Wednesday, January 6, 2016

And you thought the seasonal changes in sunrise, sunset and day length were simple?

Oh, no.  Although the shortest day/longest night for the northern hemisphere occurs at the December solstice, sunrise and sunset times don't fall so neatly into line!  It even depends where in the northern hemisphere you are!  EarthSky explains how and why better than I could.

Here in the Boston area, the sun has risen at about 7:12am eastern standard time every morning since the beginning of the year.  Sunsets, on the other hand, have been getting slowly later since before the middle of December, when they bottomed-out at about 4:12pm est.  Here's the NOAA site where you can see for yourself. 

Meanwhile, Earth made its closest approach to the sun on Jan 2!  (That three-million mile difference between closest (perihelion) and farthest (aphelion, July 5) amounts to about three percent of the 93-million-mile average distance--not enough of a difference to overwhelm seasons caused by earth's axial tilt.  I do wonder, though, if it eases our winters a bit, while worsening Australia's summers!)

From what EarthSky tells us, perihelion and aphelion will line up with the equinoxes in 6430.  With the greatest and least distance (and therefore light intensity) completely out of sync with the seasons, I wonder if those seasons will be a bit milder worldwide.  And, if so, what difference did these milder climates mean to the people of 2400 BCE???

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