Hi Denis,

(For those wondering what Denis and I are talking about:

http://www.projectpluto.com/new.htm#better_refraction

I don't quite follow where some of the problems are, and
please let me know what remains unclear after reading the
following. But I can make at least a few comments:

When you set the new 'Include Refraction' check-box in the
Locations dialog, the _only_ things that are affected are the
readout of altitude in the legend area, and (sometimes) the
altitude shown when you click on an object. Nothing else (the
position of the horizon, rise/set times, etc.) changes.

The horizon is _always_ shown adjusted for altitude and
refraction, and rise/set times are also _always_ shown adjusted
for these things. With the check-box set, you get "refracted"
altitudes; these are the altitudes one could measure with, say,
a sextant. If that check-box isn't set, you get unrefracted
("airless") altitudes.

The reason for all this is simple. There are quite a few
situations that call for airless altitudes. But I couldn't think
of any that call for showing "where would the horizon be if the
earth had no air", or "when would this object rise and set if
the earth had no air". (But if you want to see what that would
be like, set the "pressure" entry in the Locations dialog to
zero.)

Another result of all this is that if you click on an object
and get its rise/set time, and then set Guide's date/time to one
of those instants, you should see the object on the horizon.
Which again seems reasonable: whether you measure the altitude
in one system or another, the rise/set time ought to be the same,
and the object ought to be on the horizon at that time.

"... If I located the cursor on the horizon line, I should
read zero with not refraction selected and the humidity,
pressure and temperature should not be a factor."

I suppose that would indeed be so, if refraction were ignored
for the location of the horizon. It isn't (again, the horizon is
_always_ refracted and corrected for height above sea level.) So
you get an altitude of about -.6 degrees or so, with that amount
depending on humidity, pressure, temperature, and your height
above sea level.

Just to make things more confusing: when you _do_ include
refraction, the altitude of the horizon _still_ will not usually
be zero degrees. That's because of your height above sea level.
Only if you set that to be zero, _and_ turn on refraction, will
the cursor read zero as you reach the horizon.

-- Bill