Sunflowers are a sign of summer. But, astronomers can tell you the exact day and even the moment that summer officially begins. This moment in time is called summer solstice .

In 2009, in the Northern Hemisphere, summer solstice is on June 21 at 1:45 EDT (eastern daylight time).

While this is called the longest day of each year, it is only about one minute longer than the previous day. Actually summer solstice marks the point when daylight length begins to decrease.

Day length preceding the summer solstice increases. Day length following the summer solstice decreases until December 21 (first day of winter), when it starts to increase again. In the Southern Hemisphere, these dates are reversed, with summer starting in December and Winter in June.

For information about why the amount of daylight changes during the year, see SUMMER SOLSTICE.