Bill,

now it makes a lot of sense. I was missing the fact that horizon always
include the refraction. The function is therefore only for the legend
display. Would it make be easier to grasp if it were in the legend
selection, let say as "Include refraction for Alt/Az" right after the
Alt/Az selection?

Something to think about.

Thanks for clarifying the issue.

Denis.

Bill J Gray wrote:

> 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
>
>
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