Although often associated with the Winter Solstice, Tekufas Teves is an independent date. How is Tekufas Teves, or the other Tekufos (?) determined?
There are actually two different calculations used in Jewish literature for the tekufos. (They are described in Rambam's Hilchos Kiddush Hachodesh, chs. 9 and 10.)
The one that DoubleAA described is called Shmuel's tekufah, and it is the one used for most halachic purposes, including the date when we start saying "vesein tal umatar" outside of Israel, and the date of Birkas Hachammah. Its starting point is indeed Tuesday night at 6 PM, but actually the date is 22 Adar of the year 1, which is not the time of the sun's creation. (Shmuel agrees with the opinion of R. Eliezer, that the world was created in Tishrei - i.e., beginning on 25 Elul and ending with Adam's creation on 1 Tishrei; on that assumption the sun would have been created on 28 Elul, a Wednesday, and that tekufah would have occurred at 9 AM. Calculating backwards two tekufos, 182 days and 15 hours, gets us back to 22 Adar at 6 PM.)
There is also Rav Adda's tekufah, which assumes that a solar year is exactly 1/19 of a machzor katan (19 Jewish years, containing 235 lunar months). This comes out to 365.246822 days - which is somewhat more accurate than Shmuel's value, but still gains about 4.5 days per millennium, which is why the dates of the Yamim Tovim slowly move later in the secular year. The first tekufah for this calculation is also Tuesday at 6 PM, but a week later than Shmuel's - on 29 Adar (because he holds with R. Yehoshua that the world was created in Nissan). Because R. Adda's year is shorter than Shmuel's, nowadays his tekufah is about 11 days earlier.
Tekufot are the solar equinoxes and solstices. To calculate the tekufa, you must know that the sun was created at 6 pm on Tuesday of year 1 of creation (ie the beginning of the fourth day). The, add 91 days 7.5 hours to get the next tekufah and so on. This number is considered one fourth of a solar year of length 365.25 days. Since a solar year is actually 365.242581 days, our calculated tekufa shifts forward ever so slightly every time.