A: We Earth folk know good and well what an equinox and a solstice are. An equinox is when the day length roughly equals night length (occurring about March 21 and September 23). The two solstices (on or near June 21 and December 22) give us, respectively, the longest daylight and the longest night in the year. (Related: Why day, night not equal on equinox).

The tricky stuff is imagining a spaceman’s view as shown in the figure. (Related: figure of Earth orbiting the Sun.) 

But it’s basically simple. A spaceman can see that Earth’s rotation axis tilts. As Earth orbits Sol, the direction of the tilt remains fixed. So, sometimes one hemisphere tilts mostly toward the Sun and gets more sunshine than the other one. The first hemisphere knows summer and the other experiences winter. In between, neither hemisphere tilts much sunward and that’s spring or fall.

We get almost equal length days and nights on the equinoxes because then neither hemisphere tilts toward the Sun. The tilt effect is gone. The summer’s solstice gives the longest day because then our hemisphere tilts directly toward the Sun and we see the Sun the longest. At the pole, we see the Sun all day (and a day lasts the summer). Likewise, the winter’s solstice harbors the longest night because then our hemisphere tilts directly away.

USA Today: Earth’s seasons graphic

(Answered May 30, 2003)