# Why is the change in sunrise/sunset not symmetrical? **URL:** \ **Category:** Factual Questions\ **Created:** [December 19, 2004, 11:56pm UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705 "2004-12-19T23:56:25Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [December 19, 2004, 11:56pm UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/1 "2004-12-19T23:56:25Z")
OK, I don’t exactly know the right terminology, so here’s the question: **Why does the winter solstice, being the shortest day of the year, not have both the earliest sunset and latest sunrise of the year?** According to the [US Naval Observatory Sunrise/Sunset table](http://aa.usno.navy.mil/data/docs/RS_OneYear.html) , the earliest sunset of the year occurs around December 9th, but the latest sunrise of the year occurs around January 4th. (This is for coordinates E142:23, N44:55, GMT -9). My flawed layman’s math tells me that the sun’s shadow across the earth makes a parabola of daylight. Parabolas are symmetrical. Therefore, the solstice should have both the latest sunrise and earliest sunset, and then every day before or after would have an equally changed sunrise and sunset. But this is definitely not the case. Please take this as a cry for help in understanding basic 3-D geometry. I have never been good at understanding the prorated volume of an abraded frenulum or whatever it is you mathemeticians call problems of this type. ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [December 20, 2004, 1:57am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/2 "2004-12-20T01:57:24Z")
> [@NattoGuy](#): > > Please take this as a cry for help in understanding basic 3-D geometry. It is a problem in astronomy. > [@](#): > > I have never been good at understanding the _prorated volume of an abraded frenulum or whatever_ it is you mathemeticians call problems of this type. Whats that? I’ll post the answer furnished by a well know astronomet to the sunrise/sunset after others have an opportunity to supply the answer. The equation of time is NOT the answer. You forgot to start a new thread on another question in the same theread you hijacked this topic from. Namely: > [@](#): > > - > > Prior to some time shortly after 1983 an annual edition of “he Astronomical Almanac” was published in Washington DC and London. This was THE BOOK for navigators on ships and planes. Also used extensively by land surveyors. > > QUESTION FOR YOU: What takes it’s place today?\* ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [December 20, 2004, 2:41am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/3 "2004-12-20T02:41:26Z")
> [@PinGear](#): > > You forgot to start a new thread on another question in the same theread you hijacked this topic from. Namely: I started a thread on a question I wanted answered from [a different thread](http://boards.straightdope.com/sdmb/showthread.php?t=292505). If you have a question you want answered, you’re free to start your own. Oddly enough, I think the question I’m asking is actually a question that you originally raised on [that very thread](http://boards.straightdope.com/sdmb/showthread.php?t=292505)… > [@PinGear](#): > > New Question: " Why are the sunrise and sunset curves shifted and not colinear? I brought your question into this new thread… so what’s your complaint? Lastly, a “hijack” is when you jump on someone’s topic and use it to talk about unrelated topics. I started this topic specifically to avoid hijacking the other guy’s thread. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [December 20, 2004, 5:12am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/4 "2004-12-20T05:12:01Z")
[This](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980116c.html) should answer your questions. Parabolas have nothing to do with it! After you read the link, just as a little educational exercise, make it a point to start noticing when the Sun hits the north/south meridian where you live. This is easy for me because I work in downtown Chicago where the streets are laid out on a perfect north/south and east/west grid. Chicago is two degrees east of the central line of the Central Time Zone (90 degrees West), so “average” high noon is 11:52 a.m. The “equation of time” means that high noon comes between 16 minutes early and 14 minutes late, or between 11:36 a.m. and 12:06 p.m. I always go to lunch around 11:45. And, depending on time of year, the Sun will either be just east of the North/South streets, or directly on the meridian, or just west. (In summer, of course, it’s shifted by an hour.) I notice little stuff like that. I’m weird that way. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [December 20, 2004, 6:01am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/5 "2004-12-20T06:01:35Z")
> [@PinGear](#): > > The equation of time is NOT the answer. It’s not??? The answer is this. You could define “one day” to be the amount of time between when the sun is at its peak on one day, to its peak on the next day. If the Earth went around the Sun in a perfectly circular path, this would be simple. But the path isn’t circular, it’s an ellipse. When we’re closer, generally in the winter in the northern hemisphere, the Sun’s position moves a little farther each day than in the summer, shifting “local noon” earlier or later than you would expect it. So we define the day as the average of all the days in the year, and some are a little longer and some are a little shorter. I thought that this explanation was abbreviated the “equation of time,” but maybe I’m wrong. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 20, 2004, 8:33am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/6 "2004-12-20T08:33:57Z")
The middle of the daylight period shifts throughout the year; this is pretty much the main reason for DST-type clock adjustments; it’s down to the tilt of the Earth’s axis - it isn’t exactly parallel to the day/night terminator (which is exactly perpendicular to the rays of the sun) - in spring, the northern hemisphere is ‘leaning’ toward the terminator, so locations further north (on the same longitude) experience sunrise and sunset earlier than locations further south - dawn gets earlier quicker than sunset gets later. In autumn, the northern hemisphere is ‘leaning’ _away from_ the terminator, so locations further north (on the same longitude) experience sunrise and sunset _later_ than locations further south - sunset gets earlier quicker than dawn gets later. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [December 20, 2004, 4:43pm UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/7 "2004-12-20T16:43:45Z")
