# ⓘ Magnitude, astronomy. The apparent magnitude of a celestial object is a number that measures its brightness as seen by an observer on Earth. The brighter an obj ..

## ⓘ Magnitude (astronomy)

The apparent magnitude of a celestial object is a number that measures its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value. The Sun, at apparent magnitude of −27, is the brightest object in the sky.

Magnitude is a logarithmic measure. It is measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths. A full moon is −13 magnitude and the brightest planet Venus measures −5. The brightest man-made objects, Iridium flares, are ranked at −9 and the International Space Station measures −6.

## 1. History

The Greek astronomer Hipparchus first invented our system of apparent magnitude. He gave the brightest stars a magnitude of 1 and increased the numbers for dimmer stars. About 300 years later Ptolemy of Alexandria created an encyclopedia of stars based on Hipparchus work. The Persian astronomer Al-Sufi took Ptolemys work and gave Arabic names to the stars some 800 years later. Medieval European astronomers simply translated Al-Sufis work into Latin which is why many stars today have Arabic names.

## 2. Magnitude systems

• Absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard distance from Earth. The standard distance is 32.6 light years from Earth. The concept of absolute magnitude allows astronomers to compare the brightness of objects without regard to their distance. When measured using absolute magnitude the sun measures less than Sirius at only 4.8.
• Photographic magnitude is different from visual magnitude because photographic film is more sensitive to blue light and radiation the human eye cannot see.
• Apparent magnitude m is a measure of the brightness of a celestial body as seen by a person on Earth without the earths atmosphere. The brighter the object appears, the lower the numerical value of its magnitude. For example, Alpha Centauri has higher apparent magnitude i.e. lower value than Betelgeuse, because it is much closer to the Earth.
• Visual magnitude means the amount of brightness of a star on a numbered scale. It is the magnitude based on the sensitivity of the human eye.