In this one, we define the terms brightness and luminosity. We also define the magnitude system, both apparent and absolute. We don't shy away from equations anymore! We also derive how Pogson saddled all the future of astronomy with a Log-base-10 humbug. This is part of my intro Astronomy class taught at Willam Paterson University and CUNY Hunter. 0:00 Introduction 0:42 Brightness and Luminosity 2:54 The Inverse Square Law of Brightness 4:34 Measuring Apparent Brightness 7:08 Modern Magnitude System 8:39 Pogon's Definition of the Magnitude System 9:59 The Magnitude of Vegas 10:41 All stellar magnitudes reference Vega 13:01 The Science of Flux Photometry 14:25 Measuring Luminosity 15:12 Practical Issues 15:59 Luminosity, not-Brightness 16:31 Luminosity, Brightness and Distance 16:42 The Luminosity Distance 19:11 Now do it for just one star 20:42 The Distance Modulus Equation.