these days there is an equal amount of day and night. The Sun rises due east and sets due west.
As the summer solstice approaches on June 21st, the days get longer and the Sun rises higher in the sky at noon. It is also rising and setting further north of east and west. At this time, the northern hemisphere is pointing toward the Sun.
As the winter solstice approaches on December 21st, the days are shorter and the Sun is lower in the sky at noon. The sun is also rising and setting south of east and west. During this time, the Earth is pointed away from the Sun.
Copyright © 1997 Kathy A. Miles and Charles F. Peters II