SEATTLE - The Earth tilts on an axis as it orbits around the sun. In the Northern Hemisphere, Dec. 21 is the day when we are tilted the furthest away from the sun -- thus the sun appears lowest in the sky (about 19 degrees above the horizon at noon for Seattle). That means the sun spends the least amount of time above the horizon that day.
It's exactly opposite in the Southern Hemisphere, where the Earth is tilted closest to the sun, so for them, today's the first day of summer. So while we're bundling up, Australians hit the beach during the holiday season.
It works the opposite for summer on June 21. That's when we're tilted closest to the sun, and get the longest day of the year.