Magnitude: how star brightness is measured

Astronomers describe how bright something looks in the sky with its apparent magnitude. The scale is old and a little counter-intuitive, but easy to use once you know two rules.

Rule 1: lower is brighter

The scale goes back to the star catalogues of antiquity, associated with Hipparchus and Ptolemy, which sorted stars into six classes: the brightest stars were “first magnitude” and the faintest visible ones “sixth magnitude”. The modern scale kept that direction, so:

• a star of magnitude 1 is brighter than one of magnitude 2
• very bright objects have negative magnitudes.

Rule 2: 5 magnitudes = 100 times

In 1856 Norman Pogson made the scale precise: a difference of 5 magnitudes is a factor of exactly 100 in brightness. One magnitude is therefore the fifth root of 100, a factor of about 2.512.

Difference	Brightness ratio
1 magnitude	about 2.5 ×
2.5 magnitudes	10 ×
5 magnitudes	100 ×
10 magnitudes	10,000 ×

How bright things are

Object	Magnitude
Sun	−26.7
Full Moon	−12.6
Venus at its brightest	−4.9
Jupiter at its brightest	−2.9
Sirius, the brightest star at night	−1.44
Vega	0.03
Polaris, the North Star	1.97
Faintest stars visible to the eye under a dark sky	about 6

The Sun and full Moon values are from the American Meteor Society, the planets from Astronomy Engine for 2026 to 2035 and the stars from the HYG catalogue (based on Hipparcos).

What you can actually see

Under a dark sky, far from towns, the naked eye reaches roughly magnitude 6, the faintest class of the old catalogues. Light pollution and moonlight brighten the sky background and hide the fainter stars, so from a town you see far fewer.

On Astrotempo a planet is marked as visible to the eye when it is brighter than magnitude 5. Fainter planets (Uranus and Neptune) are marked as needing binoculars or a telescope. The 3D sky view draws stars down to magnitude 6.5, each sized and coloured by its catalogue magnitude and colour.

Absolute magnitude

Apparent magnitude depends on distance: Sirius looks bright partly because it is close. Absolute magnitude is how bright an object would look from a standard distance. For asteroids a different standard is used: the brightness at 1 au from both the Sun and Earth, seen fully lit. Orbit catalogues list it as the “H” value. Astrotempo uses JPL’s H values to estimate how bright asteroids look tonight.

Sources

• Krisciunas (2001), A brief history of astronomical brightness determination methods at optical wavelengths (Hipparchus and Ptolemy, Pogson’s definition)
• NASA JPL Solar System Dynamics glossary: H (absolute magnitude H)
• American Meteor Society: Fireball FAQ (magnitudes of the Sun and full Moon)
• Star magnitudes: HYG database v4 (CC BY-SA 4.0), see sources. Planet magnitudes: Astronomy Engine.