Expert: Tom Whiting - 1/28/2010

Question
Hi Tom,
my father (83y.o.) keeps talking about a line that runs diagonally around the world & through Australia. He says the line is where the Earth actually faces the sun. I thought it might have been the ecliptic but I think I'm on the wrong track entirely. Could you help please?
Thanks,
Alan

Answer
Hi Alan,
There is a special line that runs right through the middle of Australia (just misses Alice
Springs) but it's not a diagonal line... it's due East-West and it is the Tropic of Capricorn or also known as latitude 23.5 degrees South.  It represents the location of the sun in our sky on the solstice of December 21st (or is that the 22nd in Australia?).  This represents the line on the Earth where the sun passes directly overhead on that date... it's southern-most declination
of the sun before it starts moving back northward.  (Crossing over the Equator on the equinoxes of March and September). So it, that date, represents Australia's first day of summer... opposite of that of the Northern Hemisphere where December 21 is the first day of our winter.

This has nothing to do with the ecliptic... the plane of the entire Solar System, which is
the plane formed by the center of the Earth and the center of the sun.  The ecliptic also represents the path of the sun across our sky... so for that one instant in time on the December Solstice, the Tropic of Capricorn and the ecliptic plane are one and the same plane. BUT, at any other time of the year, the ecliptic is north of latitude 23.5 south. At the March and September equinoxes, the ecliptic is right over our Earth's equator, and on the June solstice the ecliptic plane is over the Tropic of Cancer at 23.5 N latitude.  (The moon and all the planets maintain their positions within a few degrees of this plane too, so the sun's path in our sky is where, at night, you'll find the moon and all the planets... since we're all basically on the same plane.  (Not exactly, but within a few degrees).

This changing aspect of the ecliptic is due to the Earth's 23.5 degree axial tilt from the
vertical position. If the Earth's axis were exactly vertical, then the ecliptic, the plane
of the Solar System, would always be right over the Equator... but then we've never have
any seasonal change either.  And the sun would always follow it's equinox path across our
sky at every point on the Earth's surface, and the amount of daylight and nightime would
always remain the same.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA