Logarithm decibels. In a similar vein, the peak signal-to-noise ratio is commonly used to assess the quality of sound and image compression methods using the logarithm. The strength of an earthquake is measured by taking the common logarithm of the energy emitted at the quake. This is used in the moment magnitude scale or the Richter magnitude scale. For example, a 5.0 earthquake releases 32 times and a 6.0 releases 1000 times the energy of a 4.0. Another logarithmic scale is apparent magnitude. It measures the brightness of stars logarithmically. Yet another example is pH in chemistry; pH is the negative
Apparent magnitude brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is usually measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V ("visual") filter band would be denoted either as "m" or often simply as "V", as in ""m" = 15" or ""V" = 15" to describe a 15th-magnitude object. The scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six "magnitudes". The brightest stars in the night sky were said to be