Take the 2-minute tour ×
Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It's 100% free, no registration required.

Everybody knows that the shortest and longest days of the year occur on the solstices. However, examination of sunrise/sunset tables shows that the maxima and minima of sunrise/sunset times are offset from the solstices by about 10 days.

According to timeanddate.com for my location (Portland, OR), for the winter solstice, the earliest sunset is about December 10 (16:27), and the latest sunrise is around January 1 (07:51).

This is counterintuitive... what's the explanation?

share|improve this question

1 Answer 1

up vote 2 down vote accepted

The point here is that sunrise and sunset does not only move towards or away from noon, but noon itself moves around 12:00. There is a difference between "civil time" (the one counted by wall clocks) and "solar time" (the one marked by sun clocks). This difference is called the "Equation of Time" and its geometrical shape is called the Analema.

So in order to have the earliest sunset you need a combination of both a short day and a noon moved before 12:00. Same reasoning for latest sunrise.

You can see more about the Equation of Time on Wikipedia: http://en.wikipedia.org/wiki/Equation_of_time

share|improve this answer
    
Yes, I noted in a comment to an answer that has now been deleted that I'd found that Wikipedia page and the one on Solar Time, explaining the reasons for the oscillation (eccentricity of the Earth's orbit and the effect of axial tilt). Thanks. –  Jim Garrison Jan 4 at 0:39

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.