The moon’s magnitude varies greatly depending on what phase it is in.
Photo credit: Dave Dyet
Magnitude is a term used by astronomers to describe how bright an object appears in the sky. The lower the number, the brighter the object; the higher the number, the dimmer the object. It’s tricky to make a table of objects based on their magnitudes because many solar system bodies’ magnitudes can vary greatly. The moon is a great example. On nights when it is full, it is so bright that it obscures stars in its vicinity. But on nights when it is a slim crescent, it is merely an accent on the starry scene. Planets, satellites, asteroids, and comets also change magnitude depending on where they currently are in their orbits.
Stars do not tend to change their magnitude unless they are variable stars. Some variable stars are intrinsic, such as Mira and Betelgeuse, which have surfaces that can change size or be covered with spots, thereby altering their light output. Other stars are extrinsic variables, such as Algol and Beta Lyrae, which are occasionally eclipsed by companion stars.
Astronomical Magnitude Scale
-13 Full Moon
-6 Crescent Moon
-1 Sirius (the brightest star)
0 Bright stars such as Vega
+2 Polaris, the North Star
+4 Ganymede, Jupiter’s largest moon
+5 Dimmest stars visible under a dark sky, also Vesta, brightest asteroid
+6 Uranus, usually requires binoculars to see
+8 Neptune, requires binoculars or a telescope
The magnitudes listed above are apparent magnitudes. Sometimes you will also see an object’s absolute magnitude listed beside its apparent magnitude. Apparent magnitude is the distinction most pertinent to stargazers; it describes how bright an object appears to us here on Earth. Absolute magnitude is a more scientific label used to illustrate how bright the object would be if it were located 10 parsecs (32.6 light-years) away from Earth. Absolute magnitude allows for a scholarly comparison of objects in space.
Check out this Solar System Magnitude Scale for a list of objects that can be observed with your eyes alone.
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Kelly Kizer Whitt loves clean, clear, and dark skies. Kelly studied English and Astronomy at the University of Wisconsin-Madison and worked for Astronomy magazine. She is currently the Feature Writer for Astronomy and Space at Suite101.com. You can follow her on Twitter at twitter.com/Astronomommy