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The winter solstice in the Northern Hemisphere in 2011 is on December 22.

But if I look at the sunset times for a location such as Washington, DC on the USNO site, the sunset time starts reversing much earlier (around December 6th).

Shouldn't the sunset time start reversing (from earlier to later) on the same date as the solstice?

Omega Centauri's explanation is correct.

  • The Royal Observatory gives a similar account of this oddity on this page under the section titled "The apparently odd behaviour of sunrise/set times near the winter solstice".
  • Cornell University provides more details here.
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This is because the earth doesn't have a circular orbit, so some times the angular velocity of the earth is greater than average (and also the reverse). Perihelion is in early January, so the planets orbital velocity is greater (i.e. the Northern hemisphere winter is shorter than half a year long). So the affects the length of a day, making it differer slightly from 24hours depending upon the season. So solar time versus clock times drifts.

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Sorry but it's still not clear. The date of the solstice and the date of the sunset times are both on the same "clock" so to speak (using the same calendar). I understand the date of the solstice drifting from year to year but shouldn't the sunset time reversal happen at the same time as the solstice? – Anna Karenina Oct 26 '11 at 1:09
I agree that the above isn't quite clear. However, one thing to consider is that the USNO site may not be exactly correct. I went into some astronomical software (Starry Night Pro) and just went to sunrise and sunset times for today (Oct. 26). They differ by 1-3 minutes from what the USNO site says. – Stuart Robbins Oct 26 '11 at 6:55
@StuartRobbins, thanks. The USNO times match with a few other sites including the NOAA which claims accuracy to within 1 minute. Though I don't think it explains why the two events would calculate out to such different dates. Actually, I've found an explanation similar to Omega Centauri's on the Royal Observatory's site. I'll add a link to it in the question. – Anna Karenina Oct 26 '11 at 13:00
Anna, the reason is that when the earth is closer to the sun than its mean distance, the approximately 4minute difference between the time the earth takes to make a single revolution and the time the sun takes to make an apparent trip around the earth varies. This difference is caused by the the angular speed of the earth around the sun. The difference between the angular speed at apihelion, and perihelion is roughly 7%. This means that the time of solar noon can drift by several seconds per day. – Omega Centauri Oct 28 '11 at 20:06

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