You are here

Moon, Saturn, Antares

July 26, 2015

The gibbous Moon anchors a pretty triangle this evening. The triangle’s other points are the planet Saturn, which is to the right of the Moon, and the star Antares, about the same distance below the Moon.

Antares is the 15th-brightest star system in the night sky. In the stellar brightness scale, it’s ranked at almost exactly first magnitude.

Astronomers have been using that scale for many centuries, although they’ve refined it to take advantage of the precise measurements possible with modern instruments.

Originally, the brightest stars of all were described as first magnitude, the next brightest were second magnitude, and so on. Today, thanks to those precise modern measurements, the brightest stars are actually in negative numbers.

The scale is logarithmic, so that a difference of five magnitudes equals a hundred-fold difference in brightness. On that scale, a first-magnitude star is roughly two-and-a-half times brighter than one of second magnitude.

The faintest objects visible to the unaided eye depend on sky conditions, light pollution, and your own eyesight. But under especially dark, clear skies, those with very good eyes can see all the way down to about sixth magnitude — stars that are just one percent as bright as Antares.

Again, look for first-magnitude Antares below the Moon as night falls, with the slightly brighter planet Saturn to the right of the Moon.

Tomorrow: electric-blue clouds in the twilight.

Script by Damond Benningfield, Copyright 2015

Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.