Latest Sunrise/Earliest Sunset/Shortest Day
- tomtierney
- 1 minute ago
- 2 min read
I've been trying to wrap my head around something that I'm sure most readers here already understand, but that has always confused me - which is why is the shortest day not also the latest sunrise and/or earliest sunset?
I downloaded the times for sunrise and sunset in my town, and used a well-known AI model to turn that into a graph. I'm not sure how easy it is to read - but the data showed me that the earliest sunset happened back on December 13th (at 4:06pm) - and the latest sunrise won't happen until Dec 30th, (at 8:38am)
I then got it to draw a graph of the length of each day, which shows the solstice occurring right on schedule on Dec 21st. (the jaggedness is just due to times being rounded to the nearest minute).
So for the next week, sunsets are getting later as you would expect - but sunrises are also getting later. Not that most of us will notice over Christmas.
But why is the whole thing so skewed? I learned somewhere in primary school that the solstice - along with the entire pattern of winter and summer - is caused by the tilt of the earth. And I know that wobbles in the earth's tilt, can cause the exact date of the solstice to vary. But why isn't the shortest day aligned with the earliest and latest sightings of the sun?
As best as I can figure it out, the reason is this: the distortion comes from our clocks rather than from the motion of the earth.
We have long since agreed on a 24 hour day and that is how our clocks are set - and it would be a brave man who suggested we make some days a little longer and some a little shorter over the course of the year, even though that is exactly what happens in reality: the earth is closer to the sun in December than it is is June - and, being closer, it moves a little faster.
The difference isn't huge, but it's not tiny either. it seems to be the difference between 147,000,000 km and 152,000,000 km - a variation of over 3%. This can cause a variation of up to 16 minutes in the true length of the day.
So, because we keep our clocks fixed even when the sun speeds up and slows down, the timing of astronomical events gets a little distorted.
During December, solar noon gradually shifts later relative to our clock time. At the same time, the Sun's maximum height is changing—getting lower until the solstice, then higher again.
Coming up to the solstice, these two effects work against each other, giving us our earliest sunset. And after the solstice they work together, pushing our latest sunrise later.
Or at least, that's as close as I can come to understanding it all. Please let me know if I have it all wrong!
Whatever the exact dates and timings, I hope you all have a lovely solstice, and indeed Christmas!
These are the links where I was reading up on it: