Summer Solstice 2013

The northern hemisphere summer solstice occurs today, 21 June 2013 at 0504 UT (which is actually 0604 BST in the UK).

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 0504 UT on 21 June 2013.

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 17h35m12s today (21 June), a full one second longer than yesterday, and six seconds longer than tomorrow.

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