Thursday, July 7, 2016

Earth at Aphelion

I'm a couple of days late, but on July 4th, 2016, Earth was at aphelion in its orbit around the Sun.  Aphelion is Earth's farthest point from the Sun in a single orbit.  Since Earth's orbit around the Sun is an ellipse, and not a circle, the distance between the Sun and the Earth is continually changing.  Notice that aphelion takes place in the summer...not the winter.  Thus the Northern Hemisphere experiences summer when Earth is FARTHEST from the Sun.  That goes against common sense.  There's a common misconception that the closer Earth is to the Sun, the warmer it is on Earth.  Not true.  For starters, this ignores the opposite seasons in the Southern Hemisphere.  This means Earth's elliptical orbit is not the main driver of temperature changes on Earth.

So what does cause Earth's season?  The answer is Earth's 23.5 degree tilt on its axis.  This changes the directness of the Sun's rays and changes the amount of energy the surface of Earth receives as it makes its way around the Sun.  When the rays are more direct, more energy is received and Earth's surface warms, giving us summer.  When the rays are less direct, less energy is received and Earth's surface cools, giving us winter.  Axial tilt is the reason for the seasons!

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