Summer arrives in the northern hemisphere on June 21st -- the summer solstice, which is also the longest day of the year.
But strangely enough, the solstice does not mark the day that the Sun rises the earliest. In fact, the date of earliest sunrise is about a week before the solstice. And the latest sunset occurs about a week after the solstice.
The mismatch occurs for two reasons.
First, Earth's orbit around the Sun is not a perfect circle. Instead, it's slightly elliptical, so Earth travels at different speeds during the course of a year: It moves fastest when it's closest to the Sun, and slowest when it's farthest away. Right now, Earth is near the far point of its orbit, so it's traveling more slowly than average. But it spins on its axis at the same rate all the time, no matter where it is in its orbit. This difference in orbital motion and rotation affects the precise time of sunrise and sunset.
And second, Earth doesn't stand perfectly upright as it rotates. Instead, its axis is tilted 23-and-a-half degrees. This also affects the time of sunrise and sunset.
As a result, the earliest sunrise of the year doesn't occur on the summer solstice. Instead, it happens before. The exact date depends on your latitude. At mid-northern latitudes -- a line from around Baltimore to Kansas City to San Francisco -- the earliest sunrise occurs about a week before the solstice -- which means it's coming up in the next few days.
Script by Ken Croswell, Copyright 2010
For more skywatching tips, astronomy news, and much more, read StarDate magazine.