From: utzoo!decvax!cca!hplabs!hao!menlo70!sytek!zehntel!teklabs!ucbcad!ucbvax:mhtsa!alice!npoiv!npois!cbosgd!djb@sri-unix
Newsgroups: net.followup,net.misc
Title: Re: speaking of calendars
Article-I.D.: cbosgd.2798
Posted: Wed Nov 10 09:15:49 1982
Received: Tue Nov 16 05:10:55 1982

You are quite right.  The Earth is closest to the Sun in January!
Why isn't it warm then?  We're in the "wrong" hemisphere. In
Australia (for example) it's pretty warm in Janurary.

Your question points up a fairly common misconception.  The seasons
are not produced by the variation in Earth's distance from the sun.
They result from the Earth's equatorial inclination.  As you've all
seen (look at the globe in your library) the Earth's equator is 
tilted about 23.5 degrees, relative to its orbital plane.  This
means that as the Earth goes around the Sun, the observed angular 
elevation of the Sun changes gradually.  At one point in the orbit,
our hemisphere is "aimed" directly at the Sun, and the Sun reaches
its maximum angular elevation ("highest in the sky").  Six months 
later, the southern hemisphere is pointed toward the sun and we are 
pointing away, and the Sun is at its lowest angular elevation
("lowest in the sky").  Because of this angular tilting, and the
fact that the Earth is spherical, when the Sun is at its highest
elevation it shines most directly on our hemisphere.  The light and 
heat are spread over a smaller area of the Earth's surface, making it
warmer here (SUMMER).  Obviously, when the Sun is at its lowest elevation, 
its light and heat strike us most obliquely, resulting in a minimum heating
(and lighting) effect (WINTER).  Since the Sun's elevation angle
changes continuously, the heating factor changes, giving rise to the
seasonal variations we experience.

So why isn't it warmest on the day that the Sun is at its highest?
(the Summer Solstice, around June 21st)?  And coldest when it is lowest
(Winter Solstice, around December 22nd)?  Well, the thermal interia
of the Earth and atmosphere introduce a time lag of several weeks, making
the Dog Days of July or August, and the blizzards of January and February.


						David Bryant
						 cbosg!djb