Light ratio in astronomy (6)
I believe the answer is:
albedo
'light ratio in astronomy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for albedo that I've seen before include "Whiteness" , "Reflecting power" , "Measure of reflectiveness, doable (anag.)" , "Proportion of astronomers" , "Measure of planet's reflection" .)