multiple locations on Moon surface with build_latlon_degrees

[roe-dl]

Is it possible to pass arrays for latitude and longitude to build_latlon_degrees?

I tried pc.build_latlon_degrees(frame,numpy.array([-90.0,90.0]),numpy.array([0.0,0.0])), but there were errors processing the result.

[brandon-rhodes]

I don't think I've ever successfully used array broadcasting with anything but Time objects. It's possible that someday a rewrite of Skyfield internals could make it possible to quietly pass additional array dimensions through Skyfield's calculations, but the last time I tried, I couldn't quickly get the code into shape to accept them.

[roe-dl]

I tried to get the apparent angle of the Moon's axis on the Sky. To do so, I took the locations 90.0°,0° and -90°,0° on the Moon's surface and let Skyfield calculate altitude and azimuth for them. So I have 2 locations at once, but one timestamp only.

        frame = frames[body]
        eph = ephemerides[body]
        south = eph + planetaryconstants.build_latlon_degrees(frame, -90.0, 0.0)
        north = eph + planetaryconstants.build_latlon_degrees(frame, 90.0, 0.0)
        altaz1 = observer.at(time_ti).observe(south).apparent().altaz()
        altaz2 = observer.at(time_ti).observe(north).apparent().altaz()
        axis = position_angle_of(altaz1,altaz2)
        return Angle(degrees=axis.degrees if axis.degrees<=180.0 else axis.degrees-360.0, preference=axis.preference, signed=True)

The same applies to wgs84.latlon(...). You can write something like observer = eph['Earth']+wgs84.latlon(numpy.array(...),numpy.array(...)) and get the expected result. But if you then write observer.at(t).observe..., you get an exception, even for one single timestamp.

On the other hand, I used arrays for the observe method, for example to plot a circle of right ascension:

        # hours of right ascension (one star day)
        hours = numpy.arange(24)
        # set declination according to the latitude the map is drawn for
        dec = numpy.array([(90-abs(almanac_obj.lat))*(1 if almanac_obj.lat>=0 else -1)]*24)
        # Below 20° of latitude the circle is too small for text marks
        above20 = abs(almanac_obj.lat)>20
        # create fictive stars holding the positions according to the 
        # coordinates defined before
        body = Star(ra_hours=hours,dec_degrees=dec,epoch=time_ti)
        # get the current altitudes and azimuths for those positions
        apparent = observer.at(time_ti).observe(body).apparent()
        alts, azs, _ = apparent.altaz(temperature_C=almanac_obj.temperature,pressure_mbar=almanac_obj.pressure)

That works fine. It draws a circle around the circumpolar area of the Sky, labelling right ascension, as the following example shows:

(The green dots are GPS satellites, the blue squares weather satellites.)