[chris (OP)]

This question was covered on the Naked Scientists a few years back:

https://www.thenakedscientists.com/articles/questions/why-does-it-get-dark-quickly-winter

I don't know about sunset, the answer supplied leaves me completely in the dark!

I'd like to improve it and replace what is written there with something much clearer. Looking around the web, the quality of answers to this question are not good on the whole, so I think we have an opportunity to improve things.

Therefore can everyone here help me to arrive at an answer that is both clear and accurate: Suggestions to the best answer below please to "Why does it get darker quicker in winter?"

Thanks!

[Bored chemist]

What do you mean by "the sun sets faster"?
There's less time between sunrise and  sunset in Winter, but that's not quite the same thing.
If we are talking about the interval between the Sun first touching the horizon and the Sun dispersing below the horizon then I don't think the season affects it much.

[chris (OP)]

Yes, I phrased the forum question differently; the original question answered on the show - if you look at that link - is "Why does it get dark quickly in winter?" implying that the rate of sunset is faster in the winter than during the summer, when sundown seems to last much longer. The answer appears to agree but is otherwise impossible for me to follow.

So, firstly, is sundown a more drawn-out process in summer compared with winter?

[Bill S]

I’m not going to try to answer this, but a few thoughts in the mix might help.

Let’s consider the Northern Hemisphere.  Not that it’s any different, really, but that’s where I am so it’s easier to visualise. 

The answer treats the sun as though it were moving round the Earth.  Could that where the problem lies?

The Earth is moving, the sun is following, roughly, lines of latitude.  These lines form circles, and the further North one goes, the smaller these circles become.  Naively, one might think that the sun has further to travel when nearer the equator, so it takes longer to set; but that doesn’t work because the Earth rotates at the same speed, whatever the season, so the sun should appear to cover the same number of degrees of the circle in a given time.

What about the fact that the curve is gentler when the circle of latitude is bigger? Could the light linger longer after sunset because the curve around which it has to refract is gentler?

Could atmospheric conditions have any effect?

Has anyone actually measured the time between the start and finish of the disappearance of the sun’s disc?

Is this similar to the perennial question as to why the moon looks bigger when near the horizon than when higher in the sky? 

[jeffreyH]

The ecliptic moves due to the tilt of the earth so that during daylight hours it is lower in the sky during the autumn and winter months. The angle at which the sun crosses the horizon is also changed. It is much nearer vertical in the summer months. The combination of a lower ecliptic and the angle with the horizon makes sunset a less dramatic affair. Out of sight, out of mind.

[Bill S]

So, psychological aspect, as with the moon?  Do we over-complicate things?

[chris (OP)]

It sounds like, from what you have written, @jeffreyH , that sunset does not occur more abruptly in winter? If the solar path is more vertical in summer then it should sink, and plunge us into darkness, faster in the summer than in the autumn...?

[evan_au]

The Sun sets fastest on the equator, where the length of the day is a fairly constant 12 hours throughout the year.
 
And the fastest of the fastest sunsets is on the equator at the dates of the equinox, when it descends at 90° to the horizon. It spends little time near the horizon (or just below it), so there is little twilight, but a very rapid transition from day to night.

The Sun subtends an average angle of 0.53 degrees as seen from the Earth.
- The Earth rotates 360 degrees in 24 hours,
- so the Sun will set in 2.12 minutes
- measured from when the disk touches the horizon to when the disk disappears (for a spherical Earth in a vacuum!).

For all other locations (and dates), the Sun approaches the horizon at an angle, and will take longer to drop below the horizon.

But to calculate this, I need @alancalverd to do the navigation in polar coordinates!

Note: Do not stare at the Sun to try and measure this - it will damage your eyes. Use a pinhole camera or burn out the image sensor on your smartphone....

[alancalverd]

The rate of travel of the sun is a constant 15 degrees per hour, and the angle subtended by the sun's disc is pretty well constant, so the time elapsed between the lower and upper edge of the sun reaching the horizon is constant so the duration of the true sunset phase is constant. However the variation in surface illumination between noon and the beginning of sunset is much greater in summer, so we tend to think that sunset begins earlier and therefore takes longer.

