It doesn't make sense that stars shining at magnitude 31.5 are among the dimmest the Hubble Space Telescope can see, while one shining at 0 is one of the brightest in the sky. And objects with magnitudes below 0 are even brighter! How can the lower number mean a brighter body? It starts making sense when you understand how this measurement system got started more than 2,000 years ago.

And find out what's happening in the sky the week of July 18 through 24, 2021.