[Ighsaan]

Ighsaan asked the Naked Scientists:

How long does it take from the time that the sun touches the horizon to the time it disappears below the horizon at sunset?

What do you think?

[Soul Surfer]

The length of time it takes can vary from a small value to a very long time depending on the angle that the sun is crossing the horizon.  

Let us first look at the mininmum in the tropics where the sun can set vertically down from the horizon. Neglecting the effects of atmospheric refraction the diameter of the sun is half a degree and the sun moves fifteen degrees an hour (360 / 24) so the sun sets in on thirttieth of an hour or two minutes.  Whereas at the north pole the sun circles the horizon at the same level and so   it sets as its latitude drops half a degree below the equator which takes about 48 hours.

[turnipsock]

Do you not have to take into account the size of the sun?

The bit we see rising first is not the same bit as we see setting last. So even at the equator, at the equinox, the sun will be in the sky for more than 12 hours.

(That is unless, we are treating the sun as a point.)

[Soul Surfer]

Turnipsock the day is extended by much more than this because the deviation at the horizon due to atmospheric refraction is about three degrees or six times the width of the sun.

[lyner]

Your personal horizon is affected by your altitude, remember.
Nautical Almanacks have the specific information you want which takes everything into account - latitude, altitude, day of the year etc..

[LeeE]

Umm...

just thought I'd point out that it's the horizon touching the sun rather than the sun touching the horizon.

[lyner]

you mean Plato is wrong?

[LeeE]

I believe 'was' is the correct term.

[lyner]

is he not arround then?
I thought I saw him in Sainsbury's the other day.

[LeeE]

Probably just someone from one of those celebrity look-alike agencies doing their shopping.