A magnitude scale is a numerical tool of reference, most often used to describe either the strength of an earthquake or the brightness of a star as seen from earth. The scale that is most commonly used to denote the brightness of stars, or their "apparent magnitude," is called the astronomical magnitude scale. For the description of earthquakes, the Richter scale and the moment magnitude scale are used.
The astronomical magnitude scale defines the magnitude of stars based on the amount of light they give off as perceived by an observer on earth. The higher a star's magnitude number, the dimmer it appears. For example, the brightness of the sun, our closest star, is about a magnitude -26, while the full moon is assigned a magnitude of about -13.
An observer in an urban area will be able to see some stars at night, but none dimmer than a magnitude three. Someone in a rural area can see stars as dim as magnitude six or seven, and binoculars bring the number almost to ten. Telescopes allow us to see much dimmer stars, up to a magnitude 30, in some cases. It is important to note that although the astronomical magnitude scale can be said to measure brightness, a comet of magnitude three will not be as bright as a star of magnitude three, because a comet's light is spread over a larger area.
Those who have lived in earthquake-prone areas, or who have studied them to any degree, may be somewhat familiar with the Richter scale, used to measure the magnitude of earthquakes. The Richter magnitude scale assigns a single number from one to ten to represent the total energy released by a quake. It is a logarithmic scale with a base of ten, meaning that an increase of one unit represents ten times more energy released. for example, a magnitude 7.0 quake releases ten times more energy than one measuring 6.0.
The amount of energy released in a quake, as measured by the Richter scale, closely correlates with the amount of its destructive potential. For this reason, it is the most widely understood scale for measuring earthquakes. Closely related to the Richter scale is the moment magnitude scale. It is also logarithmic, but with a base of 30 rather than 10.
The moment magnitude scale measures energy release as a function of the rigidity of the earth, multiplied by the amount of displacement that takes place along a fault, as well as the size of the area which was displaced. Recently, the moment magnitude scale has begun to replace the Richter scale as the most commonly used of the two. In practice, the moment magnitude of a quake is often numerically similar to its Richter scale value, causing this switch to go mostly unnoticed.