Sunday, Dec. 21, had the least daylight of the year in the Northern Hemisphere. In Washington, D.C., there were 9 hours, 26 minutes, and 18 seconds of daylight, about 2 seconds less than the day before or than on Monday.
Being the winter solstice, Sunday was also the day when the sun was lowest in the sky — it was only 27.7 degrees above the horizon in the district at its highest. Now, if you were to measure the sun’s height at its peak every day for the next week (about 12:06 p.m. on Monday, and about 12:10 pm a week from now), you would notice that while the 21st was its lowest altitude, it’s not really getting higher in the sky very fast at all.
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The sun’s maximum altitude has been 27.7 degrees above the horizon since Dec. 17, and it won’t get to 27.8 degrees until Dec. 25. It won’t hit 28 degrees until Dec. 30. (This daily change, by the way, steadily accelerates and around the equinox in March, the sun’s altitude increases by 0.4 degrees higher every single day.)
So, on these questions — the length of daylight and the altitude of the sun — we’ve reached the low point. From Monday on, things will get brighter and sunnier. But on one score, things will keep getting darker for a couple more weeks.
Specifically, sunrise will keep getting later for two more weeks here in Washington, D.C., and in Miami, it’s not until Jan. 12 or 13, 2025, that the sunrise starts getting earlier.
Why is that?
Well, two of the details above are hints.
First, note that solar noon (when the sun is due south and at its highest point) in the district is creeping later this time of year. On Dec. 1, solar noon in the district was 11:57. On Monday, it is 12:06 p.m. On Dec. 31, it will be 12:11 p.m.
That means that each solar day this time of year is slightly longer than 24 hours. So, in that sense, these are the longest days of the year.
Come summertime in the district, the solar days are each shorter than 24 hours.
This explains why the solstice isn’t the latest sunrise of the year: While we get about 10 seconds of daylight from Sunday to Monday, solar noon is about 40 seconds later. As a result, sunrise moves 30 seconds later (40 minus 10), while sunset moves 50 seconds later (40 plus 10). (The numbers in this paragraph are made up, but you get the point.)
This raises the question of why solar days vary in length. There are two reasons:
- The Earth’s orbit is elliptical, not round, and when it’s closer to the sun (which it is in December and January), the Earth moves faster through its orbit.
- The Earth’s axis is tilted relative to its orbit, and for complicated reasons, this makes the solar day longer closer to the solstices.
The underlying dynamic in both of these factors is that the Earth always has to rotate a bit more than 360 degrees to get from solar noon to solar noon, because it is also orbiting the sun in the same direction. Imagine the sun is directly between you and the star Alpha Leonis, and it is noon. Then the Earth rotates 360 degrees. You will once again be facing Alpha Leonis, but you will need the Earth to rotate slightly more for you to be facing the sun again, because you also moved along the orbit a bit, and the sun is no longer directly between you and Alpha Leonis.
How much extra little spin is needed? That depends on how fast the Earth is moving.
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The Earth moves faster along its orbit when it is closer to the sun (which is in December and January), because gravity pulls it harder. That means the Earth needs a little extra spin, which means the solar days are longer.
Also (and this is too confusing to go into here), the angle of the Earth’s axis adds to the length of the solar day close to the solstices. The net effect is that days are longest around the winter Solstice, second-longest around the summer solstice, and shortest around the equinoxes.
In sum: Because our days (measured from solar noon to solar noon) are longer than normal, our sunrise will keep getting later even as our days (measured from sunrise to sunset) are getting longer.