|MadSci Network: Astronomy|
The short answer is The Tropic of Capricorn; or -23d 27' latitude
(that's minus 23 degrees, 27 minutes).
Conversely, +23d 27' latitude is the Tropic of Cancer, and represents the Sun's declination on the Summer Solstice.
For a more involved answer, with a cool quicktime movie, please see Eric Weisstein's Treasure Trove of Astronomy on the Winter Solstice.
Thanks for using Mad Scientist,
Try the links in the MadSci Library for more information on Astronomy.