Can a star's brightness be a number?

That time, that place

He watched the sky from Rhodes, and he could get his hands on the observations Babylon had accumulated over centuries. The sky seen through other people's eyes became his data too.

Why this question

Stars differ in brightness. But bright and dim are words that carry nothing precise to another person, and they will not let you compare last night's star with tonight's. To record and to compare, you need a scale.

What was found

So he set the brightest stars at first magnitude, the barely visible at sixth, and divided the range between. What the eye felt became a number. Then he catalogued the positions and brightnesses of some 850 stars.

What followed is the remarkable part. Comparing his catalogue against records from 150 years earlier, he noticed the stars were drifting — not individually but all together, very slowly. A motion no single lifetime could reveal, visible only when generations of records are laid over one another.