Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso

Phase, Orbital

  • David W. LathamEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_1177-3


For an object in a periodic orbit, the phase is the fraction of period that has passed since the time t = 0. Mathematically, the location of t = 0 can be any arbitrary point on an orbit; however, in Keplerian orbits, the point of periastron is considered as the location of t = 0. Usually, the phase is measured from the most recent periastron passage, but sometimes the phase will include an integer count for the number of orbital cycles since a fiducial periastron passage. For a circular orbit, the usual convention is to define phase zero as the time of maximum radial velocity of recession for the more massive component.

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© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Harvard-Smithsonian Center for AstrophysicsCambridgeUSA