Sunset is 1 minute later today...

According to the U.S. Naval Observatory, sunset here in beautiful Kailua Kona (Eat your heart out: Webcam) will occur today at 5:45 pm, one minute later than yesterday. Hoorah!

But I'm trying to figure out why sunset and sunrise aren't perfectly symmetrical. I mean, I get that local noon isn't at clock noon (except for at one longitude within a time zone) but it all seems like constants to me:

• local noon = sun hits zenith (by definition)
• local noon's offset from clock noon (according to John Harrison)
• speed of the sun
• distance from sunrise point to zenith == distance from zenith to sunset point

So how can it be that (noon - sunrise) != (sunset - noon) ?

Update: Last night was an open house at the Canada France telescope and I got the lowdown from a professional. Noon is defined as when the sun is at the zenith, but because the Earth has moved in its orbit, it has to rotate slightly more than 360 degrees to bring the sun directly overhead. Not only that, because the Earth's velocity in its elliptical path varies based on how far from the sun we are (Kepler's laws of planetary motion), the amount of rotation needed to get from noon-to-noon (in other words, the actual time of a day) varies very significantly -- 15 minutes between the solstices! So those are your two asymmetries.