If you’re an early-riser, you may have noticed something peculiar happening this month. Though the days have slowly been getting longer for more than three weeks, nearly all of that extra daylight has been tacked onto the evening — not the morning hours.
Sunset in Washington, D.C., is now 21 minutes later than it was on Dec. 21, the winter solstice. Sunrise, however, hasn’t budged at all. In fact, Friday’s 7:25 a.m. sunrise was actually two minutes later than sunrise on the shortest day of the year. It doesn’t make sense — shouldn’t the sun be rising earlier now that the days are getting longer? And why wasn’t the latest sunrise on the winter solstice?
Let’s start by looking at sunrise and sunset times this month. For the first 11 days of January (highlighted in yellow), sunrise in D.C. is “stuck” at 7:27 a.m., the latest of the year during standard time. Calculated down to the second, the latest sunrise in the District happens around Jan. 5.
If you need to commute in the dark, the good news is the sun is finally beginning to rise earlier again. But if you look at the table, note how a full month after the solstice, sunrise hasn’t really changed. Meanwhile, we’re gaining a solid minute of evening daylight each day. By Jan. 20, D.C. will have about 30 minutes more evening light than in early December.
Why the mismatch? Here’s where solar noon comes into play. It shows how the sun is out of sync with our 24-hour clocks.
Though we commonly think of a day as exactly 24 hours, this is really just an average of how long it takes the sun to appear in the same position from day to day due to Earth’s rotation. A solar day, however, can be longer or shorter than 24 hours by as much as 30 seconds.
Since the sun’s position changes with the seasons, a solar day measures how long it takes for the sun to return to its highest point in the sky from one day to the next. That highest point is called solar noon. It’s the moment when the sun crosses our local meridian, or line of longitude, and appears due south in the sky.
At any location on Earth, the time of solar noon can differ from average by as much as 30 minutes throughout the year. These variations explain why sundials never match the time on your watch (or phone). If you plot the number of minutes the sun is running fast or slow for every day of the year, we get the wavy line seen in the graph below. Astronomers and timekeepers call this the equation of time.
In early November, the sun runs 16 minutes ahead of our clocks, while in early February the sun runs 14 minutes behind. On only four days of the year does the time on your watch match your backyard sundial: Apr. 15, Jun. 13, Sep. 1 and Dec. 25.
The table below shows the time of solar noon in Washington, D.C. for every day of the year (add one hour during daylight saving time). On average, solar noon in D.C. is at 12:08 p.m. Notice how the time of solar noon changes dramatically in December and January, ticking later by more than 10 minutes each month.
Why does solar noon change in the first place?
Two important tenets of astronomy explain why the sun’s position appears to change throughout the year. First, the Earth is tilted on its axis of rotation by about 23.5 degrees (if it weren’t, we’d have no seasons, and everywhere on the planet would get about 12 hours of daylight year-round).
Second, Earth’s orbit around the Sun is an ellipse, not a perfect circle. This means our distance from the sun varies by roughly three million miles over the course of a year. Earth reaches perihelion – its closest approach to the sun – in early January.
So how do these two astronomical quirks – Earth’s tilt and our variable distance from the sun – affect when we see sunrise and sunset?
This is where Kepler’s second law of planetary motion come into play. When the Earth is closer to the sun, as it is in January, our planet moves faster in its orbit and therefore covers more distance. From our viewpoint, the sun appears to move farther west in the sky, taking more than 24 hours to reach the same position from one day to the next (see here for a helpful image). This is what causes solar noon to move continually later in December and January.
Meanwhile, in the weeks before and after the solstice, the sun’s declination – or height with respect to the horizon – doesn’t change much. If you took a photo of the sun’s position at the same time of day for an entire year, the sun would trace an asymmetrical figure 8 in the sky. This path is called an analemma, and it’s the unique result of Earth’s elliptical orbit and planetary tilt. If it looks familiar, you’ve probably seen it on that dusty globe sitting on your bookshelf. (Note that if we traced the analemma at sunset, its orientation would flip, but the overall shape would still look the same).
This animated graphic shows the sun’s position in D.C. at 8 a.m. in one-week intervals throughout the year. If the Earth weren’t tilted and followed a perfectly circular orbit around the sun, the sun would always appear in the same position at 8 a.m. each day.
Take a look at the bottom of the figure 8 in the diagram. The sun’s lowest point on the curve isn’t until after the winter solstice. For nearly a month, the sun’s daily position moves noticeably westward, while its height above the horizon hardly changes. This is visual evidence of sunrise staying nearly constant for several weeks.
So, how does all this explain January’s late sunrises?
When the sun starts running behind the clock — as it does in December and January — the time of solar noon keeps getting pushed later. This delays the time of sunrise in the morning and causes sunset in the evening to be later than we would otherwise expect.
The discrepancy between clock time and solar time also explains why our earliest sunset was around Dec. 7, two weeks before the winter solstice. All told, Washington’s earliest sunset and latest sunrise occur nearly a month apart.
Fortunately, as we move deeper into winter and on toward spring, the sun’s apparent motion in the sky from day to day becomes more north-south oriented instead of east-west. As a result, sunrise begins to move earlier at a less sluggish pace. So if you’re looking forward to more morning light, you won’t have too much longer to wait.