That faint glow filling in the crescent's dark side is a measurement. Track it for years and it tells you how reflective Earth has become.

Earthshine is sunlight bounced off Earth onto the Moon and back to us, so its brightness follows Earth's albedo, mostly cloud, ice and ocean. André-Louis Danjon began measuring it in the 1920s and observatories have run long earthshine programmes since, because a single number for the whole planet's reflectivity is otherwise surprisingly hard to obtain.