How is it that the earliest sunset of the year and the winter solstice are different days?

Question

How is it that the earliest sunset of the year and the winter solstice are different days?

In: 1

Answer 1

This is only true if you use “mean” time, as we have been doing for around 200 years, since clocks in people’s homes became good enough. If you use sundial time, earliest sunset, latest sunrise and the solstice occur within 24 hours of each other.

The issue is that the length of time from one sundial noon to the next varies; some times of the year it’s a little more than 24 hours and sometimes a little less. With accurate clocks you want every day to last exactly 24 hours and doing that means that clock noon can be up to 16 minutes away from sundial noon.

Since clock noon drifts around from the real noon, sunrise and sunset times also drift. Around the solstice, the times of sunrise and sunset actually change very little due to the length of the day so pretty well all of the daily change is due to the way noon is drifting. This is what pushes clock-time sunrise away from the solstice.

As for why the length of the day varies, there are two reasons:

* The most obvious is that the earth orbits in an ellipse around the sun, orbiting faster when it’s closest in early January. That makes the sun move faster backwards (eastwards) in the sky which in turn means it take longer than 24 hours for its main westward movement to make a complete trip from noon to noon.
* Secondly, near the solstices (December and June) the sun is moving directly eastwards across the sky, and east-west movement is what counts for the time of noon. Near the equinoxes (March and September) the sun is also travelling somewhat north-south, and this means it’s not going east as quickly; it’s going kind of diagonally instead of straight.

“Mean” time is using the word in the sense of “average”. That’s where “Greenwich Mean Time” (GMT) comes from: we use the same, average length for every day, instead of allowing them to vary. Mean time came into common use for civil time even before time zones were invented. Ships’ navigators have been using it even longer.