There is also a difference in cloud height between summer and winter days. You can get a long twilight period if the cloudbase is high as sunlight is reflected from high clouds around the horizon long after the disc has disappeared.

[jeffreyH]

Evan and Alan seem to have covered it in depth.

[chris (OP)]

Thank you all.

So, can I summarise by saying that the claim that the sun sets faster in winter - i.e. it gets darker quicker at that time of year - is a myth. It doesn't set quicker, but the duration of the time the Sun is sinking in the sky is longer in Summer because it was higher to begin with. But the actual disappearance of the Sun and the onset of darkness is not going to occur more quickly in winter (ignoring any other effects, such as clouds)?

[wolfekeeper]

Yes, it's a myth. The actual sunset happens more slowly in winter because even though the sun moves at a constant angular rate because of the constant rotation of the Earth, the angle of the line motion of the sun is shallower to the horizon. This effect becomes more pronounced the further north or south you are from the tropics and when you are inside the Arctic or Antarctic circle, sunset is EVER so slow!

[David Cooper]

Some people appear to be confusing two different issues. If you're at the equator and the sun is directly overhead at noon, it will pass the horizon at 90 degrees to the horizon, minimising the time taken for sunset. At the poles, the sun will be visible just above the horizon (from both poles at the same time at the equinoxes because the atmosphere causes a lensing effect allowing you to see the whole sun when it is technically just below the horizon), and it will travel round the horizon 360 degrees without crossing it. Neither of these effects sheds direct light on the issue in question.

What we're really interested in is the difference between summer and winter. At the poles, summer puts the sun higher up and it doesn't reach the horizon at all, while in winter it never appears above it, so we can't make any useful comparisons from there. At the equator it's much better, because half way between equinoxes when the sun doesn't set directly to the west, we can make comparisons with the angle at which it goes down relative to the horizon. It's still hard to visualise this, but we can exaggerate the process by imagining a greater amount of tilt in the Earth's axis. Suppose it was tilted eighty degrees instead of twenty-ish. Remember that the sun isn't the one doing the moving, so you can get a globe and orient it towards a light source, then spin it. At the equinox the sun will still cross the horizon at ninety degrees in this scenario, but what does it do in the middle of summer for someone still on the equator when the axis is pointing nearly to the sun? They will see the sun set, not in the west, but close to the north, and it will have stayed close to north in the run up to this for a full twelve hours of daytime, its path following the edge of a circle (half of which it follows during the night). When this path meets the horizon, it will cross it at ninety degrees, although that angle only applies for a short time and will change a fraction during the time the disc of the sun is disappearing from view. Once it is below the horizon, this change of angle will increase and prolong the dusk a little, making it take a bit longer to become fully dark, and it may not even become fully dark.

This suggests that the sun might cross the horizon at the same angle in summer and winter for any given point on the Earth, but the transition to night can still lengthen due to the sun not going so low below the horizon, and indeed, I live far enough north that the sky glows to the north all night long at the start of the summer. (A bit further north, in Shetland, they call this the "simmer dim" because it's never fully night [simmer=summer]). Whichever fixed point on the Earth you view the sun from, it follows a circular path on the sky which intersects a plane at some angle, and the diameter of the circular path varies throughout the year. It's easiest to visualise this happening with the exaggerated example from before - if the axis points at the sun, the path of the sun during 24 hours will be a point rather than a line. If the axis points near to the sun, the path becomes a small circle. When the axis is pointing at 90 degrees away from the sun, the sun's path across the sky forms its largest circle of the year. Viewed from the equator, all these circles have the sun cross the horizon at 90 degrees to it. Viewed from the poles, all the paths are parallel to the horizon, so again there is no change in this throughout the year. Can the angle change for different times of year at locations in between? I doubt it, but I can't work out how to calculate it for specific latitudes to prove it (other than the equator and poles).

