Apparent magnitude
A number that describes how bright an object looks from Earth; the smaller the number, the brighter it appears.
Astronomers measure brightness using the apparent magnitude scale. It works backwards: lower numbers are brighter. The idea goes back to ancient Greek astronomers, who called the brightest stars 'first magnitude' and the faintest they could see 'sixth magnitude'. Today the scale is exact: a difference of 5 magnitudes means 100 times brighter, so each single step is about 2.5 times.
Very bright objects get negative numbers. Sirius, the brightest night-time star, is about −1.5; Venus can reach about −4.6; the full Moon is about −12.7; and the Sun is about −26.7. From a dark country site the faintest stars you can see with the naked eye are around magnitude 6.
Magnitude matters for meteors, too. Observers estimate each meteor's magnitude by comparing it with nearby stars. A meteor brighter than about magnitude −4, as bright as Venus, is called a fireball, and the very brightest, which can rival the full Moon, sometimes drop meteorites, as happened at Winchcombe in 2021. To practise, learn the magnitudes of a few familiar stars and planets and use them as your measuring sticks. Never look at the Sun, even to compare brightness.
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Level: Explorer Ages 10 and up