<p>If you follow the sunset timing for the equator you will notice that although the day remains pretty much the same all year round, the sunset moves up and down twice a year. The biggest difference is about a half hour.</p>

<p>This is due to the changing speed of the orbit of Earth. Earth's orbits is slightly oblong. Therefore on the longer parts the planet picks up speed. Which means that although Earth made a full rotation, since it is further in its orbit it is still seeing the sun. </p>

<p>This happens twice, on both longer stretches of the orbit. However the effect on one is more pronounced than the other. This is because in one leg Earth is speeding away from the sun, which slows it a bit, and in the other it is coming toward the sun.</p>

<p>The longest and shortest days are on both points of the oblong circle but the later sunset is before that point. </p>

CC BY-SA 4.0
2021-12-12T10:28:44.017Z
So just to verify what you're saying (because the question is not a topic I've looked into that much), it's due to Kepler's Second Law (the law of equal areas swept out in equal times for elliptical orbits)?  And the part of the year it's moving faster shifts the entire day later (because we move faster, we move "further past the sun", so we must [rotate a bit further to get back to it being the same position in the sky](https://lco.global/spacebook/sky/sidereal-time/))?
JeopardyTempest
CC BY-SA 4.0
2021-12-12T21:43:41.110Z
@JeopardyTempest, yes. That is how I understand it. There is a limit to how well you can describe it without an animation.
HaLeiVi
CC BY-SA 4.0
2021-12-12T23:52:27.477Z
Great idea!  Cue [Minute Physics](https://www.youtube.com/watch?time_continue=168&v=nZMMuv0Ltyo&feature=emb_title)!!!  That helps me at least :-)
JeopardyTempest
CC BY-SA 3.0
2015-12-14T17:27:48.323Z
2015-12-14T17:27:48.323Z
HaLeiVi
<p>A brief explanation is that Earth is not a perfect sphere moving in a perfect circle. Our planet is tilted, and its tilt does not align with its orbit’s apses (its perihelion or aphelion, its nearest or farthest positions from its sun).</p>
<p>A result of these imperfections can be seen in the analemma (something commonly drawn on well-made globes). <a href="https://www.lexico.com/en/definition/analemma" rel="nofollow noreferrer">Oxford</a> Dictionary describes the analemma thus:</p>
<blockquote>
<p>the asymmetrical figure-eight curve that can be traced in the sky at a given place showing the position of the sun at the same standard time (usually twelve o'clock) on successive days of a year.</p>
</blockquote>
<p>The analemma isn’t just drawn on globes, but it can be observed in the sky. The below diagram visualizes the effect. It comes from analemma.com’s <a href="https://www.analemma.com/phenomena.html" rel="nofollow noreferrer">Phenomena</a> page, which has several more diagrams on separate pages to illustrate the effect.</p>
<p><a href="https://i.sstatic.net/dSk33.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/dSk33.jpg" alt="Diagram of analemma at sunrise and sunset." /></a></p>

CC BY-SA 4.0
2021-12-08T16:44:31.967Z
2021-12-08T16:44:31.967Z
5260452
<blockquote>
  <p>So, it seems that the earliest sunset time is a few weeks ahead of the shortest day. Why is that?</p>
</blockquote>

<p>It's because we measure time by a clock rather than by a sundial. That the latest sunrise and earliest sunset don't occur on the winter solstice pretty much (but not completely) vanishes when one looks at time as measured by a sundial.</p>

<p>From early December to mid January, a solar day (the time from one local noon to the next) is 24 or more seconds longer than is a day as measured by a clock, with the discrepancy peaking at about 30 seconds on December 25 or so. While the timing of sunrise and sunset is more or less symmetrical about local solar noon, this discrepancy between the length of a day as measured by a sundial versus the 24 hour day as measured a clock means that in the northern hemisphere, the earliest sunset (as measured by a clock) occurs sometime before the winter solstice and that the latest sunrise occurs sometime after the winter solstice. (How much depends on latitude.)</p>

<p>This discrepancy between solar time and clock time is a consequence of the Earth's axial tilt and the eccentricity of the Earth's orbit. How these two two effects combine is called <a href="http://aa.usno.navy.mil/faq/docs/eqtime.php" rel="noreferrer">the equation of time</a>. The axial tilt results in a sine wave with a period of six months and zero crossings at the solstices and equinoxes. The eccentricity results in a sine wave with a period of a year (plus a tiny bit) with zero crossings at perihelion (when the Earth is closest to the Sun) and aphelion (furthest from the Sun).</p>

