The magnitude is the unit in wich the brightness of a celestial body is expressed. The magnitude scale is reverse and logarithmic, that means that the brighter the object, the smaller the magnitude. In the ancient Greece, the brightest stars had magnitude 0 or 1, while objects wich are just visible with the naked eye, belonged to the 5th magnitude. A difference of five magnitudes means a hundred times brighter/fainter; one magnitude means a difference in brightness of 2.5 times. Some brightnesses: