The nearest rich cluster of galaxies which contains more than a thousand known galaxies, is about 20 million light-years in diameter, and lies about 280 million light-years away in the constellation C
oma Berenices.
A combination of two time systems of mass-balance measurement, usually of the stratigraphic system with either the fixed-date system or the floating-date system. As originally defined, the combined sy
stem accounted rigorously for differences between the stratigraphic and fixed-date systems, but this rigorous accounting has been found impractical in most measurement programmes and various simplifications have been adopted.