In → photometry of
→ binary stars or → two-body systems,
the number of whole
or fractional orbits completed, from the point the photometry begins.
The point is conventionally chosen as the position at which the
→ primary star eclipses the
→ secondary star, and therefore
the → light curve
is at a minimum. The phase keeps counting
indefinitely, thus the secondary star gets eclipsed at phase 0, 1, 2, 3, and so on.
At these phases the primary lies between the secondary and the observer.
An orbital phase of 0.5 corresponds to halfway through the binary orbit,
0.75 is three-quarters the way through, and so on.
See also: → orbital; → phase.
