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At some point in the distant past, this question was answered for me but I have lost that penguin. My father in law brought me this question last week and I don't have the answer (annoying!)
There is a discrepancy between the date of the earliest sunset and the date of the shortest day length (in the Northern Hemisphere). The shortest day length occurs, as expected, on the Solstice - Dec 21st. However, my earliest sunset occurs centered on Nov 26th ( ±8days on either side). So, after Dec 4th, sunsets slowly get later but sunrises are adjusting fast enough to keep shortening the days until Dec 21st.
All times are local based on data from the USNO. I thought it might have something to do with the fact that I am 7.5° degrees west of the prime meridian for this time zone. But the more I think about this the less likely I believe this explains the anomaly.
Here is the data:
Can anyone out there reasonably explain what's going on here?
Thanks,
JT
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Searching the skies since 1966. "I never met a scope I didn't want to keep."
There are two things going on that contribute to the changes of the length of the day and the exact timing of the sunrise.
One is the direction of the tilt of the Earth's axis - which gives us a cyclical variation in the length of the day (with the shortest being on the Winter Solstice)
The other is the date of the Perihelion - which gives us a cyclical variation in the timing of the Sun crossing the Meridian (and hence of sunrise/sunset) relative to that of the 'mean' .
The two cycles give us the Equation of Time
Because the date of the Solstice is 21/22nd December and the date of Perihelion is around 4th January, these two cyclical variations are not in sync, so the date of the shortest day does not coincide with the date of the earliest sunrise.
If you visualize the analemma (a graphical representation of the equation of time) positioned at the sunset horizon, it is clear that the earliest sunset occurs not at the solstice, but at a point tangent to the curve of the analemma. Since the analemma will be at a different angle depending on the observer's latitude (horizontal at the equator, vertical at the poles), the position of the tangent point varies with latitude.
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Thanks for the graphical representation, Kathy. That makes it perfectly clear to me without having to wade through a major thought process. Lord knows I need to save as many of those processes as I can!
