Everyone knows that earth is tilted over on its axis by 23½°. That is to say, that the axis of rotation is out of alignment by the axis perpendicular to the planet’s plane of orbit about the sun. This is what causes the seasons, and what delineates the major climactic zones. On June 21st the North Pole is inclined toward the sun as much as it ever is, and summer begins. It seems as if on the line of latitude 23½° North, the sun passes directly overhead. Similarly, on March 21st the sun is directly over the equator (heading north) while on September 21st it’s in the same place again (southbound). On December 21st the South Pole is pointing most closely toward the sun, and the Northern Hemisphere is enjoying winter.

Thus it is that the major climactic regins of earth are defined as the Tropics, from the equator to 23½° North or South latitude, and the Arctic and Antartic, from the poles to 66½° North or South latitude. (The Arctic regions stretch from the poles down toward the equator 23½°.) The Temperate zones are everything between the tropics and the arctics.

This explains why it is in the polar regions that one experiences the midnight sun, or sunless day. In the polar summer the sun never sets because–since the earth is tilted so far toward the sun–the sunlight reaches up over the pole and still provides daylight, even at nighttime hours. The limits of this phenomenon are the Artic and Antarctic Circles, because this is how far the sunlight can reach. By extension from the foregoing, the closer one lives to the arctic regions, the longer one’s day gets in the summer time, and the longer are the nights in the winter time.

So it is that all of this also explains why June 21st is the longest day of the year.
I speak from a Northern Hemispheric point of view. Basically, all this works for the Southern Hemisphere, too, except that you have to rotate your calendar half way around, and stand on your head while doing the calculations.

But there is a catch here. “Most” of us live in the Temperate zone. So it makes sense that June 21st is the longest day of the year. For, even though the sun doesn’t get directly overhead on that day–it’s still to the south of us–it gets as close as it ever will. We have the most sunlight when the sun is as close to our position as it’s going to get.

What about people who live in the tropics? By definition, on June 21st, the sun will be past them, that is, it will pass north of straight overhead. So if you live in the tropics, what is the longest day of the year for you?