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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Reflections on the Night's Brightest Star
by Geoff Chester, USNO Public Affairs
18 January 2022
Orion and Sirius, imaged 2014 March 27 from Paris, Virginia
with a Canon EOS Rebel T2i DSLR camera.
The Moon wanes as she moves into the morning sky and the rising springtime constellations. Last Quarter occurs on the 25th at 9:41 am Eastern Standard Time. Late-night skywatchers can see Luna approach the bright star Regulus, the heart of Leo, the Lion, after she rises on the evening of the 19th. Early risers on the morning of the 24th can spot the Moon just north of the bright star Spica in the constellation of Virgo.
Stargazing at this time of year takes a certain amount of fortitude. The blasts of arctic air that pour down from the north can make an evening under the stars a test any person’s mettle. However, the cold air usually means low humidity and no haze, so we often experience nights with the highest sky transparency. I have found that crisp cold winter nights allow me to see stars that are about a magnitude fainter than those I can see in summer nights. The trade-off is that I have to bundle up from head to toe to enjoy a modicum of comfort. Fortunately we have modern materials to insulate us, unlike astronomers from the “classical” era of visual observing. One of America’s leading astronomers of the day, Edward Emerson Barnard, worked at the Yerkes Observatory in Williams Bay, Wisconsin, spending countless winter nights observing with the great 40-inch refractor located there. He habitually observed in sub-zero conditions, much to the dismay and discomfort of his night assistants. A reporter once asked him how astronomers kept warm on long January observing sessions; his answer was “We don’t.”
The bright stars of the boreal winter sky offer little respite from the cold, but their cheerful glow and varied colors make cold weather observing a bit more tolerable. Foremost among these bright luminaries is Sirius, which trails the distinctive constellation Orion across the sky. Sirius is the brightest star in the sky, and, as such, has been deeply embedded in human culture since ancient times. The ancient Egyptians identified the star as Sepet, the soul of the goddess Isis, one of the oldest deities in their pantheon. The annual rising of Sirius just before the Sun preceded the annual flooding of the Nile, which sustained their civilization for 3000 years. Although it is often referred to as the “Dog Star” from its association with the constellation Canis Major, the star’s name derives from the ancient Greek word that means “the scorcher”. Indeed, when the star is near the horizon, atmospheric refraction causes the star to flicker through the colors of the rainbow, giving the impression of a dancing flame. Sirius owes its brightness to its proximity to the solar system at a distance of just 8.7 light-years. The star is about 23 times as luminous as the Sun and glows with a distinctive blue tint. Sirius was one of the first stars to have its motion across the sky measured. Edmond Halley (of comet fame) discovered its changing position in 1718 by comparing his measurements of the star’s position with those of Ptolemy and Hipparchus. In the decade between 1834 to 1844 the German astronomer Friederich Bessel noticed irregularities in the star’s proper motion and attributed them to an unseen companion circling the bright star. The companion, now known as Sirius B or “The Pup”, was discovered on January 31, 1862 by the American telescope maker Alvan Graham Clark while testing an 18.5-inch lens ground and polished by his father. It was the first white dwarf star to be discovered.
The only bright planet now easily visible in the evening sky is Jupiter, which pops out of the twilight glow in the southwestern sky about 15 to 20 minutes after sunset. Old Jove only spends an hour in the sky after evening twilight ends, and his proximity to the horizon makes it difficult to see fine details in his turbulent clouds.
You might be able to glimpse Saturn as twilight deepens, but the ringed planet now sets before the end of twilight. He is now far too low to observe with a telescope, but you should be able to spot him in binoculars.
Ruddy Mars may be glimpsed in the pre-dawn sky, low in the southeast, but you will likely need binoculars to see him. Half an hour before sunrise, look to the southeast for the bright glow of Venus if you have a low horizon. The dazzling planet will become much more prominent as the month ends.
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