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I began reading your message, then rushed out to the back of the house as it
was near the time of sunset. At 34 deg South at an altitude of about 100m
above sea level - I have never  worked this height out -  the sun then
appeared as a fireball sinking into the South Atlantic, today almost as
close to north as it ever will be in the yearly cycle. I would never look
into it, for a short while I saw black spots just looking near it.

 Douglas, I hope this is not something I should be worried about.  I spent
about 20 of my 32 years in this southern region and I guess I had a good
deal of exposure to the local sun, but I'd still say it was in limits, being
aware of the UV-light  issue (am knocking on wood). The ozone hole expanded
closer and closer to South Africa and may even already have reached the
"sky" over South Africa in the late eighties when I was growing up.

>>"0d 49!6 to -0d
>> 49!8"
>>
>> What units are these in? I'm thinking you are referring to to
>> semi-diameter,
>> but not sure.

>If you note the refraction for a star on the horizon or a planet split by
> the horizon (planets can be larger than a point source) is - 33.8 minutes
> of
> an arc.  The correction for the upper limb of the Sun as it kisses the
> horizon rages from 49.8 to 49.6 minutes of an arc based on season.

.....the sun kisses the horizon in that moment, yes, but the observer sees
sun's centre half-way through the horizon. So 49.8 or 49.6 includes
semi-diametre, right?

> The Sun does not approximate a point source. The point being the upper
> limb
> of a Sun with a nominal SD of 15.9 arc minutes (31.8 arc minutes diameter)
> is already 33.8' minutes below the the horizon and the center 49.7' below
> the horizon (33.8 + 15.8).

Okay.

> Put another way, if we could see a star kiss the horizon it would already
> be
> 33.8 arc minutes below the horizon in a vaccum but is lifted by that
> amount
> by air refraction. As a side note, list members have pointed out in the
> past
> that almost any point source would be "extinguished" by the atmosphere at
> that elevation (amount of air between the observer and the body) and could
> not normally be observed.

An exception being the sun, right? So this is why you can't see stars (not
"our" sun) exactly on the horizon, if I understand you correctly.

Thanks for your latest posting Bill and thanks also for your note Douglas,
this online forum really is a great help.

Christian Scheele






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