Summer Solstice 2011
The northern hemisphere summer solstice occurs today, 21 June 2011 at 1816 BST (1716 UT).
But surely the summer solstice is just the longest day. How can it “occur” at a specific instant?
That’s because we astronomers define the summer solstice as the instant when the Sun gets to its furthest north above the celestial equator. Or to put it another way, the instant when the north pole of the Earth is tipped most directly towards the Sun.
And this happens at exactly 1816 BST on 21 June 2011.
It’s important to remember though that while we are in the midst of summer, the southern hemisphere are experiencing their winter solstice, and their shortest day.
And how much longer is our “longest day”? In Glasgow, my home town, the Sun will be above the horizon for 17h35m15s today, a full 6 seconds longer than yesterday, and two seconds longer than tomorrow.