Can anyone help me to understand why the earliest sunset occurs one or two weeks before the winter solstice, and the latest sunrise occurs one or two weeks later? I imagine that the slight change in orbital position from day to day must have some impact, but I still can't visualize it.

Thanks for any help.

Because of both the tilt of the Earth's axis and the eccentricity of the Earth's elliptical orbit, the sun does not travel through the 360 degrees of sky at a constant rate of 15 degrees per hour. During parts of the year, it is "ahead of" where it would be if it moved at a constant rate, and at other times it is "running late". The slowing down and catching up affects the sunrise and sunset times, just as the length of the day does. For a more complete explanation, read about the "Equation of Time" at http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980116c.html

Thanks so much for the response- exactly what I was looking for. As a bonus, I now understand the 'Sundial Correction' column in the Farmer's Almanac, which varies from zero to 32 minutes in Boston.