Wednesday was the first day of summer, also known as the summer solstice. It's the longest day of the year (and the shortest night).
The actual moment of the solstice occurred about 7:09 p.m. Wednesday, while the sun sat directly above the Pacific Ocean to the west of Hawaii.
As you may remember from your grade school science lessons, the seasons and the changing lengths of the day and night throughout the year are a result of the Earth's axial tilt.
Try to visualize the Earth's orbit around the sun as an elliptical path on an imaginary plane in space. As the Earth rests in that plane, its north and south poles—the ends of its axis—do not point straight "up" and "down." The axis is instead about 23.4 degrees off the "vertical."
As a result, the northern and southern hemisphere do not receive equal amounts of sunshine throughout the year. Right now, the northern hemisphere is "leaning" towards the sun. From now until the winter solstice on Dec. 21, as the Earth continues around the sun, that tilt in the planet's axis will be "leaning" our hemisphere less towards the sun each day.
If not for the tilt of the Earth's axis, we would not have seasons. The day and night would be exactly the same length, year round. The northern and southern hemispheres would share the sun's light equally. Right now, that only happens on the days of the spring and fall equinoxes (March 20 and Sept. 22, this year).
Perhaps appropriately, as we bask in a whopping 15 hours and two minutes of daylight, Wednesday was expected to be the hottest day of 2012 in some areas of the U.S. Today could be hotter still.
If the heat gets to be a bit too much, consider the flip side of the solstice: For our friends in the southern hemisphere, it's the winter solstice. In Punta Arenas, Chile, the high Wednesday was predicted in the mid- to upper-30s.