Summertime, summertime!

Welcome to summer!^* Today is the summer solstice, the longest day of the year. Today the sun will set about 15 hours and 11 minutes after it rises. Compared to 9 hours and ten minute back in December at the winter solstice, that's 6 extra hours of day! A whole quarter of a day that is daytime instead of nighttime - amazing!

What's also amazing is the lack of a good explanation on the web for WHY the days are longer in the summer. I googled it, and basically everybody just says, "Because of the tilt of the Earth's axis." That's clear as mud, huh?

So, here we go - I will do my best to give an explanation that is clearer than mud.

First, find your location on the globe, and find the closest latitude line (latitude lines run around the globe east-west). As you spin the globe, your location will go around, basically following the latitude line, thus we will use the latitude line to represent your path as the Earth turns. You travel this path at a constant speed, taking one 24-hour day to go all the way around.

Second, realize that, at any time, the sun is shining on half of the globe. If the globe is not tilted toward or away form the sun^**, then exactly half of any path (any latitude line) is lit by the sun, as in the image here. Since half of your path is lit, half of your 24-hour day is daytime. This is what happens at the equinoxes (spring and autumn).

At the summer solstice, however, your hemisphere is tilted toward the sun. The northern hemisphere is currently tilted toward the sun, as in the second image. Look at your path now - each latitude line in the northern hemisphere is more than half in the light. Thus you spend more than half of your day (in fact, about 63% of your day today here in the Quad Cities) as daytime.

It's also interesting to note what happens at the equator - no matter which way the Earth is tilted, someone on the equator always spends half their day (12 hours) with the sun up.

So, happy summer! Enjoy your extra daylight, and don't forget that you are actually enjoying astronomy!

_{*in the northern hemisphere
**The axis is always tilted, just in this case it is tilted "sideways" neither toward the sun nor away from it.}