happy solstice everyone! the longest day of the year if youre in the north, and the shortest if youre in the southern hemisphere! the equator is not exactly lined up with the plane of earth's orbit around the sun. if the earth was lined up with this plane, we would not experience seasons in the strong way that we do. the line between the north pole and the south pole is tilted from the line connecting the earth and the sun by an angle of 23.5 degrees. this angle, relative to the plane of earth's orbit around the sun, is always the same! this means that once a year the north pole points towards the sun (summer solstice) and it points away from the sun 6 months later (winter solstice in the northern hemisphere).
imagine that you were on the sun, watching the earth as it moved around you. here's a quicktime movie showing what you would see (of course the earth also should spin 365.25 times during the course of this movie, but that would be confusing, so just focus on the wobbling axis!)
the tropic of cancer and the tropic of capricorn are defined as the latitudinal lines that are 23.45 degrees north and south of the equator, respectively. when standing anywhere on the tropic of cancer today, june 21st, the sun will appear to be at the zenith... directly overhead. the sun appears at the zenith when standing at the tropic of capricorn on the winter solstice!
another interesting question to ask is why the earth spins at all? it has to do with the conservation of angular momentum in the original gas cloud as it collapsed as our sun began to form along with all the planets around it.
my sister always has great solstice parties in her small ohio town of yellow springs. i usually try to be home for the rockin winter solstice party and maybe i will be this year!