Friday, January 2, 2009

Happy Perihelion

Not quite yet I know, but as everyone else is wishing happy New Years, it seems to be the season to wish good will.

On or around the 4th of January we (earth that is) will be closest to the sun. That the calendar year just started is only a coincidence. Due to orbital variations, our date of closest approach varies a little bit over time. That is the time/season of perihelion will gradually shift over thousands of years. Earth's axis is slowly but continuously changing, with a cycle of approximately 25,765 years.

Currently the distance between the earth and sun varies between 98.3– 101.7% of its average distance. At its average distance sunlight amounts to about 1365.5 watts/m^2. Being at perihelion, the sun’s intensity is greater of course. However, since intensity varies by the inverse square of the distance, its intensity is now about 1412.3 watts/m^2. That is a 6.7% increase over where it was just last summer!

Ever wonder how our climate would be if perihelion occurred in June instead of January? It won’t happen for another 12,000 years or so, but when it does Northern hemisphere summers would be warmer and winters colder. Just the opposite will happen in the southern hemisphere and there is another difference too. It not just that most of us live in the north, but there is much more land than the south.

The extra land of the north provides a big platform for seasonal snow, and snow feeds back into the climate through the change in albedo. That is the amount of sunlight that is absorbed. Less snow means more sunlight is absorbed and more warmth. More snow leads to less absorption and cooler temperatures.

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