What does this statement mean: "Apparent magnitude of a star, X, is m." ? [itex] m_2 - m_1 = -2.5 log (B_2/B_1) [/itex] Apparent magnitudes are defined relatively, right? We can talk about differences in apparent magnitudes. If i know the ratio of the brightnesses, i can find out the difference in apparent magnitudes. Do we have some kind of reference magnitude, relative to which we can make the statement in quotes?