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The Sky This Week, 2020 March 10 - 18

Cats and dogs the sky?
Orion and his Dog, imaged 2017 March 2 near Boulder, Colorado.
Orion and his Dog
imaged 2017 March 2 near Boulder, Colorado with a Canon EOS Rebel T2i DSLR

The Moon wends her way through the morning sky this week, diving southward along the ecliptic to join the rising stars of summer.  Last Quarter occurs on the 16th at 5:34 am Eastern Daylight Time.  Early risers can watch Luna approach three planets in the pre-dawn sky, culminating in a beautiful close approach to Mars and Jupiter on the morning of the 18th. 

The early evening is still graced by the bright stars of winter surrounding the distinctive outline of Orion, the Hunter.  At 9:00 pm Orion is west of the meridian, but just to the southeast is the brightest star in the entire sky, Sirius.  Unlike most of the stars in Orion, Sirius gets its brilliance from its relative proximity to us, located just 8.6 light-years away.  With twice the mass of our Sun, Sirius shines with about 25 times the radiant power of Old Sol.  In our skylore it is seen as a jewel in the collar of Canis Major, one of Orion’s hunting dogs.  Its name derives from the ancient Greek word for “scorching”, since its first appearance just before sunrise corresponded to the hot days of mid-summer.  This “heliacal rising” was noted in many ancient cultures, but perhaps none paid as much attention to this event than the ancient Egyptians.  The sight of Sirius rising just before the Sun coincided with the annual flood of the Nile river, which deposited nutrient-rich silt in the long valley that sustained their civilization for some 3000 years.  It also served to calibrate their civil calendar, which consisted of 12 months of 30 days with 5 intercalary days tacked on at year’s end.  This calendar didn’t account for the extra 0.2422 days in a tropical year, so the start of each year “slipped” relative to the rising of Sirius.  However, every 1460 years the two calendars would once again synchronize.  This “Sothic Cycle” was diligently noted by Egyptian priests, and when the calendars coincided it was widely celebrated.  Their civilization lasted long enough to observe the festival three times.  In Roman times the heliacal rise of Sirius heralded the “Dies Canicularum”, which today are still known as the “Dog Days” of early August.

Of course, if there are dogs in the sky, then there must naturally be cats.  By 11:00 pm the sky’s biggest cat, Leo, the Lion, is edging his way toward the meridian.  Leo’s brightest star, Regulus, sits at the base of a semicircular asterism popularly known as The Sickle.  Regulus is around 80 light-years away and is actually a quadruple star system.  The main component, Regulus A, is the brightest component of the system, with a luminosity of nearly 300 Suns.  It is unusual due to its very rapid rotation, spinning once on its axis in just under 16 hours.  By contrast, the Sun rotates at a leisurely pace of once in about 24 days.  The rapid rotation of Regulus distorts it into a flattened sphere; if it rotated much faster it would tear itself apart from its equator.  Leo’s second-brightest star, Algeiba, lies just north of Regulus.  If you look at the two stars with binoculars you will note a perceptible color difference between them, with Algeiba sporting a striking yellow tint.  Point a small telescope at Algeiba and you will see that it is actually a closely-spaced double star.

Two other cats occupy the rather blank patch of sky between the twin stars of Gemini, Castor and Pollux, the Sickle of Leo, and the outlying stars of Ursa Major.  These are Leo Minor, the Lion Cub, and Lynx.  They were introduced into the sky by the Polish astronomer Johannes Hevelius, who plotted them on his grand star atlas, Firmamentum Sobiescianum.  Hevelius abhorred the idea of blank spaces in the sky, and a number of his “new” constellations survive to this day.

Venus is making rapid progress through the setting autumnal constellations.  Watch her progress over the next several weeks as she closes the gap with the Pleiades star cluster.  She will pass through the cluster during the first week of April.

Early risers can watch a planetary dance taking place in the morning twilight sky.  As the week opens, Mars, Jupiter, and Saturn are evenly spaced in the southeastern sky, but with each successive morning ruddy Mars closes the gap with bright Jupiter.  Mars will overtake and pass the giant planet next week, and on the morning of the 18th the waning crescent Moon will join the pair.  Saturn waits for Mars to whiz by at the end of March.  Jupiter won’t catch up to the ringed planet until next year.