Is it true that the earliest sunset isn't on the winter solstice?
― A short day and an early dusk are two different things
The day with the shortest daylight really is the winter solstice. But the day the sky goes dark earliest in the evening is not the solstice at all. In Tokyo it's about two weeks earlier, in early December. And the latest sunrise of the year doesn't turn up until after New Year. What's driving the gap is "solar noon," which slips a little later every single day.
It's early December. It's not even 5pm, and it's already pitch dark outside. "The evenings are getting dark so early this year," you think.
Then the solstice passes and the New Year arrives. You'd expect the days to be getting longer now — but the mornings, if anything, stay dark. Commuting or heading to school in January can feel even darker than in December.
It's not your imagination. The calendar's "shortest day" and the "earliest dusk" and "latest dawn" we actually feel happen on completely different days.
There are just two reasons for the gap
Daylight is shortest at the winter solstice. Near the bottom of any curve, the slope is gentle. For about two weeks either side of the solstice, day length shifts by only a few tens of seconds a day.
The moment the sun climbs highest isn't locked to 12:00 on the clock. Through December it slides almost 30 seconds later every day. Even while day length is stuck, this keeps moving.
Add these two together, and evening and morning end up shifting in opposite directions. Let's take them one at a time.
First, look at day length
In Tokyo, day length is shrinking by nearly a minute a day in late November. But as the solstice approaches, that shrinkage keeps easing off. On the solstice itself, it's essentially flat.
It's the same as a ball thrown straight up: at the very top, it seems to hang still for a moment. Near a turning point, change slows right down. For about a week either side of the solstice, day length shifts by only one or two minutes total.
In short: around the solstice, day length is effectively flat. Which means sunset time is now being driven entirely by the other factor.
The culprit: "solar noon," running late every day
Our clocks mark off each day at exactly 24 hours. But the time from one solar noon (the sun's highest point) to the next isn't exactly 24 hours. It's longer some days, shorter others.
Two things cause this. First, Earth's path around the sun isn't a perfect circle but a slightly squashed ellipse — Earth moves faster when it's closer to the sun. Second, Earth's axis is tilted, so the direction the sun cuts across the sky changes with the seasons.
Combined, these two effects make solar noon creep back and forth over the course of a year. This shift is called the equation of time. And early December happens to be exactly when solar noon is sliding later at its fastest pace all year.
Sunset time equals solar noon plus half of day length. If day length is flat but solar noon keeps getting later, sunset gets later too. Take a look at Figure 1.
Why the morning shifts the opposite way
Sunrise time equals solar noon minus half of day length. The key difference from evening: this one is a subtraction.
Before the solstice, shrinking daylight is the stronger effect, so sunrise keeps getting later. After the solstice, days start lengthening again, but only by a tiny amount at first — while solar noon is still sliding later. So sunrise keeps getting later for a while even after the solstice has passed.
The upshot: sunset turns around before the solstice, and sunrise turns around after it. Figure 2 shows that addition and subtraction at work.
It's easy to assume days start getting longer only after New Year, but if you look at evenings alone, they've already bottomed out by early December. By the solstice itself, sunset is thought to be already about 4 minutes later than on the very earliest day.
This gap shrinks the further north you go. At high latitudes, day length swings so much around the solstice that it overwhelms the lag in solar noon. Conversely, the closer you are to the equator, the bigger the gap becomes.
Summary
"The solstice is the shortest day" is correct. "The solstice is the day it gets dark earliest" is not. The two get mixed up all the time, but they're different things. Day length flattens out around the solstice while solar noon just keeps sliding later — so evening's turning point comes early, and morning's turning point comes late.
The solstice only decides how "long" the day is.
What decides the "time" it gets dark is solar noon, which is perpetually running late.
For why the seasons exist at all, see Why do we have seasons?; for the mismatch between the year's true length and the calendar, see Why does the date of the autumn equinox change from year to year?.
- Using the National Astronomical Observatory of Japan's ephemeris calculator page, look up sunrise and sunset times for your own town from December 1 to January 31, and write them down.
- Read down the sunset column alone and find the earliest time. It should fall before the solstice.
- Next, in the sunrise column, find the day with the latest time — this time it should fall after New Year. If you also calculate the exact midpoint between sunrise and sunset for each day, you can watch it creep later day by day.
Try comparing the same figures for a southern island and for Hokkaido, and you'll see the size of the gap change too.
Want to go deeper? ― Terms, formulas, and where this fits in the curriculumMarked by level, from junior-high science to university-level courses
- Jr. HighCovered in junior-high science
- High SchoolCovered in high-school Earth Science Basics
- High School+Advanced high-school content, or textbook sidebar material
- UniversityNot covered in high school — university-level celestial mechanics / positional astronomy
- ResearchNot yet settled even at university level — an active research question
Jr. HighTerminology: this phenomenon has names
- Solar transit (culmination): the moment the sun reaches its highest point for the day, due south. That moment is called the transit time.
