Geometric albedo node of their orbit, and hence lie on the ecliptic. In practice, measurements at small nonzero phase angles are used to derive the parameters which characterize the directional reflectance properties for the body (Hapke parameters). The reflectance function described by these can then be extrapolated to zero phase angle to obtain an estimate of the geometric albedo. For very bright, solid, airless objects such as Saturn's moons Enceladus and Tethys, whose total reflectance (Bond albedo) is close to one, a strong opposition effect combines with the high Bond albedo to give them a geometric albedo above unity (1.4 in the
Incompressible surface compressing "S" along "D". A non-negative complexity measure on compact surfaces without 2-sphere components is "b"("S") − "χ"("S"), where "b"("S") is the zeroth Betti number (the number of connected components) and "χ"("S") is the Euler characteristic. When compressing a compressible surface, the Euler characteristic increases by two, while "b" might remain the same or increase by 1. Thus, every properly embedded compact surface without 2-sphere components is related to an incompressible surface through a sequence of compressions. Sometimes we drop the conditions that "S" be compressible or that "D" be anything more than a disk that intersects "S" only along