Magnitude scales are used to measure the strength of earthquakes and the brightness of stars. The astronomical magnitude scale measures the amount of light emitted by stars, with higher numbers indicating fainter stars. The Richter scale measures the energy released by an earthquake, with each unit representing ten times more energy. The moment magnitude scale is similar but with a base of 30. It measures the stiffness of the earth, displacement along a fault, and the size of the displaced area. It is becoming more commonly used than the Richter scale.
A magnitude scale is a numerical reference tool, most often used to describe the strength of an earthquake or the brightness of a star as seen from Earth. The scale most commonly used to indicate the brightness of stars, or their “apparent magnitude,” is called the astronomical magnitude scale. The Richter scale and the moment magnitude scale are used to describe earthquakes.
The astronomical magnitude scale defines the magnitude of stars based on the amount of light they emit as perceived by an observer on earth. The higher a star’s magnitude number, the fainter it appears. For example, the luminosity of the sun, our closest star, is about a magnitude of -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 of magnitude three. Someone in a rural area can see stars as faint as magnitude six or seven, and binoculars bring the number nearly to ten. Telescopes allow us to see much fainter stars, down to a magnitude 30 in some cases. It is important to note that although the astronomical magnitude scale could be said to measure brightness, a magnitude three comet will not be as bright as a magnitude three star, because the light from a comet is scattered over a larger area.
Those who have lived in, or studied earthquake-prone areas in some way, may be familiar with the Richter scale, which is 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 an earthquake. It is a logarithmic scale with base ten, which means that an increase of one unit represents ten times more energy released. for example, an earthquake with a magnitude of 7.0 releases ten times more energy than one with a magnitude of 6.0.
The amount of energy released in an earthquake, measured on the Richter scale, is closely related to the amount of its destructive potential. For this reason it is the best known 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 the release of energy as a function of the stiffness of the earth, multiplied by the amount of displacement that occurs along a fault, as well as the size of the area that has been displaced. Recently, the moment magnitude scale has begun to replace the Richter scale as the more commonly used of the two. In practice, the moment magnitude of an earthquake is often numerically close to its Richter scale value, causing this switch to go largely unnoticed.
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