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I live about 45 degree north latitude. Other then at the Equinox, daylight is seldom 12 hours. Sometimes it is more sometimes less. The summer sun can last 16+ hours.

I understand how and why there is only a single sunrise and sun set at the North and South Poles. But I am trying to get my mind around daylight actually lasting 12 hours every day all year. It seems like the Earth's tilt and our journey around the sun would make it so the amount of daylight is always changing no matter where you are on the Earth.

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    Perhaps an Astronomy SE question? – Timmy Jim May 1 '17 at 14:30
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    At first it sounds like you are asking a question about places receiving a total of 12 hours over the entire year. Later it sounds like you are saying 12 hours per day every day of the year. If the latter: no, that does not happen. If the former: only if there are nearby features such as cliffs which block out most of the sunlight. I saw an article once about a town in a valley surrounded by steep hills which could go for weeks without any direct sunlight; during the day it would be an early-dusk level of darkness. – Loduwijk May 1 '17 at 23:02
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If you neglect the influence of the terrain/buildings, the atmosphere and the height above sea level, both the time with and without sunlight is very close to 12 hours on the equator (other minor deviations may arise from the non-spheric form of the earth, non-constant rotational speed of the earth and probably some more minor influences). This is due to the equator line always crossing the day-night-border exactly in the middle. This article on latitude has three images (10.-12.) which illustrate this nicely.

These two links also provide explanations for this:

In the Northern Hemisphere, the length of the day is longer during the months when the North Pole is tilted towards the Sun and shorter during the months when it's tilted away from the Sun. The reverse is true for the Southern Hemisphere. The Equator is exactly halfway in between the poles. So it wouldn't make any sense for a day on the equator to be longer when one of the poles is tilted towards the Sun, and shorter when the other one is.

http://curious.astro.cornell.edu/about-us/161-our-solar-system/the-earth/day-night-cycle/195-is-the-sun-always-up-for-exactly-12-hours-at-the-equator-beginner

If you do not consider the atmosphere's influence and calculate sunrise and sunset when the true solar disk is on the horizon, then day and night are equal on the equator.

http://islandnet.com/~see/weather/whys/day_equator.htm

In reality the refraction happening in the atmosphere extends the time in which it is light, depending on models this is estimated at 6-8min.

  • This is wrong. It is only uniform when the sun is directly overhead. The sun is only directly over the equator two days a year. The equator experiences the smallest deviation over the course of a year. If there is difference at 40 degrees there is a difference at 0.004 degrees. – paparazzo May 1 '17 at 19:06
  • I never said the sun was directly over the equator all year, there still is 12h of sunlight all year. Can you back your claims up with sources? I agree, mine aren't very strong, but I don't want to invest time into searching a good source over this purely geometrical question. Look at the images in my first link, they are enlightening. – imsodin May 1 '17 at 20:04
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    @imsodin It would not be so easy to find fault with your answer if you at least removed the word "exactly," as that does allow for easy argument. It is not exactly 12 hours year round. It would be closer to that than any other latitude, and it does average to 12, but it is not exactly that all year. – Loduwijk May 1 '17 at 23:08
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    That argument I can't win, you are obviously right - maintaining something is exact is impossible. So I made my statement not absolute and added other minor factors that can contribute to a deviation (thought those are tiny). – imsodin May 2 '17 at 7:08
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Imagine for a minute that the Earth is perfectly spherical, that sunlight arrives "from infinity", and that any given spot is either lit or unlit. Then, at any given time, exactly half the surface is in "daytime" and the other half in "night-time". The line separating the two is a Great Circle - a circle with the same centre and radius as the solid sphere.

It should be clear that any other Great Circle on this ideal Earth must intersect this penumbral line and be divided exactly into two equal parts of light and dark.

Now consider the path that any given place on the sphere travels as it rotates. It will move around a line of latitude (i.e. always eastwards). This path forms a circle parallel to the Equator. There is a line of latitude that is a Great Circle, and that is the Equator itself.

So we can deduce that a point on the Equator, in ideal conditions, will spend exactly half the day on the light side and half the day on the dark side of the Earth (except for the case we ruled out where the Sun is exactly over one of the poles).

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It is very close to a 12 hour day of light every day in the Southern Philippines. The mountains behind were I live have some effect on shorter afternoon. & the sun rises over the S Pacific. We have about 5 months the sun is in the North side of the house. About 7 months the sun is south side. But on a boat at sea at the equator it could be done. But still on land only 1 day a year were we would have the exact 12 hour day.

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At the equator there is about 12 hours year round. This is due to the tilt of the earth. The equator more directly faces the sun than any other part of earth.

http://articles.chicagotribune.com/2011-07-08/news/ct-wea-0708-asktom-20110708_1_equator-length-daylight

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    "This is due to the tilt of the earth." False. The variation is due to the tilt, even if it is only a 2 minute annual variation. If there were no tilt, it would be exactly the same amount year round. "The equator more directly faces the sun..." Again false. The area getting the most direct sunlight varies during the year due to the tilt. At equinox this is the equator, but at the solstices the most direct sunlight is at one of the tropics. – cobaltduck May 1 '17 at 13:20
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    This answer would be correct if it ended both of its sentences with "on average." It is only true if you average the results over the year. – Loduwijk May 1 '17 at 23:04
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Give the check to avanalfen.

Day light is effected by tilt of the earth. The tilt of the earth is what causes seasons.

When the sun is directly overhead absent other factors the split would by 50% 50%.

Only between the tropics is the sun ever directly overhead.

The equator has the least deviation from sun directly over head. At the equator the deviation is about 2 minutes.

But there are other factors. Daylight is extended by the atmosphere refracting the light and gravity bend light (a very small amount). This adds about 7 minutes which is greater than 2 minutes so there is a range around the equator that is more than 12 hours a day year round.

  • What is it with those 2 minutes? Is this because of the earth being a potato rather than a sphere? – imsodin May 1 '17 at 16:16
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    @imsodin1 The 2 minute deviation at the equator is due to the tilt. – paparazzo May 1 '17 at 16:21
  • That alone doesn't really work out: When you are exactly on the equator of a perfect sphere, day and night will have the exact same length. Regardless of the tilt, the equator-line will always intercept the day-night-border on the plane defined by the centers of sun and earth (sphere). – imsodin May 1 '17 at 17:05
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    @imsodin My answer clearly covers that in the last paragraph. As does the link in the answer from avanalfen. – paparazzo May 1 '17 at 17:22
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    Tried to remove incorrect info from this answer but that would change it too much, so I won't touch it – anatolyg May 10 '17 at 11:30

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