Source: The Daily Gazette
http://www.dailygazette.com/news/2009/jun/21/0621_solstice/
For the sun, it’s the big day out.
Today is the summer solstice, the longest day of the year and the beginning of summer on the Gregorian calendar. Some will greet the great day with celebrations; others may just be interested in all of the scientific facts under the sun.
According to the National Weather Service in Albany, the sun was set to rise at 5:16 a.m. and set at 8:37 p.m. for a total of 15 hours and 21 minutes of sunlight. In the Northern Hemisphere, the solstice was observed at 1:45 a.m.
“That’s the moment the sun reaches its highest point against a stellar background,” said Richard Monda, a Schenectady astronomer whose “Star Talk” column is published each month in The Sunday Gazette.
The solstice is easy to understand. Monda said June 21 is the time when, in the Northern Hemisphere, the sun reaches its highest point in the sky.
“Every day, the sun makes a little higher arc in the sky until the first day of summer, then it makes a lower arc across the sky until the first day of winter,” Monda said. “Like a child on a swing, when it reaches the end of its travel, it seems to stay still. And that’s where the word ‘solstice’ comes from.”
“Solstice” is derived from Latin words that mean “sun” and “stand still,” according to published reference sources.
“The sun is overhead on the first day of summer at the Tropic of Cancer, and that’s as far north as one will ever see the sun overhead,” Monda said. “North of the Tropic of Cancer, the sun will never be overhead.”
The Tropic of Cancer, one of five major circles of latitude that mark maps of the Earth, indicates the northern edge of the tropical zone, crossing Mexico. All of the United States is north of the line.
Astronomy students know other interesting numbers come at solstice times.
“If you go north of the Arctic Circle on the first day of summer, anywhere above the Arctic Circle, you’ll have 24 hours of daylight,” Monda said.
The daylight and darkness durations will switch six months from today. On Dec. 21, the winter solstice and the first day of winter, nine hours of sunlight and 15 hours of darkness will be observed.
Hugh Johnson, a meteorologist with the National Weather Service in Albany, said available summer sunlight will begin to decrease after the solstice. At first, people won’t notice seconds and then minutes they’re losing at dusk and dawn.
“By the end of July, it [sunset] will be 8:15 p.m.,” he said. “In August, we really go into free-fall. In the beginning of the month, it’s 8:15 p.m.; by the end of the month, it’s 7:32 p.m.”
Sunrise will also arrive later and later, as summer fades into autumn and autumn changes to winter. By September, Johnson said, the sun will come up around 6:20 a.m.
Monda said the sun’s solstice time does not mean the Earth is at its closest point to the star.
“On July 4, we are at our farthest from the sun, 94 million miles,” he said. “For the Earth, it’s the tilt that causes the seasons, not our distance from the sun. If the Earth were not tilted as it rotates, we would have no seasons. We would have the same kind of climate all year round.”
Summer celebrations
Some groups are taking advantage of the light today to celebrate the season.