What does 10f mean in the line >>> print('%.10f' % yh.earth_distance)?
Here is an example where it is used:
>>> line = "C/2002 Y1 (Juels-Holvorcem),e,103.7816,166.2194,128.8232,242.5695,0.0002609,0.99705756,0.0000,04/13.2508/2003,2000,g 6.5,4.0"
>>> yh = ephem.readdb(line)
>>> yh.compute('2007/10/1')
>>> print('%.10f' % yh.earth_distance)
14.8046731949
>>> print(yh.mag)
23.96
This has nothing to do with ephem, this is formatting for python strings. In this particular case — "use 10 digits after floating(decimal) point"