- Equation of time: the gap between the time set by the sun's actual position and the steady time kept by clocks. Over a year it's said to swing between roughly minus 14 minutes and plus 16 minutes.
- Perihelion: the point where Earth comes closest to the sun. Earth passes it in early January — the depths of winter in Japan.
Jr. HighHigh SchoolWorking the numbers: how much later is the solstice sunset than the earliest sunset?
Sunset time equals solar noon plus half of day length. Let's track rough daily figures for early December in Tokyo. No symbols — just keep the units consistent: seconds for time shifts, days for day counts.
| Daily lag in solar noon (early December) | ~25 seconds |
| Daily shrinkage in day length (early December) | ~25 seconds |
| Days from the earliest sunset to the solstice | ~17 days |
| How much day-length shrinkage pulls sunset earlier (only half counts) | 25 ÷ 2 = 12.5 (seconds) |
| Net daily delay in sunset | 25 − 12.5 = 12.5 (seconds) |
| Over the 17 days to the solstice | 12.5 × 17 = 212.5 (seconds) |
| Converted to minutes | 212.5 ÷ 60 ≒ 3.5 (minutes) |
That gives an estimate of the solstice sunset being about 3–4 minutes later than the earliest sunset. Tokyo's actual figure is close to 4 minutes, so the estimate holds up. Put another way: the difference is only a few minutes — far too small to notice by eye, and only visible once you put it in a table.
High SchoolHigh School+Why does the equation of time happen at all?
High SchoolEarth's orbit is a slightly squashed ellipse, and Earth moves faster when it's closer to the sun. During that faster stretch, it takes Earth a little extra time to turn back to face the sun again, so the solar day runs slightly long. This is the equation of time's first component, and it shows up as a wave that repeats once a year.
High School+The second component comes from the axial tilt. Because the sun's path is tilted relative to the celestial equator, its motion — once you resolve it into purely east–west progress — speeds up or slows down with the seasons. This component shows up as a wave that repeats twice a year. Add the two waves together and you get the actual equation-of-time curve. Early December happens to be one of the steepest points on that combined curve, anywhere in the year.
UniversityThe mean sun, and how "time" itself is defined
In positional astronomy, rather than tracking the real sun, astronomers define a fictitious "mean sun" that moves along the celestial equator at a constant rate. Its transit sets what's called mean solar time — the time our clocks actually follow. The equation of time is, in effect, exactly the difference between the real sun and this mean sun. Because Earth's rotation rate also fluctuates very slightly, official time standards today are set not by Earth's spin but by atomic vibration, adjusted as needed with leap seconds.
ResearchWhat's still not fully understood
- Fluctuations in Earth's rotation speed In recent years, days on which Earth's spin speeds up slightly have been observed. Exactly how much the core, mantle, oceans, and atmosphere each contribute is still under debate.
- Long-term changes in orbital shape The ellipticity of Earth's orbit and the tilt of its axis both drift slowly over tens of thousands of years. The shape of the equation of time should shift along with them, but how far that can be tied to past climate records is still an open research question.
- How this maps onto how it actually feels The sense that "it's getting dark early" depends not just on sunset time but on the length of twilight and the weather. How people actually perceive the seasons sits at the boundary between psychology and meteorology, and remains unresolved.
In other words, even this article is only "the explanation as currently understood." The figures vary slightly by location and year, so check your own location's almanac for exact times.
Where this fits in the curriculum (by level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| Jr. High | Science — Earth and space (the sun's daily motion) | The term "solar transit," and how sunrise/sunset times are determined |
| High School | Earth Science Basics — Earth's motion | The elliptical orbit, and how the sun's motion changes with the seasons |
| High School+ | Earth Science — the celestial sphere and time | The two components of the equation of time, and how they combine |
| University | Positional astronomy / celestial mechanics | The mean sun, mean solar time, leap seconds |
| Research | Geodesy / Earth rotation | Fluctuations in rotation speed and disentangling their causes |
| ― | Everyday connections | How light it is coming home, morning darkness, outdoor work and club-activity schedules |
- National Astronomical Observatory of Japan, Ephemeris Computation Office (国立天文台 暦計算室) — sunrise, sunset, and transit times by location
- National Astronomical Observatory of Japan (国立天文台) — explanations of the calendar and astronomical phenomena
- National Astronomical Observatory of Japan (ed.), Rika Nenpyō (『理科年表』, Chronological Scientific Tables), Maruzen Publishing — annual editions, calendar section tables of sunrise/sunset and the equation of time
- Nagasawa Ko (長沢工), Calculating Sunrise and Sunset — How to Find the Times Celestial Bodies Rise and Set (『日の出・日の入りの計算 ― 天体の出没時刻の求め方』), Chijin Shokan
※This article is a general-audience science explainer. The figures given are approximations meant to illustrate the underlying mechanism. Actual sunrise and sunset times vary by location and year, so check an official source such as the National Astronomical Observatory of Japan for precise values.