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The Sky This Week, 2019 September 10 - 17

It's time for the Harvest Moon
Saturn and six moons, imaged 2015 July 12 at the U.S. Naval Observatory.
Saturn and six moons, imaged 2015 July 12 at the U.S. Naval Observatory
Composite image made with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor.
and a ZWO ASI120MC color imager. Note Titan to the upper right.

The Moon climbs through the dim autumnal constellations this week, reaching her Full phase on the 14th at 12:33 am Eastern Daylight Time.  As the Full Moon that occurs closest to the autumnal equinox, this particular Full Moon is almost universally known as the Harvest Moon, at least in Northern hemisphere climes.  It gets this name due to the unusual geometry of the Moon’s path through the sky at this time of the year.  If you follow the Moon’s progress around the sky each month, you will note that she advances about 13 degrees on successive nights.  On average, the time between successive moonrises in northern temperate latitudes is about 50 to 60 minutes, but around the times of the equinoxes the angle that the plane of the Moon’s orbit makes with the eastern horizon reaches its extremes.  In the spring it intersects the eastern horizon at a high angle, so times of successive moonrises of the nearly Full Moon differ by up to 80 minutes.  In the fall the opposite effect takes place, with Luna’s orbit intersecting the horizon at a much shallower angle.  Now the times of successive full moonrises differ by just 30 minutes.  This effect becomes more pronounced as you move to higher northern latitudes.  Throughout most of Europe, where the annual harvest season is in full swing, the Full Moon rises only 20 minutes later each night.  If you venture north of the Arctic Circle, the Moon will actually rise earlier on successive nights.  This annual effect was not lost on farmers in bygone times.  Having the nearly Full Moon rise at nearly the same time each evening at peak harvest time enabled them to work well past sunset to bring in their reapings.

The bright Moon washes out all but the brightest stars in the evening sky.  Most of the eastern sky appears blank, populated by the dim constellations of Capricornus, Aquarius, Cetus, and Pisces.  With the exception of the second-magnitude star Diphda in Cetus, none of these star patterns boast stars brighter than third-magnitude.  However, there is one star that stands out from the retinue of faint stars.  You’ll find it located low in the southeast during the later evening hours, a lonely beacon in an otherwise near-empty sky.  This star is Fomalhaut, brightest star in the obscure constellation of Pisces Austrinus.  It is the 19th brightest star in the sky, and its isolated position makes it a useful navigation reference for interplanetary spacecraft.  For many years its spectrum has been used as a standard reference to study other stars.  Located at a distance of about 25 light-years, it began to garner intense interest by astronomers in the mid-2000s.  Infrared studies revealed that Fomalhaut was surrounded by a pair of flattened discs of cool material that were thought to be proto-planetary in nature, similar to the Kuiper Belt that encircles the Sun beyond the orbit of Neptune.  In November, 2008 it was announced that the Hubble Space Telescope had imaged a planetary companion to the star, the first direct image of a planet orbiting a star other than the Sun.  Since that time new observations indicate that at least two other planets also circle Fomalhaut, and it joins the star Castor as the only first-magnitude stars known to host planetary systems.

While Fomalhaut skirts the southern horizon, the bright stars of the Summer Triangle pass across the zenith.  Vega, Deneb, and Altair should be easy targets to spot despite bright moonlight and urban light pollution.  You can use these stars to find a number of interesting things to look at with a small telescope despite the brightness of the sky.  Near the center of the triangle, about halfway between Vega, the brightest of the trio, and Altair, the southernmost, lies a third-magnitude star that you may be able to glimpse with the naked eye on a crisp evening.  This is Albireo, the star that marks the “head” of Cygnus, the Swan.  Point a small telescope at Albireo and you will be rewarded with a view of a wide double star whose components exhibit a striking color contrast.  The brighter component shines with a golden yellow hue, while the fainter member is distinctly blue.  Because of this I like to call Albireo the “Navy Double”.  This is a sight that’s best enjoyed in small telescopes.  In larger instruments the colors tend to wash out.

Jupiter continues to dominate the southwestern sky in the early evening.  The giant planet is gradually inching eastward against the stars of Ophiuchus, the Serpent Bearer.  In addition to the changing surface features of the planet itself, the comings and goings of his four bright Galilean moons are also interesting to watch.  If you happen to be watching Old Jove in your telescope at 8:06 pm EDT on the evening of the 16th, watch the largest moon Ganymede emerge from the planet’s shadow well to the east of the bright planetary disc.

Saturn crosses the meridian at around 8:30 pm, offering several hours of telescopic inspections before he sinks toward the southwest horizon.  Haze and bright moonlight will hamper your search for some of the planet’s bevy of icy moons, but you should be able to spot Titan, the largest of these companions.  Titan is unique in the solar system because it has a dense atmosphere, but you probably wouldn’t want to visit.  Titan’s weather consists of downpours of liquid methane and blizzards of frozen ethane, and it is very cold at 94° Kelvin (-290°F)!

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