Rising and Setting of the center of a body

[roe-dl]

Is there an easy way to calculate the rising and setting (or crossing some angle below the horizon) of the center of a body instead of its upper limb? There is such a function in PyEphem, but I do not find it here.

[brandon-rhodes]

Yes! Provide a parameter horizon_degrees as shown in this section, and Skyfield won't use the planet's radius, but will assume that you built it into the horizon number you are providing:

https://rhodesmill.org/skyfield/almanac.html#computing-your-own-refraction-angle

If that solution works for you, I suppose I'll have to re-word that section of the docs to make clearer that horizon_degrees shuts off Skyfield's own target-radius calculation.

[roe-dl]

I thought that refraction() method was primarily focussed on the influence of temperature and pressure. If using that method switches off Skyfield's own target-radius calculation, the question is the other way round: Is there an easy way to use both Skyfield's target-radius calculation and temperature-pressure-influence calculation?

Background: I have to change an application for the use of Skyfield instead of PyEphem. That application provides 4 parameters: the horizon, a flag whether to use the center or the upper limb, the temperature and the pressure. That I have to use to calculate risings and settings.

[brandon-rhodes]

I thought that refraction() method was primarily focussed on the influence of temperature and pressure.

You are right, it is entirely focused on temperature and pressure.

If using that method switches off Skyfield's own target-radius calculation…

I might have created a misunderstanding. Neither find_risings() nor find_settings() ever knows whether you have called the refraction() routine. They only know whether you have decided to provide a horizon_degrees parameter or not.

So it sounds like I should update the documentation to explain that if you don't provide horizon_degrees, then Skyfield itself uses a standard amount of atmospheric refraction, plus it tries to add in the radius if the target is the Sun or Moon, so that it's giving the rise/set of the upper limb of the target body. — But that if you do provide horizon_degrees, then you are entirely responsible for accounting for both of those effects, if you so choose, but that Skyfield stops trying to adjust for them?