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The Sky This Week, 2018 January 16 - 23

More to learn about Sirius.
Orion and Sirius
imaged from near Boulder, Colorado on 2017 March 2.

The Moon returns to the evening sky this week, appearing as a waxing crescent in the southwestern sky by the evening of the 18th.  Each succeeding night you’ll find Luna’s growing crescent coursing through the barren star fields of Aquarius and Pisces.  First Quarter falls on the 24th at 5:20 pm Eastern Standard Time. 


As the Moon climbs higher on the sky, look for the phenomenon of “Earth-shine” between the cusps of the waxing crescent.  It should be easily seen during the earlier crescent phases, but you should still be able to see it up to the time of First Quarter if you have a really clear sky.  The effect is caused by sunlight reflecting off the Earth and falling on the portion of the Moon that’s not in direct sunlight.   Popularly known as “the Old Moon in the arms of the New Moon”, the delicate blue shading hints at what our fair planet must have looked like to the few people who have had the opportunity to see it from a distance.  Those of us with our feet firmly rooted on terra firma can only speculate on that sight and enjoy the slow creep of the terminator from night to night as it reveals more details on the Moon’s battered ancient surface.

Last week we discussed the bright, luminous blue stars in the constellation of Orion.  This week we’ll another look at Sirius, which trails Orion through the winter night skies.  The stars of Orion are bright due to their extremely high luminosities, but Sirius owes its brilliance to its close proximity to our solar system.  Sirius lies at a distance of just 8.6 light-years, which makes it the sixth-closest star system to us.  It was one of the first stars to have its distance determined by measuring its annual parallax, and small irregularities in its motion against more distant stars led the astronomer Friederich Bessel to determine that it had an unseen companion in 1844.  In 1862 Bessel’s hunch was confirmed by optician Alvan Graham Clark, who discovered a faint companion star while testing an 18.5-inch diameter telescope lens.  This companion turned out to be the first “white dwarf” star to be discovered when its spectrum was isolated in 1915.  White dwarf stars are the “end results” of the demise of normal, sun-like stars.  When such stars exhaust their nuclear fuel, the battle between gravity and radiation pressure in the star’s core breaks down, allowing gravity to compress the remnant matter into a degenerate state where electrons no longer repel atomic nuclei.  This material, consisting of tightly packed atomic nuclei, is incredibly dense; the white dwarf companion to Sirius packs a solar mass into a sphere the size of the Earth.  Known as Sirius B, the companion star is visible in backyard telescopes of 8-inches or more diameter if you have steady air and know just where to look.

Sirius was another very important star to the ancient Egyptians, who noticed that when it rose just before the Sun each summer the annual life-giving flood of the Nile would follow.  This observation served as a marker for their civil calendar, which “synched” with the heliacal rise of Sirius every 1460 years.  Their civilization lasted long enough to observe this event three times!

If you’re on the hunt for bright planets you’ll have to get up before the Sun to find them.  Jupiter should be easy to find during gathering morning twilight.  The giant planet is the brightest object you’ll see in the southeastern sky at this time.  He is trailed by the much fainter, red-tinted Mars, who is moving steadily eastward toward the rising stars of Scorpius.  If you have a clear view of the southeast horizon start looking for the yellow glimmer of Saturn about 45 minutes before sunrise.

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