Q.87: How are star magnitudes measured? When I was about fourteen or fifteen years old, my friend Miffy (a.k.a. Paul Smith) and I would amuse ourselves in our French class by doing calculations using some of the complicated looking formulas in Sir James Jeans' book Astronomy and Cosmogony [4]. While outdated (even then) as a textbook of astrophysics, it was fascinating both historically and scientifically, and I still have my copy of it. Since I was pretty good at French, I had done my home- work, and as long as we were quietly working on the back row, our teacher probably had no idea that we were budding astrophysicists . . . besides, the French books we were using were old and dog-eared, and my lovely Dover edition of Jeans' classic 1928 work was crisp and new. It was very exciting to skim through its pages and encounter very impressive formulae; at the time I had no idea what most of them meant, but I determined that one day I would (and one day I will.) The first of the formulas in chapter two of the book (The Light from the Stars) enabled one to calculate the differences in brightnesses (apparent mag- nitudes) two stars would have in terms of the light received from those stars (assuming that they are both equally distant from us). The fainter a star ap- pears, the larger is its magnitude. Let us examine this concept in terms of a standard logarithmic model of the magnitude scale. As noted in question 41, the observed brightness of stars is expressed in terms of their apparent mag- nitudes m on a numerical scale that increases as the brightness decreases: L , m ¼ 6 À 2:512 log 10 L 0 (87:1) where L is the light flux (luminosity or brightness) of the star (or planet) and L 0 is the brightness of the faintest star visible to the (average human) naked eye. Thus, when L ¼ L 0 , m ¼ 6, and, therefore, since