Re: East/West walls and the North/South divide

[Albert Franco]

 

Quote:

Since the sky appears to rotate around you in 24 hours, anything on the celestial equator takes 12 hours to go from exact east to exact west.

 

The above quote aligns with my way of thinking on this.  It seems to me that even though the sun is still above the horizon after it travels north of exact east or exact west, it has moved beyond the point where it would be shining on a south facing wall.

 

The part that I'm not fully getting is the fact that the wall's angle to the Sun's orbit will change (vertically) with latitude.  I'm not fully getting my mind around exactly how this will affect the illumination of the wall face.  Even considering that, it still seems like the face wouldn't be lit.

 

AF

 

Frank King <[EMAIL PROTECTED]> wrote:

Hi Gianni,

> In my opinion the Waugh's statement is correct if we think only
> to the sundials in the North hemisphere (as Waugh did).

Yes, you (and Mr Waugh!) are quite right of course...

> For this reason it seems to me a little " trick " to consider the
> period of illumination of the South face in a sundial to the
> ANTarctic polar circle :-)

Yes, this is indeed one of my tricks! I like to think that every
wall has TWO sides and the `wrong' side can be very interesting.

North-facing dials are quite common but not many people realise
that it is (theoretically) possible to have more than 12 hours of
continuous sun on the same face of a wall. You just have to be
in the right place!

> For curiosity I send some approximate values...

These are the figures I was thinking about, especially:
> Latitude 66ƒ 27'
> Dawn 3h 27m
> Start of the illumination 6h 43m
> Length of the illumination 10h 33m

It is the last figure that is responsible for my `trick'. Here
the sun is on the `wrong' side for 24 - (10h 33m) = 13h 27m,
well over 13 hours.

Here is another `trick' concerned with the difference between the
north and south hemispheres...

If `summer' in northern latitudes is taken as the period
between the March equinox and the September equinox and
`summer' in southern latitudes is taken as the period
between the September equinox and the March equinox then:

a) Is summer longer in the north than in the south? or

b) Is summer the same in the north as in the south? or

c) Is summer shorter in the north than in the south?

The correct answer is (a) but what is interesting is that the
difference is OVER A WEEK. In Europe we have almost 8 more
days a year where the sun is above the horiz! on for longer
than it is below, than the poor people in Australia!

Frank King

[In Cambridge where it is raining heavily and all thoughts
of sundials are purely theoretical at the moment :-(( ]



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