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The Sky This Week, 2018 August 14 - 21

Dog Days...
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The Moon and Jupiter at dusk, Stellafane, Springfield, Vermont, 2016 August 5

The Moon waxes in the evening sky this week, gliding by three planets as she courses along the southern reaches of the ecliptic.  First Quarter occurs on the 18th at 3:48 am Eastern daylight Time.  Luna begins the week close to the bright glimmer of Venus, six degrees northeast of the dazzling planet as dusk falls on the 14th.  On the 16th and 17th you’ll find her on either side of Jupiter as twilight fades to full darkness.  She ends the week in the company of Saturn, just four degrees northwest of the ringed planet on the evening of the 20th.

The hot, muggy days of August are often referred to as the “Dog Days”, a description that conjures panting canines lolling in the shade with panting tongues hanging out.  The expression has been around for quite some time, and it commemorates an astronomical event which dates back thousands of years to the earliest traces of civilization.  The event is the “heliacal rise” of the brightest star in the sky, Sirius.  This is the time when the star first becomes visible just before sunrise, and it has long been associated with the hottest days of summer.  Ancient Egyptians noted that the event coincided with the annual flood of the Nile, and starting in the earliest Dynastic times became the anchor for their solar calendar.  However, their civil calendar of 365 days lost a full day to the solar calendar every four years.  After 1460 of these civil calendar years the heliacal rise of Sirius once again coincided with the first day of the civil calendar.  The fact that this coincidence was recorded three times through the course of Egyptian history is a testament to the durability of their culture!  The significance of the heliacal rise of Sirius continued in ancient Greek and Roman culture.  The Greeks gave the star its current name, which translates loosely as “The Scorching One”, since it was believed that the light of the star at its heliacal rise, added to that of the Sun, caused the hottest days of the year.  In Roman skylore Sirius became associated with the constellation Canis Major, the Larger Dog, and the days around the heliacal rise became known as the “Dies Canicularum”.  In Egypt the heliacal rise of Sirius occurred in late July, but at the latitude of Rome the event occurred in early August.  If you’re vacationing at a place with an unobstructed view of the southeast horizon, look to the southeast about 20 minutes before sunrise.  If you see Sirius in the brightening morning twilight, you are witnessing one of the oldest astronomical sightings ever made by humans.

If you were “skunked” by the rainy weather that washed out the peak of the Perseid meteor shower, you may still have a few more nights to look for shower stragglers.  On the nights leading up to the First Quarter Moon you may still be able to catch five to ten Perseids per hour after local midnight.   There is also an enhanced number of “sporadic” meteors during this time of the year.

Venus reaches her greatest elongation east of the Sun on the 17th.  At this time she will be 46 degrees from Old Sol, but her position along the southern ecliptic means that the best views of her will be from the southern hemisphere.  Here in the temperate northern climes she is only about 15 degrees above the western horizon as evening twilight begins to fade.  Through the telescope she resembles the Moon at first quarter phase.

Jupiter is still prominent in the southwest as twilight darkens and still dominates the early evening sky.  This week Old Jove passes just half a degree north of the second-magnitude star Zubenelgenubi.  The pair form an attractive sight in binoculars or a low-power telescope, and you still have an hour or so to get a closer view before the giant planet sinks into the horizon haze.

Saturn is the current showpiece of the night, standing above the southern horizon at around 10:00 pm.  This is the best time to view him in the telescope, and with his rings presenting their widest tilt to our line of sight, he should present a great view in almost any instrument.

Mars is receding from his close approach at the end of July, but he is still a brilliant red-tinted beacon in the southeastern sky.  His visible apparent disc is slowly beginning to shrink, but is still about 23 arcseconds across.  The global dust storm that has been hampering views of his surface features is finally beginning to abate, so you should be able to make out some of the details in four-inch and larger telescopes.

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