# Does every spot on the planet's surface get the same amount of daylight over a year? **URL:** \ **Category:** Factual Questions\ **Created:** [September 23, 2010, 12:57am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720 "2010-09-23T00:57:12Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [September 23, 2010, 12:57am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/1 "2010-09-23T00:57:12Z")
I’ve tried to figure this one out in my head, by imagining a rotating globe orbiting the sun, and it seems that over the course of a year, the average spot on the planet’s surface should see about 12 hours of daylight per 24 hours. (For our purposes, daylight is defined as the sun being above the horizon, ignoring unusual geographic features such as high mountains, deep valleys, etc.) For example, at the equator, day and night length don’t change much, about 12 hours each and every day. Average: 12 hours of daylight/24 hours over the year. Meanwhile, at the poles, it’s mostly 6 months of daylight, followed by 6 months of night, averaging to 12 hours of daylight/24 hours over the year. But is that really true? I turn to my fellow dopers to affirm/shred this hypothesis of mine… ---
**Author:** ![Simplicio](https://avatars.discourse-cdn.com/v4/letter/s/c37758/32.png) [@Simplicio](https://boards.straightdope.com/u/Simplicio)\ **Post date:** [September 23, 2010, 1:06am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/2 "2010-09-23T01:06:32Z")
Yes. Or it would if the Sun was a point, since its like a half degree in diameter you get another minute (someone check my math) of daylight then you do darkness. So theres that to cheer you up when your feeling blue, averaged out you’ll spend a couple more hours of your life under the sun then you will in darkness. ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [September 23, 2010, 1:07am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/3 "2010-09-23T01:07:12Z")
I believe this would be true if the Earth’s orbit were exactly circular. But the Earth is a bit closer to the Sun in the (northern hemisphere) winter so with a constant rotation rate, I suspect it’s not exactly true, but I can’t tell what part gets more. It should be very correct to a high approximation. Of course the quality of the sunlight is not the same. The North Pole, for example, gets its fair share of sunlight, but the sun is never higher in the sky than 23 1/2 degrees. ---
**Author:** ![GameHat](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@GameHat](https://boards.straightdope.com/u/GameHat)\ **Post date:** [September 23, 2010, 1:10am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/4 "2010-09-23T01:10:49Z")
_I misread your post_ Mods, please delete ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [September 23, 2010, 1:30am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/5 "2010-09-23T01:30:07Z")
If we are talking about lumens - no. ---
**Author:** ![Simplicio](https://avatars.discourse-cdn.com/v4/letter/s/c37758/32.png) [@Simplicio](https://boards.straightdope.com/u/Simplicio)\ **Post date:** [September 23, 2010, 1:37am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/6 "2010-09-23T01:37:44Z")
> [@komolono](#): > > If we are talking about lumens - no. > [@OP](#): > > (For our purposes, daylight is defined as the sun being above the horizon .. ---
**Author:** ![ataraxy22](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ataraxy22/32/307_2.png) [@ataraxy22](https://boards.straightdope.com/u/ataraxy22)\ **Post date:** [September 23, 2010, 1:43am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/7 "2010-09-23T01:43:04Z")
The Earth and the Moon rotate around a common center of gravity and there are [Peaks of Eternal Light](http://en.wikipedia.org/wiki/Peaks_of_eternal_light) on the Moon, so I’d use that as evidence to say that all spots on Earth do not have the same amount of daylight per year. ---
**Author:** ![Ruminator](https://avatars.discourse-cdn.com/v4/letter/r/b9bd4f/32.png) [@Ruminator](https://boards.straightdope.com/u/Ruminator)\ **Post date:** [September 23, 2010, 1:51am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/8 "2010-09-23T01:51:16Z")
> [@OldGuy](#): > > I believe this would be true if the Earth’s orbit were exactly circular. But the Earth is a bit closer to the Sun in the (northern hemisphere) winter so with a constant rotation rate, I suspect it’s not exactly true, but I can’t tell what part gets more. It should be very correct to a high approximation. I would think a larger issue than the elliptical orbit would be the fact that the period of orbit is not an exact multiple of the # of Earth axis rotations. Something like [~365.25 days per year](http://en.wikipedia.org/wiki/Year#Summary). Therefore, without doing any complicated math at all, there’s already approximately 3/4ths of the Earth’s surface that didn’t get daylight equal to the other 365 days of the year. If Earth’s year was something closer to 365.0001 days per year it seems like we could ignore the discrepancy but 0.25 is quite a bit I think. ETA: After rereading OP, I believe he is asking a different question to validate the intuition of daylight visible to the Earth’s surface as a \*\*geometry \*\*question. For that purpose, I guess we should assume 365.00 exactly to avoid over-complicating it. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 23, 2010, 1:55am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/9 "2010-09-23T01:55:19Z")
That just means that you have to average over a period longer than a single year, though. ---
**Author:** ![Ruminator](https://avatars.discourse-cdn.com/v4/letter/r/b9bd4f/32.png) [@Ruminator](https://boards.straightdope.com/u/Ruminator)\ **Post date:** [September 23, 2010, 2:02am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/10 "2010-09-23T02:02:11Z")
Right… for example, one could find the least-common-multiple of the 2 periods and use that for an average. But the OP wrote “over a year” in his title and not “over LCM of years & days” 🙂 ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [September 23, 2010, 2:02am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/11 "2010-09-23T02:02:51Z")
