accuracy of user defined horizon

Luc Desamore Jan 28, 2014

Hi all,

I am looking the user horizon feature in Guide, I plan to use it to
simulate rises on megalithic alignements on particular horizon

I used for position and refraction :
N50° W0° alt : 0m
Humidity 20, Temp 15, pressure 770mm/Hg.
and I create in horizon.dat a skyline of 10° altitude on the whole range
of 0-360°azimuts.

I noticed that the blue horizon line and the bottom of the red horizon
zone begin at about -0.57°alt to account for the value of the
refraction at the horizon.

Suppose we want to simulate the rise of Regulus today,
On the bottom blue line, it is quite correct, I find :
2014 jan 28 18:35:24 UTC and Regulus is on the line.

On the 10°line there is a problem
By calculation, I obtain 2014 jan 28 19:42:20 UTC
Indeed if I check with "include refraction = ON", the altitude of
Regulus is exactly 10° in Guide.
But Regulus is actually displayed 29 arcmin (0,48°)
above the top of the red zone !

I guess that these 0.48° are the difference between the horizon
refraction (-0.57°) and the 10° refraction (0.09°).

Is there something I could do to correct this ?