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Summer Solstice 2012

The northern hemisphere summer solstice occurs today, 20 June 2012 at 2309 UT (which is actually tomorrow in the UK, 21 June 2012 at 0009 BST).

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 tilted towards the Sun as far as it can.

And this happens at exactly 2309 UT on 20 June 2012.

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 17h35m13s today and tomorrow (20 and 21 June), a full seven seconds longer than yesterday, and eight seconds longer than 22 June.

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