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We all know the Winter Solstice comes on December the 20th or 21st, which is (by definition) the shortest day of the year.

The Winter Solstice day is not the day of the year the Sun rises later (that would be one or two weeks later), and also is not the day the Sun sets earlier (that would be one or two weeks earlier)...

Why is it like this? Why aren't sunset and sunrise times symmetric?
I.e., why isn't there a "middle of the day" time valid throughout the year?
Does it have something to do with the eccentricity of the Earth around the Sun?
Does it have something to do with the fact the Earth is not a perfect Sphere?

I would understand a technical explanation, but I am asking for a widely-understandable, simple one. Analema on a globe
Pictured: an analema on a globe.

Nico
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    Welcome to SE-Physics. This interesting question would be even more interesting if you could cite some references for the effect you refer to (Winter solstice sun rise/set). Also an explanation of the picture you shared would be helpful, too. Thanks in advance. – FraSchelle Dec 02 '13 at 10:27
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    Pictured it is the analema, the "curve representing the changing angular offset of a celestial body" (Wikipedia). And I don't see why you need references on some approximate data in a question... – Nico Dec 02 '13 at 10:45
  • Well, you said "But, if I understood my astronomy classes correctly, [...]" but the statement following is either true or wrong, and it is not related at all to your understanding. A good way to avoid this is to cite your source(s), then the reader can clarify this point. – FraSchelle Dec 02 '13 at 10:55
  • I am sorry, but it was simply an expression, a way to introduce the concept without saying "if Google is right..." or a similar stupid sentence. I will just remove it to avoid misunderstandings – Nico Dec 02 '13 at 11:22

4 Answers4

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It is happening because of the acceleration of the Earth orbital speed around the Sun (Earth is near the perihelion). Between December 13 and December 31 the Earth is speeding up and also it is normally rotating around its axis. These 2 movements (constant rotation and increasing orbital speed) add up to create the observed apparent movement of the Sun on the Earth sky. The Sun rises later and also sets later every day.

It is a bit tricky to visualize, so try it with a globe.

mpv
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  • Then, if it happens because it's on the perihelion (the closest point in the orbit to the Sun) then some 13,000 years ago, when the perihelion happened on July, what happened with the day length in the winter solstice? I know I'm getting deeper into the matter, but it's getting interesting (in my opinion) – Nico Dec 02 '13 at 20:40
  • This is only half the story. Or really, less than half the story. The seasonal effect caused by the angle between the equator and the plane of the earth's orbit has a greater impact on the length of the solar day than the annual effect caused by the variation in the earth's orbital speed. You haven't mentioned the seasonal effect at all. – Dawood ibn Kareem Mar 27 '17 at 07:00
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There are two causes of this and other effects which are basically that noon as measured by the Sun, i.e. when it is due south, is usually not the same as noon by local clock time. The two phenomena are: the speeding and slowing of the Earth in its elliptical orbit round the sun, and the inclination of the Earth's axis to the plane of the ecliptic. If we kept clock noon the same as Solar noon, then days would vary rather awkwardly in length through the year. Here at latitude 52 degrees North, noon today, 4th December, is about 11-53, and changes quite rapidly to 12-07 p.m. in a month's time. It "drags" dawn with it, so mornings stay dark well into January, while sunset begins to advance, giving us short mornings and longer afternoons.

At any particular location, the situation is made slightly more complicated by the use of time zones.

It's the reason why we have "Greenwich mean time, which is calculated as though the sun moved at a constant speed in a circular path round the Earth, to give us days of equal length, exactly 24 hours between successive noons. (almost exact-- someone will cavil)

This effect is rather curiously known as "the equation of time" and a good account is given at: http://en.wikipedia.org/wiki/Equation_of_time. There is a detailed description of time and its measurement in the book "Greenwich Time and the Longitude" by Derek Howse, published by Greenwich Observatory.

Harry Weston
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  • +1 for that hell of a good answer...! Very well explained – Nico Dec 04 '13 at 21:04
  • This is the best answer here, although I would have enjoyed a bit more detail as to WHY solar noon moves forward so much in December. If I find the time, I might write an answer myself later. – Dawood ibn Kareem Mar 27 '17 at 07:03
  • Thank you for the kind comment David Wallace. I will add another answer to explain this in detail, but other commitments must delay that a while. The basic answer is that there are the two elements to the sun's apparent drift, one occurs once a year and the other twice a year, both having peaks that coincide in December to give that effect. – Harry Weston Mar 28 '17 at 10:27
  • Effect of orbital speed: as we go round the sun we have to look further to the east to see the sun, more than a 360° rotation. When the orbital speed is high, in Northern winter when the Earth is closest to the sun, this extra time is increased, and in summer when we are further away and the speed is less, the extra time is reduced. This results in a longer solar day in winter and a shorter solar day in summer. The average speed is 66 000 mph, one Earth diameter every seven minutes (average 29.79 km/s, max 30.29, min 29.29). – Harry Weston Mar 28 '17 at 10:30
  • I've had to split this explanation into two because of the character limit for comments. The effect of the tilt is: if the Sun moved at a constant speed along the Ecliptic, when parallel to the Equator at Solstices its movement along the equator is at the same speed. But when the Ecliptic is inclined, at Equinoxes, its projection on to the Equator is shorter, so it moves more slowly relative to GMT. Greenwich Mean Time is calculated as though the Sun moved at a constant speed along the equator. Strictly the time scale is UTC, GMT is the time zone name. – Harry Weston Mar 28 '17 at 10:32
  • We don't notice the changes from April to October because the sun is high in the sky at midday, with early dawns and late sunsets outside the working day. Dull November, with mornings longer and afternoons shorter by over 20 mins ,is obscured by the change from summer time at the end of October. In the days before world war one, when people lived more outside and there was less artificial lighting, I am sure that the early dusk added to the tradition of dullness for November. Nowadays in UK we don't notice the short November afternoons because of the enormous jump caused by the end of BST – Harry Weston Mar 28 '17 at 10:46
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Solar days