> [@Mangetout](#): > > The middle of the daylight period shifts throughout the year; this is pretty much the main reason for DST-type clock adjustments; Umm . . . No, it isn’t. The Equation of Time does _not_ provide the motivation for Daylight Savings Time. > [@](#): > > it’s down to the tilt of the Earth’s axis - it isn’t exactly parallel to the day/night terminator (which is exactly perpendicular to the rays of the sun) - in spring, the northern hemisphere is ‘leaning’ toward the terminator, so locations further north (on the same longitude) experience sunrise and sunset earlier than locations further south - dawn gets earlier quicker than sunset gets later. That’s how the axial tilt drives the change in seasons. The axial tilt is also _partly_ responsible for the Equation of Time (the other factor is the Earth’s elliptical orbit), but in a different way. Memo to mods and admins: This question comes up, as regularly as the seasons, with each solstice. A complete explanation of the Equation of Time and how it changes in the course of a year, and how those changes interract with latitude to drive the earliest and latest sunrises and sunsets, would require an essay of a couple thousand words. This would be a good subject for an SDSAB report. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [December 21, 2004, 1:25am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/8 "2004-12-21T01:25:03Z")
Thanks for answering my question, FreddyThePig. If NASA says it’s due to the equation of time, I’m gonna have to go with the equation of time, PinGear’s comment notwithstanding. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 21, 2004, 1:38am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/9 "2004-12-21T01:38:45Z")
> [@Freddy the Pig](#): > > That’s how the axial tilt drives the change in seasons. I’m well aware that the axial tilt drives the seasons, but I wasn’t talking about that at all. Not even a bit. ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [December 21, 2004, 1:57am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/10 "2004-12-21T01:57:01Z")
> [@CurtC](#): > > It’s not??? > > The answer is this. You could define “one day” to be the amount of time between when the sun is at its peak on one day, to its peak on the next day. If the Earth went around the Sun in a perfectly circular path, this would be simple. But the path isn’t circular, it’s an ellipse. When we’re closer, generally in the winter in the northern hemisphere, the Sun’s position moves a little farther each day than in the summer, shifting “local noon” earlier or later than you would expect it. So we define the day as the average of all the days in the year, and some are a little longer and some are a little shorter. > > I thought that this explanation was abbreviated the “equation of time,” but maybe I’m wrong. The question was WHY not how. You have hit dangerously close, mentioning the **eliptical orbit** of the earth around the sun. The equation of time tells you how much fast or slow the time is at your meridian with respect to the sun. ---
**Author:** ![Yeticus\_Rex](https://avatars.discourse-cdn.com/v4/letter/y/a698b9/32.png) [@Yeticus\_Rex](https://boards.straightdope.com/u/Yeticus_Rex)\ **Post date:** [December 21, 2004, 2:13am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/11 "2004-12-21T02:13:38Z")
[Analemmas and the Equation of Time.](http://www.analemma.de/english/analem.html) [Another cool site with pics.](http://solar-center.stanford.edu/art/analemma.html) [And other Googled sites.](http://www.google.com/search?hl=en&q=analemma+sun+pic) It will help you visualize the what the combination of Earth’s axial tilt and eccentricity does to the apparent placement of the Sun in the sky for the same time every day, for the year…and how they end up as the “Equation of Time” analemmas that end up on globes. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [December 21, 2004, 2:14am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/12 "2004-12-21T02:14:28Z")
> [@PinGear](#): > > The question was WHY not how. > > You have hit dangerously close, mentioning the **eliptical orbit** of the earth around the sun. PinGear, either you have an explanation or you don’t. Others have provided good explanations. If you have your own explanation, you’re welcome to share it. If you don’t, please stop dancing around the issue or responding to questions with questions. Thanks. ---
**Author:** ![Yeticus\_Rex](https://avatars.discourse-cdn.com/v4/letter/y/a698b9/32.png) [@Yeticus\_Rex](https://boards.straightdope.com/u/Yeticus_Rex)\ **Post date:** [December 21, 2004, 2:26am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/13 "2004-12-21T02:26:50Z")
[Best explanation for the “Equation of Time” complete with mathematical equations.](http://www.analemma.com/Pages/framesPage.html) ---
**Author:** ![Yeticus\_Rex](https://avatars.discourse-cdn.com/v4/letter/y/a698b9/32.png) [@Yeticus\_Rex](https://boards.straightdope.com/u/Yeticus_Rex)\ **Post date:** [December 21, 2004, 5:12am UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/14 "2004-12-21T05:12:29Z")
> [@NattoGuy](#): > > OK, I don’t exactly know the right terminology, so here’s the question: **Why does the winter solstice, being the shortest day of the year, not have both the earliest sunset and latest sunrise of the year?** > > According to the [US Naval Observatory Sunrise/Sunset table](http://aa.usno.navy.mil/data/docs/RS_OneYear.html) , the earliest sunset of the year occurs around December 9th, but the latest sunrise of the year occurs around January 4th. (This is for coordinates E142:23, N44:55, GMT -9). The answer is the summation of the two effects of axial tilt and the elliptical orbit. [This graph pretty much says it all.](http://www.analemma.com/Pages/Summation/SummationEffect/Summation.html) ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [December 21, 2004, 4:03pm UTC](https://boards.straightdope.com/t/why-is-the-change-in-sunrise-sunset-not-symmetrical/280705/15 "2004-12-21T16:03:43Z")
> [@Mangetout](#): > > I’m well aware that the axial tilt drives the seasons, but I wasn’t talking about that at all. Not even a bit. Then why did you bring up which hemisphere was tilted toward the Sun at which time of year? The effect of axial tilt on the Equation of Time, which drives the asymmetry in sunrises and sunsets, is the same in both hemispheres at both solstices.