# The Naked Scientists Forum

### Author Topic: Does the sun appear to move faster at sunrise and sunset compared to midday?  (Read 13271 times)

#### AntonMaeso

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##### Does the sun appear to move faster at sunrise and sunset compared to midday?
« on: 01/06/2013 17:43:34 »
Due to the atmosphere diffracting the light at sunrise and sunset but not at midday.

#### yor_on

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #1 on: 01/06/2013 19:29:07 »
Heh :)

Now, that's a tricky one. You mean that by being diffracted they should have a further path to go, before reaching our eyes? Or is it something else you're thinking of?

#### AntonMaeso

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #2 on: 01/06/2013 20:57:31 »
I think I meant atmospheric refraction

Since the atmosphere refracts the light around the earth during sunrise and sunset (hence the sun appears there when it isn't) but not at midday.

So if I was watching the sunrise it would appear to travel a greater distance in less time since the curve is decreasing at the same time as it moves.

I hope that is more clear.
Sorry about the very nontechnical language.

#### evan_au

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #3 on: 02/06/2013 06:27:08 »
The Sun is about half a degree across as seen from the Earth (as is the Moon). The Earth rotates 360 degrees in 24 hours, so the visible disk of the Sun passes a given reference point in 24 hours/360/2 = 2 minutes. This applies whether the Sun is at the horizon or at noon.

...or it would if the Earth didn't have an atmosphere to bend the light.

It is not wise to look directly at the Sun - but the greater depth of atmosphere at sunrise & sunset means that we might risk a quick glance to see if it's reddened disk has touched the horizon or not. (Or, safer for you, look at the LCD screen of your digital camera, and burn out the CCD sensor. Do not try this on a SLR-type Camera as the sunlight is concentrated into your eye via the viewfinder!)

When the Sun is high, it's not safe (or comfortable) to even glance at it to see exactly where it is located.

When the sun is rising or setting, the horizon provides a ready position reference - "the Sun is touching the horizon" vs "the disk of the Sun has just disappeared". This takes about 2 minutes.

When the Sun is high in the sky, there is no stable reference by which to measure its motion - so it just looks "high", and half an hour later it still looks "high". Clouds don't count as a stable reference point! ...but the (somewhat blurry) shadow of a tall building might.

A better solution might be to time the passage of the Moon, since the Moon appears about the same size as the Sun, and moonlight is not damaging to your eyesight. Watch the Moon as it rises, then watch it as it passes behind the trunk of a tree or a tall building - it should be about 2 minutes in each case (the Moon travels about 3% slower across the sky than the Sun, since it orbits the Earth in about 29 days).

#### yor_on

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #4 on: 02/06/2013 13:36:39 »
Anton, been thinking some, and, well, been thinking, at least :)

Assuming that light bend to gravity, also assuming that the gravity (potential) should be highest at ground level it seems to me that you might have a point. What speaks against the assumption of refraction should be that as long as you see the source, light still has to reach you, and some of it should still be 'un-refracted', I think?

It's also so that the speed of light won't change by passing glass, even though the rays angle change (refraction) as it leaves, so by refracting I'm assuming that you think of it as meeting a lot of molecules on its way to my eye, finding new angles making its passage longer? In the end also reaching me.  I seriously doubt it's measurable though, and as I said, I won't swear to this :
=

Inside the glass you should have a delay though, exchange that for molecules :) but would that be measurable? And there should be more molecules 'in the way' of that sun ray as the sun sets and rise.. Tricky :)
« Last Edit: 02/06/2013 13:40:03 by yor_on »

#### evan_au

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #5 on: 02/06/2013 22:12:57 »
While glass changes the speed of light significantly, air does not change the speed of light by an amount that you could measure with a stopwatch.

The speed of light in a non-magnetic substance is determined by its relative permittivity:
• Glass: 4.7
• Air: 1.00058986

See the table at: http://en.wikipedia.org/wiki/Relative_permittivity

#### dlorde

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #6 on: 02/06/2013 22:52:41 »
Visually, it's easier to detect a change in distance from the horizon when the sun is close and there is a widening (or narrowing) gap between it and the horizon than when it is high in the sky and there is no nearby stationary reference point.

I'll guess it's a perceptual illusion.
« Last Edit: 03/06/2013 16:59:21 by dlorde »

#### yor_on

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #7 on: 03/06/2013 00:27:36 »
Think the permittivity is set from lights speed in a vacuum, being 1, so there is a slight difference. Now we just need to decide the amount of molecules at different angles of the sun, relative some observer, to see the delay. But we should still find rays able to pass in between (so to speak), as a guess? After all, they're 'dimension less' :) so they should find place, propagating.
=

Eh, that might balance itself out, thinking some more, the amount I mean?
Eh again, meaning that you could get it as some percentage of a average, possibly? But if you get one ray hitting your retina, unimpeded by any molecules, then it shouldn't matter what the other does, should it?