Solar noon is the time of day when the sun reaches its highest point in the sky, as seen from any particular place on the earth's surface. A solar day is the length of time that passes between solar noon on one day, and solar noon the following day.

The earth takes 23 hours, 56 minutes and 4.09 seconds to spin on its axis. During that time, it also traverses a short distance around its orbit around the sun - approximately one degree of angle - which causes a change in the apparent position of the sun in the sky. In one solar day, the earth must complete an entire rotation on its axis, plus rotate a little more to compensate for that extra (approximately) one degree of angle that the sun appears to move. This little extra rotation takes 3 minutes and 55.91 seconds on average, meaning that the average solar day is almost exactly 24 hours.

However, there are two effects that cause the time taken for the extra rotation to vary substantially. They must both be taken into account, when calculating the length of the solar day. In particular, towards the end of December, the solar days are approximately 25 seconds longer than the annual average.

Seasonal effect on solar days

There is an angle of approximately 23.5 degrees between the plane of the equator and the plane of the earth's orbit. This means that an angle of one degree in the earth's orbit doesn't necessarily correspond to an angle of one degree in the earth's rotation. Close to the solstices (so December and June) this angle between the equator and the orbit implies that to compensate for each degree of the earth's revolution around the sun, the earth must rotate approximately 1.09 degrees. That extra 0.09 of a degree takes the earth approximately 21 seconds. So this has the effect of lengthening the solar days by roughly 21 seconds close to both solstices. Close to the equinoxes (March and September) the opposite effect occurs, and to compensate for each degree of the earth's revolution around the sun, the earth only needs to rotate approxiately 0.92 degrees. This has the effect of shortening the solar days by roughly 20 seconds close to the equinoxes.

Annual effect on solar days

The earth's orbit is not perfectly circular, so generally, it's not travelling exactly at a right angle to the sun. This means that the sun is sometimes pulling against the earth's motion, slowing it down; and sometimes pulling with the earth's motion, speeding it up. The effect is very slight - the earth travels about 1.7% faster than its average when it's closest to the sun in early January; and about 1.7% slower than its average when it's furthest from the sun, in early July. This means that the extra angle that the earth has to make up, from one solar noon to the next, is greatest in December and January, and least in June and July. This has the effect of lengthening the solar days by about 4 seconds at the start and end of the calendar year; and shortening the solar days by about 4 seconds in the middle of the calendar year.

Implication for December's sunrise and sunset times

When calculating sunrise and sunset times, it's necessary to take into account both the time of the solar noon, and the length of the daylight period. Roughly speaking, half of the daylight period occurs between sunrise and solar noon; and half occurs between solar noon and sunset. Now the daylight period is pretty short (for the northern hemisphere) or pretty long (for the southern hemisphere) throughout December, and doesn't vary as much as it does at other times of the year. But the time of solar noon varies a lot, because December has such long solar days.

For about 10 days before the solstice (so typically from 11 to 21 December) the daylight period is getting shorter (northern hemisphere), but so slowly that the long solar days have a greater effect on the time of (northern hemisphere) sunset. So the sunsets move slowly later during this period, even though the daylight period is shortening.

For about 10 days after the solstice (so typically from 21 to 31 December) the daylight period is getting longer (northern hemisphere), but so slowly that the long solar days have a greater effect on the time of (northern hemisphere) sunrise. So the sunrises move slowly later during this perod, even though the daylight period is lengthening.

Dawood ibn Kareem
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Think about this :

People that live on the equator have 12 hours of day and twelve hours of night all year ! If the earth's axis was perpendicular to the plane of its orbit (ecliptic) everyone would have twelve hour days and nights all year. But that ain't the model. The axis is ~ 23.5 degrees to the perpendicular. The plane containing the Earth axis and the line from earth center to sun center is perpendicular to the ecliptic twice a year ,i.e. the solstices . These do not coincide with earth's apogee and perigee about the sun .

Currently, the Earth reaches perihelion in early January, approximately 14 days after the December Solstice. At perihelion, the Earth's center is about 0.98329 astronomical units (AU) or 147,098,070 kilometers (about 91,402,500 miles) from the Sun's center.

The Earth reaches aphelion currently in early July, approximately 14 days after the June Solstice. The aphelion distance between the Earth's and Sun's centers is currently about 1.01671 AU or 152,097,700 kilometers (94,509,100 mi).

a good read may be found at

https://en.wikipedia.org/wiki/Apsis

MAY THE FORCE BE WITH YOU !

mudbone
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