Sky Watch

by J. Alex Knoll

A Degree of Magnitude

In the case of brightness, less is more

The sun winks from view at 8:05 Friday, setting almost a minute later each following night. As the sun sets, the first light you’re likely to see emerging from the darkness is the evening star Venus high in the west.

Blazing near magnitude –4, there’s no mistaking Venus for either first-magnitude Aldebaran, the red eye of Taurus the bull below and to the right of the planet, or golden Capella of Auriga the charioteer, shining at zero magnitude above and to the left of Venus.

Perhaps counter-intuitively, the brighter a stellar object, the lower its magnitude. A truly archaic gauge, the scale of magnitude dates back to the ancient Greek philosopher Ptolemy, who divided the stars into six categories, with first-magnitude being the brightest and sixth-magnitude being those stars just discernable. Each change in magnitude represents a doubling or halving of brightness.

More recently, the system has expanded beyond six degrees of magnitude, providing negative numbers to account for the stellar luminaries, such as Sirius, the brightest star at magnitude –1.4, and the brighter planets, like Venus, Mercury, Mars on close approaches and Jupiter at best apparition. Additionally, the new system measure the sun, magnitude –26, and the moon, magnitude –12. And while Ptolemy’s original gradation of magnitude petered out at sixth magnitude, the dimmest lights visible to the unaided eye, the Hubble Space Telescope, has found stars as dim as magnitude 30.

Sunset reveals Mars almost directly overhead, and Saturn high in the southeast. Mars is high in the west come midnight and sets a little after 2am. Saturn arcs high through southern skies before finally setting around 4am.

As Saturn sets in the west, Jupiter, rises in the east. At magnitude –2.1, you shouldn’t have any difficulty spotting this gaseous giant, which is visited by the moon Sunday and Monday mornings.