Apparent magnitude is the degree of brightness of a celestial body as observed on Earth, compared to its actual brightness marked by the atmospheric absence. The brighter the object projects to us, the lower its magnitude.

The scale used to gauge the magnitude of a celestial body traces its origins back to the Hellenistic practice of categorizing stars according to how visible they are to a mere observer. It is called the system of six magnitudes wherein the brightest stars were taken to belong in the first magnitude, while the faintest were graded as sixth magnitude. Today, the system does not work anymore within the confines of the six grades and the visibility of celestial objects. The brightest stars are labeled with negative magnitudes. To exemplify, Sirius, hailed to the brightest in the celestial sphere, lists -1.4 as its apparent magnitude. The Sun and the Moon are also included; -12.6 for the Moon and -26.73 for the Sun.

Measuring the apparent magnitude of celestial bodies is also a way to measure their distance from the Earth. Stars indicating a relative change in their magnitude might suggest gradual movement, be it to or from the planet. Some faint stars become stronger over the years, while