Thursday, Dec. 7, at 4:45 p.m., the sun will set in Washington a few seconds earlier than the day before. On Friday, Dec. 8, the sun will set a few seconds later.

If you hate early sunsets, then take heart: We have turned the corner in Washington. Each day for the next six months, sunset will become a bit later. The improvement will be very slow at first, but by March, you’ll notice the extra afternoon sunshine every week.

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Why, you ask, isn’t the earliest sunset on the winter solstice, Dec. 21?

Before answering that, let’s note that unlike the solstice, which is the day of least daylight everywhere in the Northern Hemisphere, the day of the earliest sunset changes according to latitude.

In Miami, for instance, the earliest sunset was a week ago, and your sunset is already retreating a bit each day. In Dublin, the earliest sunset is a week from now, on Dec. 13 or 14.

You see the pattern? The closer you are to the equator, the earlier your sunset turns the corner.

But at the other end of the day, it’s the mirror image. Dublin has its latest sunrise on Dec. 30, and in Miami, the mornings keep getting darker until Jan. 13. If you plotted it out, you would see symmetry around Dec. 21 — in Washington, the earliest sunset is two weeks before the solstice, and the latest sunrise is two weeks after the solstice. In Dublin, it’s one week on either side, and in Miami, it’s three weeks on either side.

What’s behind all of this? Why isn’t it simple, with the latest sunrise, earliest sunset, and least daylight all on the solstice?

It would be that simple if the Earth’s orbit were a circle — because then, every day would be precisely 24 hours from midday to midday. But in reality, the length of our solar days varies.

The time from solar noon to solar noon averages 24 hours, but some days, it is shorter than 24 hours (as much as 22 seconds shorter), and other days, it is longer (as much as 29 seconds). This isn’t because the Earth changes its spin rate — that variation is negligible.

The variation in noon-to-noon time is due to the interaction between the Earth’s spin and the Earth’s orbit.

If you were to look at the North Star and time how long it took the Little Dipper to rotate 360 degrees around it, you would get about 23 hours, 56 minutes, and 4 seconds without significant variation. (This is called the sidereal day). The solar day, solar noon to the next day’s solar noon, is about four minutes longer than the sidereal day because the Earth is trucking around the sun and thus needs to rotate about 361 degrees for the same spot on the Earth to be directly facing the sun.

The faster the Earth is trucking around the sun, the more the Earth needs to rotate to clock a full solar day. Because of how gravity works, the Earth revolves around the sun faster when it’s closer to the sun, and we just happen to be closer to the sun in the Northern Hemisphere’s winter.

To recap this last part: The Earth’s eccentric orbit places it closer to the sun in December, which increases the amount the Earth needs to spin to complete a solar day.

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Our clocks are not set by the solar day but are set to 24 hours exactly — the average solar day. When the solar day is longer than the clock day, solar noon will move a bit later every day — that’s happening in the Northern Hemisphere right now.

If solar noon moves 30 seconds later from today to tomorrow, but the amount of daylight shortens by only 20 seconds, then you will have less daylight but a later sunset, which is what will happen every day for the next two weeks in Washington.