The radiance of lunar objects near opposition

Abstract

The radiance of lunar objects at phase angles |g| < 5 ° has been measured on plates taken at the Kirkwood and Yerkes Observatories during the lunar eclipse of 18 November 1956. The measurements have been combined on a uniform scale of brightness by comparison with photoelectric determinations of the radiance of the floor of Plato made for that purpose on the same night by Gehreis et al.(1) The lunation curves of Grimaldi, Copernicus and Tycho show the reality of a non-linear surge in radiance (“the opposition effect”) for lunar objects. Between g = −1.4° and g = −0.7° there is a 10 per cent increase in the radiance. By linear extrapolation to g = 0° we have found the normal albedo of 36 crater floors.