This is a question worth studying :)

« Last Edit: 03/06/2013 00:35:07 by yor_on »

#### bizerl

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #8 on: 03/06/2013 01:53:36 »
I think I meant atmospheric refraction

Since the atmosphere refracts the light around the earth during sunrise and sunset (hence the sun appears there when it isn't) but not at midday.

So if I was watching the sunrise it would appear to travel a greater distance in less time since the curve is decreasing at the same time as it moves.

I hope that is more clear.
Sorry about the very nontechnical language.

I would have thought that if there was any sort of difference with how the sun appears to moves on the horizon due to refraction, it would appear to move slower at the horizon. If you consider that (according to my sources - the TV show "QI") at sunset, the sun is actually already below the horizon when we see it first touch the horizon, it would mean the image of the sun is taking longer to set than the actual sun.

Is this the effect you were referring to Anton?

#### yor_on

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #9 on: 03/06/2013 08:56:29 »
Now, assuming a same permittivity, also assuming those rays to reach my retina in a same 'amount' over a second, no matter if the sun is under or over a horizon, will this make the sun seem slower, as it is under? Even though the light may take a longer 'path' reaching me, the information of that sun should still move at a same pace? Or??

#### AntonMaeso

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #10 on: 03/06/2013 12:02:56 »
I understand that the increased atmosphere will have a minor effect on the speed of light. What I am getting at is the perception of the movement of the sun. If there is an effect it would be hard to measure due to differences in reference points. In no way do I suggest trying to look at the sun (it didn't go well for Newton).

I have attached a basic diagram. The point that I am trying to make is that there is a reduction in the angle of refraction through out the day. Would the reduction of this angle plus the movement of the sun make the sun appear to move faster compared to merely the movement of the sun?

It might be quite a small effect since when the sun hits the horizon it would no longer be visible if there was no atmosphere.

You might be right bizerl but would you not think that the sun would appear to move faster at sunrise and slower at sunset?

#### AntonMaeso

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #11 on: 03/06/2013 12:11:07 »
Here is a diagram

#### CliffordK

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #12 on: 03/06/2013 17:35:32 »
Can I suggest a "Flat Sky Theory".

Ok,
If we perceive the earth as being flat, as well as all landmarks that we see on the earth also being flat, then why don't we also perceive the sky as also being flat, especially when comparing it to terrestrial landmarks, and thus the sun would be traveling through a straight line across the sky.

Marks on graph at every 10 degrees.  Distance to sun displayed is 100x the diameter of the Earth (I was initially playing with a thin atmosphere and atmosphere grazing satellite in my diagram).

In this case, with the "flat sky", the sun would be perceived to follow a straight line (blue).  '
Then, the distance along the straight line that the sun would travel in a fixed period of time would in fact be greater in the morning and evening, than at noon (distance between intersections of the blue and black lines).

Obviously, one would also have perspective, but humans are reasonably good at mentally correcting for perspective.
« Last Edit: 03/06/2013 17:37:29 by CliffordK »

#### dlorde

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #13 on: 03/06/2013 22:41:59 »
Can I suggest a "Flat Sky Theory".
Interesting idea.

#### bizerl

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #14 on: 04/06/2013 00:28:31 »
I think of it like looking at a scene through a back-to-front telescope. Everything looks really small and if someone walks across the field of view, they are visible for a longer time than they would be if we viewed them through an empty cardboard tube of the same diameter.

In your diagram, Anton, it seems that there is a longer time that the light from the sun is being pointed in our direction, in which case it is visible for more time than it would be without atmosphere and therefore would appear to be moving slower than it actually is.

Sunset vs sunrise is an interesting point and I'm not sure how that would resolve.

Hang on, I've just had a thought, perhaps as the sun moves into the lens created by the atmosphere, it appears slowed down, but as it leaves it speeds up so that the point we actually witness it setting, it does indeed appear to move faster than it should...

My brain hurts. I'm going to stop looking at the sun now...

#### damocles

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #15 on: 04/06/2013 07:30:41 »
My thought on this is that we are biologically "tuned" for watching birds fly. The angular velocity of a bird is much greater when it is overhead than when it is low in the sky, so we compensate when observing the flight of a bird -- a very complicated mathematical transformation that is "hard wired" into our brain. For the sun or the moon, the angular velocity is constant, so that at least partially we do this transformation, and judge them to be going faster when they are low in the sky than when they are overhead.

