For many folks, summer began on Memorial Day weekend, and to meteorologists, it started June 1. But astronomically, it arrived at 3:38 this morning local time.

To understand why that exact time is so significant, we need to go back to the reason for seasons in the first place: the earth revolves around the sun while at the same time being tilted on its axis. Here's the situation as it looked six months ago. The earth was at a point in its orbit around the sun where the axis of the North Pole was pointing away from the sun at as large an angle as possible - as a result, the Northern Hemisphere got a minimum of direct sunshine, signaling the beginning of winter here.

Now, six months later - in fact, at exactly 3:38 this morning our time - the earth had traveled around the sun and arrived at precisely the point in its orbit where the axis of the North Pole was pointing most directly toward the sun. As a result, the sun today got as high in the sky as it ever gets, and therefore we'll have the most hours of daylight - about 15 - of any day of year.