Here comes the sun – sunset time

Last week, my friend Maria asked me a question about sunset time that I didn’t know the answer to because it’s totally outside my field of expertise. But I knew that Dan Falk would know, and Maria knows him too, so I made her ask him instead.
It’s an interesting question, so with their permissions, here are the question and answer reproduced for everyone’s learning enjoyment.

Maria: [Looking at this website of daylight hours and sunset time]

“Why does sunset time start being later on December 12, 9 days before the solstice, but we still lose time of day, since sunrise is later and later. How come they are not the earliest/latest at the same time, on Dec. 21?”

Dan:

“That is a very good question indeed; and although the answer is well known (to astronomers!) it remains very difficult to explain in simple terms – and I admit I have some trouble wrapping my head around it (in spite of having written a book on “time”!).

The short answer is that the discrepancy in sunset time is due to the earth’s orbital axis being tilted (this is what gives us the seasons), and also to the earth’s orbit being elliptical rather than circular.

Now for a more painful analysis: The solstice, by definition, is the day on which (for us here in the northern hemisphere) the sun reaches its most southerly declination, which also gives us the shortest day of the year. If we measured time with a sundial – which gives “local solar time” – the solstice would also have the earliest sunset and the latest sunrise. But we don’t — we use “mean solar time.” (Actually UTC, but for the purposes of this discussion you can ignore the difference between this and mean solar time). (You can look up “mean solar time” in the index to my book – In Search of Time 🙂 ) Mean solar time is an average: Think of it as the time that a sundial would read, if the Earth’s axis wasn’t tilted with respect to the ecliptic, and if its orbit was a circle rather than an ellipse. But of course it is tilted, and the orbit is an ellipse. The result is that clock time differs from local time, and the difference can be quite large. The difference is called the “equation of time” (wiki-googleable), and it depends on one’s latitude as well as where one is located relative to the centre of your time zone. Anyway, as a result of these two effects, the time of sunset as read by a clock (i.e. as measured via mean solar time) is at its earliest more than a week before the solstice, and, similarly, the time of latest sunrise is more than a week after solstice.

(Additional thoughts: Notice that “solstice” is unambiguous: Regardless of how we measure time, there is no ambiguity as to which day of the year is the shortest. But “latest” sunset is relative to the system of measurement! “Late” means that our clock displays a late hour. And that depends on whether we’re using a sundial, a clock, or something else.)”