A little relief from the winter blues is imminent -- sunsets start getting later again next week, even though the days are still getting shorter.
Confused? Read on.
The year's earliest sunsets in Baltimore occur around Dec. 7, at 4:43 p.m. Sunsets are stuck at that time for about a week and a half in early December.
But after this weekend, they shift later by a matter of seconds each day. By the end of the month, sunset is comes 10 minutes later, just a few minutes before 5 p.m.
Days continue getting shorter until the solstice Dec. 21, when the sun is up about 9 hours, 24 minutes. That's because the sunrises are still getting later even as the sun begins to set later. Sunrise doesn't start arriving earlier again until early January, getting as late as 7:26 a.m.
Why don't the latest sunrise and earliest sunset occur on the solstice? Because the "solar" clock is not precisely aligned with our 24-hour clock.
Solar noon, the point at which the sun is highest in the sky, occurs at different times within 10-20 minutes of noon (or 1 p.m. during daylight savings time) throughout the year. And the length of the solar day, from one solar noon to the next, is not precisely 24 hours, nor is it constant.
Space Telescope Science Institute astrophysicist Frank Summers explains, Earth is moving more quickly around the sun as it approaches perihelion -- its closest point to the sun in its slightly elliptical orbit -- so it takes slightly longer for Earth to rotate from one solar noon to the next. Earth is currently approaching perihelion, which occurs Jan. 4.
In December, it takes about 24 hours and 30 seconds for Earth to rotate from one solar noon to the next, according to EarthSky.org. Solar noon is currently a few minutes before the clock strikes noon, while it occurs at 12:10 p.m. by the end of the month.
"If the midpoint of our day were totally constant, you would expect that halfway through the sun's path was always at 12 noon, and then of course the earliest sunset and earliest sunrise would fall on the same day as shortest day of the year," Summers said. "Because the midpoint fluctuates around that, then of course we're going to get fluctuations in our perceived time of sunset and sunrise."