# Apparent magnitude

by luuurey
Tags: apparent, magnitude
 P: 24 Hello everyone, I've been wondering if I can calculate the apparent magnitude of The Moon since I know the apparent magnitude of The Sun(say -27mag) and reflection coefficient of Moon's surface(say 0.12). We know the equation $\Delta m = -2.5 log_{10}(I/I_0)$ Where I is the flux of the light that comes from source and I_0 is flux of a reference source. I know the magnitude of The Sun, so I can say: $-27 = -2.5 log_{10}(I_S/I_0)$ Where I_S is the flux that I can measure on the sunlight. I know that the flux of the light from The Moon is just reflected sunlight and hence$I_M=0.12 I_S$, where I_M is the flux of light that comes from The Moon. So I get: $\Delta m_M = -2.5 log_{10}(I_M/I_0)= -2.5 log_{10}(0.12 I_S/I_0)= -2.5 log_{10}(I_S/I_0)-2.5 log_{10}(0.12) = -27 -(-2)=-25$ I'm sure that the apparent magnitude of The Moon is definitely not 25mag. It's around -13mag. What's wrong with my idea? Where did I make mistake? Thank you for your answer. I will be very thankful for your help!