It occurs to me though that the size of the circle the sun follows across the sky is the same size at the summer and winter solstices, so the problem can be simplified to these two cases. In one case, most of the circular path is above the horizon, whereas in the winter most of it is below the horizon. These two circles are intersecting a plane, and the part of the summer path that's below the horizon is the same shape as the part of the winter path that's above the horizon, so the angles at which they cross the plane must be the same. I suspect that this may serve as a simple proof that there is no change in the intersection angle for any given point on the Earth, at least when comparing the solstices.

[David Cooper]

... But it's not the angle alone that counts. In the case of observing from the equator, the circle of the sun's path across the sky is largest at the equinoxes, so it sets fastest at those times even though it always goes down perpendicular to the horizon. From locations far from equator and pole, the size of the circular path across the sky is the same at the winter solstice as it is for the summer solstice, so it "moves" across the sky at the same speed and must cross the horizon at the same speed (and angle). There is a big difference between the two cases though, because in winter, the angle steepens after the sun has set, whereas in summer it gets more shallow, so the average angle during the half hour (or hour) following sunset is steeper in winter than in summer - it is only identical at the point when it's crossing the horizon.

[Petrochemicals]

The sun sets very fast on the Mediterranean and earlier than here in summer. But i would not say it sets quickly here in winter .Reason being that the suns light is refracted by clouds and the atmosphere and travels round the planet long after the sun has passed beyond the horizon vizible from ground level. At the equator there is no land higher or lower to refract or dispurse the light . Maybe the winter effect has something to do with light quality of the surrounding area also being poor also. The sun sets at about 3 15 today on a clear day it still takes a good long while to darken.

The poles are an exam0le of that, whilst it gets light some days for a long time , the sun does not rise above the horizon. Slowly the sun rises until it never goes away until next winter, that is a long sunset.

As for the sun rizing, i have watched it f4om the top of a hill, the sun shoots up from the horizon at a rate of knots, and slows down until around noon it barely moves, and then increaces in speed once more. Due to the perspective of angular observation

[evan_au]

At the equator there is no land higher or lower to refract or dispurse the light

I don’t understand what you mean - please clarify.
- There are places in the tropics where the sun sets over the sea. There are places in the UK where the Sun sets over the sea.
- There are placeds in the tropics where the sun sets over a mountain. There are places in the UK where the Sun sets over a mountain.
- I can’t see how higher or lower altitude can be compared
- These effects would be easier to compare with sunset (or sunrise) over the sea, since mountains are too variable.

Or do you mean higher or lower latitudes?
- in reality, any single point on Earth in temperate zones will experience a change in Sun angle of ±23°
- the range is reduced in the tropics, because the Sun has two positions with the same angle
- the range is reduced at the poles, because the Sun doesn’t rise in winter.

[Petrochemicals]

At the equator there is no land higher or lower to refract or dispurse the light

I don’t understand what you mean - please clarify.
- There are places in the tropics where the sun sets over the sea. There are places in the UK where the Sun sets over the sea.
- There are placeds in the tropics where the sun sets over a mountain. There are places in the UK where the Sun sets over a mountain.
- I can’t see how higher or lower altitude can be compared
- These effects would be easier to compare with sunset (or sunrise) over the sea, since mountains are too variable.

Or do you mean higher or lower latitudes?
- in reality, any single point on Earth in temperate zones will experience a change in Sun angle of ±23°
- the range is reduced in the tropics, because the Sun has two positions with the same angle
- the range is reduced at the poles, because the Sun doesn’t rise in winter.

And at the equin9xes ?

The second one Ewan lattitudes, if you are shining a light, face on to something you get a small area illuminated and reflects straight back, if you shine the light at an angle, the light is reflected and spread in the oppposite direction, it is drawn out, lenghthened and elongated. If tye sun is the same the light is not as strong but is more spread. Hope that gives you a view of what i hav3 tried to say. As for the land its to do with light at an angle reflecting off it.

The winter thing is to do with the angle, we direct sun, and when the sun sets  we get the reflecti9ns of lots of landmasses to our west, because 9f the winter tilt the size of these land masses is greatly reduced in winter due to the angle of the earth. Again this is illusion, just there is less light perceptable

In the summer it only gets truly dark for about 3 hours, 11-2am m after there is a tinge of light in the air The equatorial regions, south america africa are noted for how quick night set in.