Monday, December 21, 2009

This year's solstice - The longest day of the year

I'm not crazy. I swear. I'm also not making some point about the meaning of existence. Actually, it's all really quite simple.

We hear about the winter solstice being the shortest day of the year, and we take it to mean we have less sunlight hours today than any other day of the year. The tilt of Earth's rotation around it's axis causes the northern hemisphere to be tipped away from the Sun. So rather than rising high in the sky, it skips along the horizon. This means less daylight hours today than yesterday.

Today, however, December 21st, 2009, is longer than yesterday. In fact, this year's winter solstice has more daylight than any other winter solstice in history.

The simple reason is: the Earth's spin is slowing down.

Tidal friction is one of the leading causes.

The moon's gravity pulls not only the oceans towards it, causing tides, but it actually stretches the Earth.

These distortions cause friction which saps energy from the Earth. When the Moon was formed about 4.5 billion years ago, the day was about six hours long! And this isn't just sunshine day, this is six hours for a full day/night cycle.

It's not a speedy decrease, though. We've picked up only about two milliseconds in the last 190 years. Right now we get about nine hours of sunlight at the winter solstice in mid-latitudes. Next year, it will be a tiny little bit more.

So, if you are one of those who want to find meaning in everything... take solace in the fact that even on the darkest day of the year, it will only get brighter.

*images courtesy of Museum Victoria, and the Department of Physics at the University of Oregon*

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