<p>Right now, perihelion occurs a couple of weeks after the northern hemisphere winter solstice. (This is just coincidental. In 5000 years or so, perihelion will occur around the time of the spring equinox, and in 10000 years, around the time of the summer solstice.) The difference between length of day as measured by a sundial vs a clock currently reaches its peak in late December because of that coincidence.</p>

CC BY-SA 3.0
2015-12-15T15:37:46.380Z
2015-12-15T15:37:46.380Z
David Hammen
<p>The length of the day is the distance between sunrise and sunset. Sunrise and sunset around the longest and shortest day of the year do not go in the same "direction" as the earth makes the "turn" around the closest and farthest points of the oval orbit. That is, the sunrise and sunset times are a function of both the rotation of the earth as well as the movement of the Earth in its orbit around the sun (and the angle of the axis to the plane of the orbit).</p>

<p>Thus, there are times when sunset starts getting later (which would make the day longer) but sunrise is still getting later. For example, in Baltimore Maryland sunrise/sunset on December 5 was 7:11/4:43, December 12 was 7:17/4:44, December 19 is 7:22/4:46, and December 26 will be 7:25/4:49</p>

<p>The earliest sunset in Baltimore was at 4:43 from December 4 through December 12, while sunrise while sunrise continues getting later until January 10 (7:27 A.M.)</p>

<p>This means that the length of the day continues getting shorter until sunrise gets later less than sunset is getting later and then (sunrise) starts getting earlier.</p>

CC BY-SA 3.0
2015-12-14T20:26:22.417Z
2015-12-20T21:13:03.170Z
2015-12-20T21:13:03.170Z
sabbahillel
<p>I viewed sunset times in some major Eastern U.S. cities including Boston, New York, Philadelphia, Boston / D.C. and Miami. It seems that the earliest sunset time starts around the first week of December, stays the same for the next week, and starts to get later again by December 18.</p>

<p>The winter solstice is around Dec. 22, which is considered the shortest day (i.e. length of sunlight) of the year. So, it seems that the earliest sunset time is a few weeks ahead of the shortest day. Why is that?</p>

<p>Chicago sunset/sunrise from <a href="http://www.sciquill.com/analemma/" rel="noreferrer">http://www.sciquill.com/analemma/</a>
<a href="https://i.sstatic.net/gp6EQ.gif" rel="noreferrer"><img src="https://i.sstatic.net/gp6EQ.gif" alt="http://www.sciquill.com/analemma/"></a></p>

CC BY-SA 3.0
2015-12-15T01:57:30.620Z
I really like the explanation from http://www.sciquill.com/analemma/page2.html. It is quite intuitive. Also, the page http://www.analemma.com/ provides a great explanation and a very cool set of animations of how this effect occurs in other planets of the Solar System.
arkaia
<diurnal-cycle>
Why does the earliest sunset time not coincide with the shortest day of the year?
DanF
# Why does the earliest sunset time not coincide with the shortest day of the year?

(Asked by: username_0 on 2015-12-14)

I viewed sunset times in some major Eastern U.S. cities including Boston, New York, Philadelphia, Boston / D.C. and Miami. It seems that the earliest sunset time starts around the first week of December, stays the same for the next week, and starts to get later again by December 18.

The winter solstice is around Dec. 22, which is considered the shortest day (i.e. length of sunlight) of the year. So, it seems that the earliest sunset time is a few weeks ahead of the shortest day. Why is that?

Chicago sunset/sunrise from <http://www.sciquill.com/analemma/>
[![http://www.sciquill.com/analemma/](https://i.sstatic.net/gp6EQ.gif)](https://i.sstatic.net/gp6EQ.gif)

**Tags:** diurnal-cycle

> **Comments:**
> **username_1**: I really like the explanation from http://www.sciquill.com/analemma/page2.html. It is quite intuitive. Also, the page http://www.analemma.com/ provides a great explanation and a very cool set of animations of how this effect occurs in other planets of the Solar System.  

---

## 4 answers

### Answer by username_2 on 2015-12-14. Score: 5

If you follow the sunset timing for the equator you will notice that although the day remains pretty much the same all year round, the sunset moves up and down twice a year. The biggest difference is about a half hour.

This is due to the changing speed of the orbit of Earth. Earth's orbits is slightly oblong. Therefore on the longer parts the planet picks up speed. Which means that although Earth made a full rotation, since it is further in its orbit it is still seeing the sun.

This happens twice, on both longer stretches of the orbit. However the effect on one is more pronounced than the other. This is because in one leg Earth is speeding away from the sun, which slows it a bit, and in the other it is coming toward the sun.

