10:16 a.m. PDT: The exact moment of the 2011 summer solstice in Woodland Hills, Calif. (Ian O'Neill/Discovery News)
Did you feel it?
Well, you probably didn't, unless you're attending a sun worshiping party (in which case it was probably the beat of bongos and strong cider you felt rather than any spacetime warping), but you just lived through a beautifully subtle celestial event: the summer solstice.
You may not know — and I didn't until I wrote about it in 2010 — that the summer solstice (and, indeed, the winter solstice) is a precise moment in time. It is generally assumed that because the summer solstice gives us the longest day of the year that the whole day is the solstice. This isn't entirely true.
The summer solstice for the Northern Hemisphere is the exact moment when the axial tilt of the Earth is at its most inclined toward the sun during its 365-day orbit — at an angle of 23° 26'. That doesn't happen at midday, nor does it happen at midnight; it happens at the exact same time for every country on the planet. It isn't like a New Year's celebration when the clock strikes midnight across the time zones in turn — this is a global time event, with the solstice occurring at the same moment.
(NOTE: The exact moment of summer solstice in the Northern Hemisphere is also the exact moment of winter solstice in the Southern Hemisphere.)
In 2011, the time at which the Earth is at its most tilted toward the sun — thereby bathing most of the Northern Hemisphere in sunlight — was at 17:16 UTC. This time is different every year, and sometimes the time shifts so the solstice doesn't occur on June 21 at all! Next year, for example, the solstice is on June 20 at 23:09 UTC.
(UTC stands for "Coordinated Universal Time." It is the standard that all global clocks set themselves by.)
For me, in California, solstice occurred at precisely 10:16 a.m. PDT. In the U.K., the solstice occurred at 6:16 p.m. BST (there's an eight-hour time difference between Los Angeles and London).
In last year's article for Discovery News, I waxed lyrical about how lucky we are to be living on a planet with a tilt when I was sitting in my sister's backyard in Coventry, U.K., bathed in post-solstice sun:
So, once again, let us "worship" our planet with a tilt. Although there seems to be a preponderance of small, rocky worlds in our galaxy, we know of only one that is so delicately tailored for Earth Brand Life™ to thrive.
June 21, 2010 at 12:28 p.m. BST: The exact moment of the 2010 summer solstice in Coventry, UK (Ian O'Neill/Discovery News)