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Earth closest to Sun on January 4

EarthSky magazine noted today that tonight the Earth will reach its closest point in its orbit to the sun, right around 1:30 am EST on the 4th.  You can read about it here.

Many people think that winter is caused by how far our planet is from the sun, but really the seasons are caused not by the shape of the orbit but by the tilt of the Earth towards or away from the sun over the course of the year (I’ve seen videos of Harvard undergrads getting this wrong on YouTube).  The difference between aphelion (when the Earth is farthest away from the Sun in July) and perehelion (tonight) is about 3 million miles compared to the distance of 93 million miles for the average orbital distance, so it is not a large amount.  But over thousands of years, orbital changes have been shown to lead to the growth and decay of the ice sheets which are associated with the Ice Ages that have occurred across Earth for the last two million years.  It’s hard to see the evidence of the Ice Ages here in the Southeast, but where I grew up in Michigan, evidence of moraines, kettle lakes and other remnants of ice sheets were all around us.

The importance of the orbital patterns was discovered by a scientist named Milankovitch, and if you are interested you can read more at http://en.wikipedia.org/wiki/Milankovitch_cycles.  The Milankovitch cycles are one of the most important drivers of climate on the time scale of thousands of years due to changes in incoming sunlight, but on the year or decade time scale don’t make any significant difference and are usually left out of climate models working on shorter time spans.

Source: NASA

Source: NASA