Incidentally, if you think about it, the optical effect of atmospheric refraction actually goes in the other direction: optical sunset is delayed from geometrical sunset, therefore the optical angular velocity is marginally lower than the constant geometrical angular velocity when the sun is low in the sky.

#### RD

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #16 on: 04/06/2013 10:16:43 »
« Last Edit: 04/06/2013 10:44:19 by RD »

#### RD

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #17 on: 04/06/2013 10:34:49 »
... For the sun or the moon, the angular velocity is constant, so that at least partially we do this transformation, and judge them to be going faster when they are low in the sky than when they are overhead.

and judge them to be growing bigger ...

Quote from: nasa
Humans perceive the sky as a flattened dome, with the zenith nearby and the horizon far away. It makes sense; birds flying overhead are closer than birds on the horizon. When the moon is near the horizon, your brain, trained by watching birds (and clouds and airplanes), miscalculates the Moon's true distance and size.
http://science.nasa.gov/science-news/science-at-nasa/2008/16jun_moonillusion/
« Last Edit: 04/06/2013 10:37:27 by RD »

#### AntonMaeso

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #18 on: 04/06/2013 11:55:57 »
RD is correct I got it the wrong way around. But I think the point still stands. It may in fact appear to speed up at sunrise with reduction of the angle and slow down at sunset with the increase of the angle.

bizerl are you suggesting that most of the refraction occurs at the point of sunrise and sunset?

The "Flat Sky Theory" theory intrigues me as well. If we lost all perspective. Does anyone know what size of an effect it would have on the speed of the sun? (or other objects in the sky).

#### yor_on

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #19 on: 04/06/2013 12:26:24 »
Hmm, Clifford? You might be on to something.

"The soldiers of truth and reason of the Flat Earth Society have drawn the sword, and ere another generation has been educated and grown to maturity, will have forced the usurpers to abdicate. Like the decayed and crumbling trees of an ancient forest, rent and shattered by wind and storm, the hypothetical philosophies, which have hitherto cumbered the civilized world, are unable to resist the elements of experimental and logical criticism; and sooner or later must succumb to their assaults. The axe is uplifted for a final stroke - it is about to fall upon the primitive sphere of the earth, and the blow will surely "cut the cumberer down!" "

Kind of like this, it's uplifting, and slightly magical :)
http://theflatearthsociety.org/wiki/index.php?title=Main_Page

#### damocles

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #20 on: 04/06/2013 14:41:17 »
RD is correct I got it the wrong way around. But I think the point still stands. It may in fact appear to speed up at sunrise with reduction of the angle and slow down at sunset with the increase of the angle.
Sorry Anton, but the effect of atmospheric refraction is to slow the optical angular velocity at either end of the day.

#### AntonMaeso

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #21 on: 06/06/2013 09:46:24 »
No need to apologize.

Could you explain in more detail please. It has been a long time since I studied physics at school.

#### damocles

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #22 on: 06/06/2013 22:07:22 »
From AntonMaeso:
Quote
No need to apologize.

Could you explain in more detail please. It has been a long time since I studied physics at school.

Consider the situation in terms of the following figures relating to an equatorial location on the prime meridian at the equinox:

because the sun's rays are bent towards the normal by the atmosphere, we have optical sunrise at 5.55 a.m. Geometric sunrise is at 6.00 a.m.

When the sun's optical elevation is at 15° the time is 6.59 a.m, but for geometric elevation of 15° the time is 7.00 a.m.

For the next few hours the optical and geometric locations of the sun are close to agreement.

But when it comes close to sunset the geometric elevation of 15° at 5.00 p.m. comes a minute before the optical elevation of 15° at 5.01 p.m.

And geometric sunset occurs at 6.00 p.m. a full 5 minutes before optical sunset at 6.05 p.m.

The detail of these numbers may be a little bit out, but the fact of more than 12 hours of daylight at the equinox can easily be checked on any almanac -- sunrise and sunset times are based on the optical position of the sun, while the equinox date is determined by the geometric position of the sun.

Geometrically, the angular velocity of the sun is a constant 15°/hour along its path. Optically the sun moves its first (or last) 15° in 64 minutes, that is, its apparent motion would be slower near the horizon.
« Last Edit: 06/06/2013 22:16:13 by damocles »

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##### Re: Does the sun appear to move faster at sunrise and sunset compared to midday?
« Reply #22 on: 06/06/2013 22:07:22 »