The longest and shortest days are on both points of the oblong circle but the later sunset is before that point.

> **Comments:**
> **username_3**: So just to verify what you're saying (because the question is not a topic I've looked into that much), it's due to Kepler's Second Law (the law of equal areas swept out in equal times for elliptical orbits)?  And the part of the year it's moving faster shifts the entire day later (because we move faster, we move "further past the sun", so we must [rotate a bit further to get back to it being the same position in the sky](https://lco.global/spacebook/sky/sidereal-time/))?  
> **username_2**: @username_3, yes. That is how I understand it. There is a limit to how well you can describe it without an animation.  
> **username_3**: Great idea!  Cue [Minute Physics](https://www.youtube.com/watch?time_continue=168&v=nZMMuv0Ltyo&feature=emb_title)!!!  That helps me at least :-)  

---

### Answer by username_5 on 2015-12-15. Score: 5

> So, it seems that the earliest sunset time is a few weeks ahead of the shortest day. Why is that?

It's because we measure time by a clock rather than by a sundial. That the latest sunrise and earliest sunset don't occur on the winter solstice pretty much (but not completely) vanishes when one looks at time as measured by a sundial.

From early December to mid January, a solar day (the time from one local noon to the next) is 24 or more seconds longer than is a day as measured by a clock, with the discrepancy peaking at about 30 seconds on December 25 or so. While the timing of sunrise and sunset is more or less symmetrical about local solar noon, this discrepancy between the length of a day as measured by a sundial versus the 24 hour day as measured a clock means that in the northern hemisphere, the earliest sunset (as measured by a clock) occurs sometime before the winter solstice and that the latest sunrise occurs sometime after the winter solstice. (How much depends on latitude.)

This discrepancy between solar time and clock time is a consequence of the Earth's axial tilt and the eccentricity of the Earth's orbit. How these two two effects combine is called [the equation of time](http://aa.usno.navy.mil/faq/docs/eqtime.php). The axial tilt results in a sine wave with a period of six months and zero crossings at the solstices and equinoxes. The eccentricity results in a sine wave with a period of a year (plus a tiny bit) with zero crossings at perihelion (when the Earth is closest to the Sun) and aphelion (furthest from the Sun).

Right now, perihelion occurs a couple of weeks after the northern hemisphere winter solstice. (This is just coincidental. In 5000 years or so, perihelion will occur around the time of the spring equinox, and in 10000 years, around the time of the summer solstice.) The difference between length of day as measured by a sundial vs a clock currently reaches its peak in late December because of that coincidence.


---

### Answer by username_4 on 2021-12-08. Score: 3

A brief explanation is that Earth is not a perfect sphere moving in a perfect circle. Our planet is tilted, and its tilt does not align with its orbit’s apses (its perihelion or aphelion, its nearest or farthest positions from its sun).

A result of these imperfections can be seen in the analemma (something commonly drawn on well-made globes). [Oxford](https://www.lexico.com/en/definition/analemma) Dictionary describes the analemma thus:

> the asymmetrical figure-eight curve that can be traced in the sky at a given place showing the position of the sun at the same standard time (usually twelve o'clock) on successive days of a year.

The analemma isn’t just drawn on globes, but it can be observed in the sky. The below diagram visualizes the effect. It comes from analemma.com’s [Phenomena](https://www.analemma.com/phenomena.html) page, which has several more diagrams on separate pages to illustrate the effect.

[![Diagram of analemma at sunrise and sunset.](https://i.sstatic.net/dSk33.jpg)](https://i.sstatic.net/dSk33.jpg)


---

### Answer by username_6 on 2015-12-14. Score: 2

The length of the day is the distance between sunrise and sunset. Sunrise and sunset around the longest and shortest day of the year do not go in the same "direction" as the earth makes the "turn" around the closest and farthest points of the oval orbit. That is, the sunrise and sunset times are a function of both the rotation of the earth as well as the movement of the Earth in its orbit around the sun (and the angle of the axis to the plane of the orbit).

Thus, there are times when sunset starts getting later (which would make the day longer) but sunrise is still getting later. For example, in Baltimore Maryland sunrise/sunset on December 5 was 7:11/4:43, December 12 was 7:17/4:44, December 19 is 7:22/4:46, and December 26 will be 7:25/4:49

The earliest sunset in Baltimore was at 4:43 from December 4 through December 12, while sunrise while sunrise continues getting later until January 10 (7:27 A.M.)

This means that the length of the day continues getting shorter until sunrise gets later less than sunset is getting later and then (sunrise) starts getting earlier.


---

