Herbst, T. M.; Rix, H.-W. (1999). "Star Formation and Extrasolar Planet Studies with Near-Infrared Interferometry on the LBT". In Guenther, Eike; Stecklum, Bringfried; Klose, Sylvio (eds.). Optical and Infrared Spectroscopy of Circumstellar Matter, ASP Conference Series, Vol. 188. Vol. 188. San Francisco, Calif.: Astronomical Society of the Pacific. pp. 341–350. Bibcode:1999ASPC..188..341H. ISBN1-58381-014-5. {{cite book}}: |journal= ignored (help)
Asimov uses the term "concave" to describe the Earth–Moon orbital pattern around the Sun, whereas Aslaksen uses "convex" to describe the exact same pattern. Which term one uses relies solely upon the perspective of the observer. From the point-of-view of the Sun, the Moon's orbit is concave; from outside the Moon's orbit, say, from planet Mars, it is convex.
