Found in articles
Printing Office. ISBN 978-0-7077-4082-9. , p. M3-M4 "Introduction to Spherical Astronomy: p. 8". www.physics.csbsju.edu. Retrieved 2019-02-01. USNO Nautical
Carroll 2004, Ch. 6.7 Einstein, A. (1939). "On A Stationary System With Spherical Symmetry Consisting of Many Gravitating Masses". Annals of Mathematics