Ahad's Magnitude is a theorematic formulation in astrophysics that quantifies the level of light illumination cast upon non-luminous celestial bodies by the light of the surrounding universe.

It originates from a March 2004 astronomical paper by Abdul Ahad headed The Music of the Night Sky, [1] a transcript of which was published as a letter to the editor in the Journal of the British Astronomical Association in October 2005[2]

The term 'Ahad's Magnitude' derives from Ahad's constant, as it was originally termed, to illustrate the constant amount of visible light (stellar flux) that would illuminate the Earth if the Sun were hypothetically switched off[3].

He asked these simple questions: "If we exclude all light coming directly and indirectly from the nearby Sun, how much illumination do the far away stars cast upon the Earth?"
Ahads constant public domain image

and "How much light illumination would the sky provide during a voyage out toward the stars?"

Ahad's Magnitude is the sum total of all light measured in the visible part of the electromagnetic spectrum, aggregated using the magnitude system in astronomy from the formula:

Ahad's magnitude

Where mi is the apparent magnitude of the ith star.

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