Space Book

What is apparent magnitude?

Astronomers use the term apparent magnitude to describe how bright an object appears in the sky from Earth. The idea of a magnitude scale dates back to Hipparchus (around 150 BC) who invented a scale to describe the the brightness of the stars he could see. He assigned an apparent magnitude of 1 to the brightest stars in the sky, and he gave the dimmest stars he could see an apparent magnitude of 6. He did not include the sun, moon, or planets in his system.

The magnitude scale astronomers use today is based on Hipparchus' system, but has been expanded since the invention of the telescope. In this system, the brighter an object appears, the lower its magnitude. Some of the brightest objects (including the sun and planets) visible in the sky have negative values for apparent magnitude. The faintest objects detected with the Hubble Space Telescope have apparent magnitudes of 30.

The following table gives a list of some commonly known objects and their apparent magnitudes

Apparent Magnitude Celestial Object
-26.7 Sun
-12.6 Full Moon
-4.4 Venus (at brightest)
-3.0 Mars (at brightest)
-1.6 Sirius (brightest star)
+3.0 Naked eye limit in an urban neighborhood
+5.5 Uranus (at brightest)
+6.0 Naked eye limit
+9.5 Faintest objects visible with binoculars
+13.7 Pluto (at brightest)
30 Faintest objects observable by the Hubble Space Telescope