[chris (OP)]

This is hideously difficult to explain and I'd be grateful to anyone who can give me a hand and put this into plain English without using horrendous jargon...!

Chris

[neilep]

I refer you to the explanation given on LBC 97.3FM or DAB ! on the morning of December 22nd 2007 where at 7:30am (ish) they DID give the explanation as part of the weather report in very brief laymans terms!....They always give the sunrise time and it was a surprise to me when they said that the sunrises were still extending !!...being morning...they did not mention Sunset though !

Glad I could help !............................................DOH !! []

[lyner]

Sunrise and Sunset are pretty much symmetrical about Noon time.
If the Earth had a perfectly circular orbit and there were no other planets to speed it up or slow it down, then, in one day (Noon to Noon) the Earth would  go the same distance around its orbit - like a small gear wheel running around a large gear wheel. The Sun would appear overhead each time the Earth had spun once plus 1/365 of a turn.
But the Earth's orbit around the Sun is slightly elliptical and its speed changes on the way round. This causes our gears to 'slip'. The number of degrees (the solid angle) that it moves through in 24 hours is more when it is closer and less when it is further away. This means that the Sun won't be exactly overhead when the clocks all say it should be. Sometimes Noon comes a bit earlier than average (or Mean time) and sometimes later.
The tilt of the Earth's orbit also affects things but it's more difficult to describe without diagrams.
In the Northern Hemisphere, Noon lags a bit in winter and leads summer, compared with our clocks. In the Southern hemisphere, it's the other way round. I think we get the better deal because the afternoons are getting longer already and I don't care about the mornings.
This variation used to be called 'The Equation of Time" and you can see graphs published for each year in  some Nautical almanacks. I even saw a graph on a brass plate in a little park in Geneva when I went for a walk there.
Goodle Equation of Time and you can see lots of pretty graphs.

[turnipsock]

Sunrise and Sunset are pretty much symmetrical about Noon time.

Ah, that is where you are wrong. The earlist sunset is about the 14th Dec amd the latest sunrise is around the 28th Dec. The shortest day ends up around the 21st Dec but the sunsets are getting later by then. It always seems that way when you start back at work in Jan that the nights are noticably brighter.

The effect is even more prounced when go further West from Greenwich (which is most of the UK) as the Sunset is later by about 20 mins at 5 degrees W, and so is the sunrise.

[syhprum]

This become horribly apparent if you rely on your sundial to catch a plane or train you can be as much as 20 minutes out either way

[lyner]

Ah, that is where you are wrong

I can't really be wrong if I'm referring to noon as when the Sun is overhead - unless the Earth is not spinning at a constant rate. The only asymmetry will be due to the  differential of  time change per day  (the double differential of the time).
Of course they are not symmetrical about GMT noon.

[turnipsock]

I'll check the almanac tomorrow, what is your long soph??

[lyner]

About 0; about 7 miles West of the Meridian.
But how can it be anything but a symmetrical thing about the overhead position time of the Sun (i.e. local noon)?
The Earth would have to be very non circular about the N/S axis or move in jerks for it to be any other way.
If you wanted to do it properly, you would need to know my lat, too, and I can't remember that.

[turnipsock]

If you were to actually use a sextant to get the exact time of noon at your location, then the sunset and sunrise will be equal periods of time on either side of this.

However, the sun does not transit Greenwich at noon every day but varies.

[lyner]

Yes; that's what the equation of time is all about.
Spring will be here soon(ish) and we can look forward to life after work!

[Pumblechook]

The Sun is not a point source.  Light bends over the horizon.  There is diffraction, scattering and the Earth orbits the Sun as well as spinning on its axis.  Anyway, how is sunrise and sunset defined? It is gradual ...not like flicking a switch. 

Maybe somebody could calculate what the times would be if the Earth's orbit was ignored, the Sun was a point source and there was no atmosphere. 

[lyner]

The optics of the situation are known and the times of sunrise and sunset are defined as below:

The times of sunrise and sunset refer to the times when the Sun's upper limb, as affected by refraction, is on the true horizon of an observer at sea-level. This occurs when the Sun's centre is 50 arcminutes below the true horizon, the upper limb then being 34 arcminutes (just more than the Sun's apparent diameter) below the true horizon. http://www.nmm.ac.uk/server/show/conWebDoc.345

If you ignored the orbit of the Earth (why would you want to? It would fall into the Sun.) you  would be talking about  Sidereal time. Sidereal time is the time related to the apparent passage of the Stars around the Earth - there is 1/365 day difference between the two. Astronomers are much more interested in sidereal time than solar time  it tells them where to find a star when they want it.

[Pumblechook]

I was just thinking if you simplflied to the situation to a perfect sphere with no atmosphere which rotated as the earth does and the Sun was a point source then the anomalies would disappear.

[Pumblechook]

Something here.....

imagine.gsfc.nasa.go...

Looks like noon shifts around. Presumably the time sunrise to noon and noon to sunset are always equal on any given day.

After 21/ 22 Dec noon must be a bit later than 12.00 UTC at zero degrees Long.

From the link...

Both effects combine to create an offset in the time of local noon (and those of sunrise and sunset) by as much as +/- 16 min


So if the day lengthens by 2 mins from day A to day B and noon also shifts one minute later sunrise doesn't change but sunset is 2 minutes later. 

[DoctorBeaver]

In Uganda we had 2 longest days and 2 shortest days each year.  []

[lyner]

That's what living between the tropics does for you!