The winter solstice marks the shortest day and longest night of the year in the Northern Hemisphere. It occurs between December 20th and 23rd each year. After the solstice, the length of daylight begins to increase as we head towards spring and summer. But does this mean the sun starts setting later right away? The answer is complicated and depends on a few factors. In this article, we’ll examine how the setting sun’s timing changes in the weeks and months after the solstice.
The Role of the Solstice
On the winter solstice, the Northern Hemisphere is tilted at its maximum away from the sun. This results in the year’s shortest period of daylight and longest nighttime. The solstice marks a turning point – after it passes, the number of daylight hours gradually begins increasing. However, this doesn’t immediately translate into delays in sunset times.
In the weeks immediately following the solstice, sunset times continue getting earlier. This counterintuitive effect has to do with the discrepancy between solar time and our civil clock time.
Solar Time vs Clock Time
Solar time is based on the position of the sun in the sky, which determines when sunrise and sunset occur. Our civil time kept on clocks and watches is determined by the constant 24-hour cycle of a day.
Around the solstice, solar time is ahead of clock time by up to 30 minutes. So even as days get longer in solar time, sunset keeps occurring earlier by the clock.
It takes about 2-4 weeks after the solstice for solar time to realign with clock time due to the earth’s elliptical orbit. Only then do sunset times start shifting later.
The Equation of Time
The Equation of Time describes the difference between solar time and clock time over the year. Around the winter solstice, the equation of time is positive – meaning solar time is ahead of clock time:
| Date | Equation of Time (minutes) |
|---|---|
| December 15 | +12 |
| December 22 (Solstice) | +30 |
| January 1 | 0 |
As the table shows, the equation of time gradually decreases after the solstice until solar and clock time realign in early January. This changing discrepancy is why sunset continues shifting earlier for a few weeks after the solstice.
Other Factors Affecting Sunset Time
In addition to the equation of time, a few other factors influence sunset timing:
– Latitude: The farther north you live, the more exaggerated the seasonal variation in sunset times. Near the equator, daylight length stays more consistent year-round.
– Atmospheric refraction: This bends light and makes the sun appear slightly higher in the sky than its true position. It results in sunsets occurring 2-3 minutes later than predicted astronomically.
– Topography: Mountains, hills, and buildings on the western horizon make the sun appear to set earlier. Flat terrain with no obstructions delays sunset by a few minutes.
– Daylight saving time: When clocks shift forward an hour in spring, sunsets occur at a later civil time despite solar time staying fixed.
– Leap years: The extra day in February during leap years pushes sunset times slightly later.
So in the weeks after the winter solstice, the ongoing realignment of solar and civil time is the main driver of sunset getting earlier by the clock. Location, refraction, terrain and other factors also influence the timing.
When Do Sunsets Start Getting Later?
Given these factors, how long does it take for sunset times to actually start delaying after the winter solstice?
For mid-northern latitudes, sunset times typically continue shifting earlier for 2-4 weeks after the solstice. Here are some examples for reference cities in the U.S.:
| City | Solstice Sunset Time | Sunset 2 Weeks Later | Sunset 4 Weeks Later |
|---|---|---|---|
| Chicago | 4:21 pm | 4:20 pm | 4:23 pm |
| Denver | 4:38 pm | 4:37 pm | 4:40 pm |
| New York | 4:31 pm | 4:28 pm | 4:33 pm |
For lower latitudes closer to the equator, the effect is reduced. In Miami for example, sunset times shift later within 1-2 weeks of the solstice.
Further north, the lag extends longer. In Seattle, sunset times don’t start delaying until early February, while in Anchorage, Alaska, it lasts until mid-February.
So on average, for most mid-latitude Northern Hemisphere locations, sunset begins shifting later in civil time around 2-4 weeks after the winter solstice.
The Full Cycle
To visualize how sunset time changes over the full year, here is a graph for Chicago:
| Month | Sunset Time |
|---|---|
| December (Solstice) | 4:21 pm |
| January | 4:30 pm |
| February | 5:40 pm |
| March (Equinox) | 7:00 pm |
| April | 7:40 pm |
| May | 8:15 pm |
| June (Solstice) | 8:30 pm |
| July | 8:25 pm |
| August | 7:45 pm |
| September | 6:50 pm |
| October | 6:00 pm |
| November | 4:40 pm |
As shown, sunset times gradually shift later through spring, peak in mid-summer, and then retreat earlier again in the fall and early winter. The key takeaway is that while the winter solstice marks a turning point astronomically, sunset continues getting earlier for a few weeks after until solar time realigns with civil time.
The Setting Sun’s Position
A related question is whether the setting sun’s position on the horizon starts moving northward right after the solstice.
Around the winter solstice, the setting sun is at its southernmost point along the horizon. But similar to timing, the sun’s position continues shifting southward through early January. This is again due to the discrepancy between solar and civil time.
It takes about 2-4 weeks for the sun to reach its southernmost setting point before reversing course and moving northward along the horizon through spring and summer. So both the timing and position of sunset are delayed versus the solstice itself.
Solar Noon
The winter solstice also does not mark the year’s earliest solar noon, or when the sun reaches its highest point in the sky. Through early January, solar noon keeps occurring slightly earlier by the clock.
It takes until early February for solar noon to reach its earliest time and begin delaying through spring and summer. So solar noon, sunset time, and the sun’s horizon position all shift a few weeks after the solstice itself.
Equinox vs Solstice
This lag in the setting sun also happens around the summer solstice in June. Sunset times continue shifting later for 2-4 weeks following the summer solstice.
The equinoxes work differently – they do mark an immediate change. On the spring and fall equinoxes, the setting sun’s direction instantly reverses along the horizon. And sunrise, sunset and solar noon times either begin shifting earlier or later on the equinox date itself.
The Southern Hemisphere
Everything is reversed in the Southern Hemisphere due to the opposite seasons. Here, the December solstice marks the longest day and beginning of summer.
In the weeks after the December solstice, sunset times continue shifting later in the Southern Hemisphere. It takes until mid-January for solar and civil time to realign and sunset to begin occurring earlier each day.
The June solstice marks the beginning of winter in the south. Again, sunset times initially keep shifting later before reversing direction in July. So the same principles apply on either side of the equator.
Conclusion
In summary, the winter solstice represents a turning point, after which daylight hours steadily increase. But due to the discrepancy between solar time and clock time, the onset of later sunsets and shifting solar noon is delayed 2-4 weeks after the solstice itself. This is caused by the gradual realignment of solar and civil time.
So the solstice marks the astronomical beginning of lengthening days. But it takes until early January for these effects to become apparent to our clock-driven senses. The equinoxes, by contrast, produce an instant visible change in the sun’s daily path and timing. Understanding these dynamics provides deeper insight into the intersection of astronomical events and our civil reckoning of time.
