Technology explainer
Why Does a Lower Number Mean a Brighter Object on the Magnitude Scale?
This explains why lower numbers mean brighter objects on astronomy's apparent magnitude scale, and why each five-step jump represents a hundredfold increase in brightness.
When astronomers say something has an "apparent magnitude of -16.7," the first instinct is to assume a bigger number means bigger. It's actually the opposite, a quirk baked into one of astronomy's oldest measurement systems. The apparent magnitude scale runs backwards: the lower the number, the brighter the object. The Sun registers around magnitude -26.7, while the faintest stars visible to the naked eye sit around magnitude +6.
What makes this more confusing is that each single step on the scale doesn't mean a fixed amount brighter, it means a multiplied amount brighter. Every five steps down the magnitude scale means an object is about 100 times brighter, not five times. So the gap between magnitude -16.7 and -26.7 looks small as two numbers, but it's actually the real gap between a bright artificial mirror and the Sun itself.
Astronomers use this scale to compare any two objects in the sky: a distant star against a nearby planet, or, as in a recent study, a proposed mirror satellite against the full moon. Understanding the scale is what turns a number like "four magnitudes brighter than the full moon" into something a reader can actually picture, rather than just a raw figure.
First appeared in
A Single Space Mirror Would Outshine the Full Moon
