Tuesday, June 21, 2011

Summer solstice: it's worth celebrating

Today is June 21. It's summer solstice, the longest day of the year in the northern hemisphere.

For my birthday last May, I asked for a globe. I wanted to be able to illustrate solar-system phenomena to my kids, as well as show them the true geometry of our world. For example, the shortest path from Vancouver to England passes through the arctic circle.

A couple days ago, I pulled the blinds over the window, grabbed a lamp and my globe, and showed my 3 kids what summer solstice means, why it happens, and how it's linked to the seasons. It's the day when the earth's spin axis is maximally inclined toward the sun. Here's another - more technical - way of putting it. Imagine a line connecting the sun and the earth, passing right through the centre of each. Now draw another line along the earth's spin axis, also through the centre of the earth. Those two lines form an imaginary plane. Another imaginary plane is formed by the earth's orbit around the sun. A solstice occurs when those two planes are perpendicular.

You might have heard of the arctic circle, equator, and the tropics. These earth zones are all defined by the earth's tilt and are related to the solstice.

(Taken from this video.)

The tropics are the range around the equator, from 23.5o north (Tropic of Cancer) to 23.5o south (Tropic of Capricorn), where the sun is directly above at some time of the year. We never get that in Canada... we're not in the tropics. The closest we get is today, summer solstice.

Finally, here is a picture of Tricia on our honeymoon.


What the hell does that have to do with summer solstice, you ask? Notice that her shadow is directly below her. This picture was taken in Mexico right around the time of the summer solstice, the only time I've seen the sun directly overhead.

(And Trish isn't too hard on the eyes either. :-)

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