@misc{Ball_L._de_Lehrbuch_1912,
 author={Ball, L. de},
 address={Leipzig},
 howpublished={online},
 year={1912},
 publisher={Verlag von Wilhelm Engelmann},
 language={ger},
 title={Lehrbuch der Sphärischen Astronomie},
 keywords={spherical astronomy, motion of celestial bodies},
}