Elliptical orbit causes the apparent diameter of the sun to be about 3% smaller in early July than it is in early January: [Sun at Perihelion and Aphelion](http://apod.nasa.gov/apod/ap090703.html) That’ll mess with perfect symmetry of daylength in the northern vs southern hemispheres. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 23, 2010, 2:07am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/12 "2010-09-23T02:07:10Z")
There has to be an accounting for total solar eclipses in some years when the moon is blocking the sun over some part of the earth. So there wouldn’t be an equal amount of daylight in every spot, in every year. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [September 23, 2010, 2:10am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/13 "2010-09-23T02:10:44Z")
> [@OldGuy](#): > > I believe this would be true if the Earth’s orbit were exactly circular. But the Earth is a bit closer to the Sun in the (northern hemisphere) winter so with a constant rotation rate, I suspect it’s not exactly true, but I can’t tell what part gets more. It should be very correct to a high approximation. … The Earth’s varying speed of revolution does, just as you say, skew the share of daylight time betwixt the hemispheres. Aphelion occurs around the Fourth of July (and isn’t that handy as a reminder) and the Earth is moving more slowly at this point in its orbit. (And how about a big shout-out to Kepler for figuring that out with little more than pure genius!) July 4 is not too far from the summer solstice so the Northern Hemisphere gets more than its fair share of daylight as the Earth lolly-gags along with its top end showing itself to the Sun. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 23, 2010, 5:09am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/14 "2010-09-23T05:09:55Z")
> [@](#): > > (And how about a big shout-out to Kepler for figuring that out with little more than pure genius!) Pure genius and extensive tables of the best observational astronomical data ever collected at the time, thanks to his mentor Tycho Brahe. Both deserve credit for that one. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [September 23, 2010, 7:36am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/15 "2010-09-23T07:36:49Z")
> [@Simplicio](#): > > > [@komolono](#): > > > > If we are talking about lumens - no. > > > [@OP](#): > > > > (For our purposes, daylight is defined as the sun being above the horizon > > .. Yeah, thanks **Simplicio**. Golfers may need lumens, but no sunlight… This link may help **Qadgop the Mercotan** calculate the exact hours [http://www.srrb.noaa.gov/highlights/sunrise/calcdetails.html](http://www.srrb.noaa.gov/highlights/sunrise/calcdetails.html) ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [September 23, 2010, 7:54am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/16 "2010-09-23T07:54:00Z")
> [@Simplicio](#): > > Yes. Or it would if the Sun was a point, since its like a half degree in diameter you get another minute (someone check my math) of daylight then you do darkness. There’s another effect that’s bigger than that: atmospheric refraction. Because of this, the Sun appears to rise several minutes earlier than it would on an airless planet and set several minutes later. To see this, [here’s a page](http://www.timeanddate.com/worldclock/astronomy.html?n=202&month=9&year=2010&obj=sun&afl=-11&day=1) with the sunrise/sunset/time of daylight for Portland in the month of Sept. The equinox is the 22nd when you’d expect daylight to be exactly 12 hours long. Instead it’s 10 minutes longer than that. It doesn’t get down to 12 hours until the 25th. ---
**Author:** ![Walther\_Ego](https://avatars.discourse-cdn.com/v4/letter/w/8dc957/32.png) [@Walther\_Ego](https://boards.straightdope.com/u/Walther_Ego)\ **Post date:** [September 23, 2010, 9:19am UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/17 "2010-09-23T09:19:21Z")
Also, the Earth is flatter closer to poles. That should give some extra minutes for those who live far from the equator, and particularly in midsummer. Around my latitude the sunrise is then in NNE and sunset in NNW. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [September 23, 2010, 12:25pm UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/18 "2010-09-23T12:25:16Z")
> [@Chronos](#): > > > [@](#): > > > > (And how about a big shout-out to Kepler for figuring that out with little more than pure genius!) > > Pure genius and extensive tables of the best observational astronomical data ever collected at the time, thanks to his mentor Tycho Brahe. Both deserve credit for that one. Brahe collected the data, but when it came to applying it he couldn’t see past the end of his own nose. (As a side note, a moose he owned drank too much beer and died falling down stairs. I believe I can forgive his geo- and egocentrism for that.) ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [September 23, 2010, 12:25pm UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/19 "2010-09-23T12:25:28Z")
Thanks for all the help, gang. Especially the NOAA tables, they’re fun to play with! Yes, this was essentially a geometry question, in which I had reduced orbits and globes to perfect circles and spheres, and wondered if said globe got uniform “sun over the horizon” time over the course of a year. Also a big shout-out to Kepler and his silver-nosed teacher, too, for having done the background work. 😉 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 23, 2010, 7:03pm UTC](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720/20 "2010-09-23T19:03:38Z")
> [@](#): > > Brahe collected the data, but when it came to applying it he couldn’t see past the end of his own nose. Sure, but the end of his nose could be pretty far away, if he forgot to put it on that morning. [Next page](https://boards.straightdope.com/t/does-every-spot-on-the-planets-surface-get-the-same-amount-of-daylight-over-a-year/554720.md?page=2)