Today is 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 will occur at about 6:09 p.m. this evening, while the sun sits 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 tonight until the winter solstice on December 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 September 22, this year).
Perhaps appropriately, as we bask in a whopping 15 hours, two minutes of daylight, today is also expected to be the hottest day of 2012 to date in our area. Tomorrow could be hotter still.
If the heat gets to be a bit too much, consider the flip side of today's solstice: for our friends in the southern hemisphere, today is the winter solstice. In Punta Arenas, Chile, the high today will be in the mid to upper 30s.
If you're interested in astronomy, check out...
- Milwaukee Astronomical Society, 18850 W. Observatory Rd., New Berlin
- Wehr Astronomical Society, 9701 W. College Ave., Franklin
- More groups around the state at wisconsinastronomy.org.
The observatory is on the roof of the Pewaukee Public Library. Public presentation are the second Saturday of every month.