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The Sky This Week, 2018 May 29 - June 5

Gazing at a "normal" star?
Jupiter, 2018 May 25, 04:26 UT
Imaged from Mollusk, Virginia, USA
with a Celestron 23.5-cm (9.25-inch) Schmidt-Cassegrain telescope.

The Moon wanes from the full phase this week, scudding along the southern horizon as she passes through her gibbous phases.  Last Quarter occurs on June 6th at 2:32 pm Eastern Daylight Time.  Luna passes through the stars of Sagittarius early in the week, then sets sail across the dim “water” signs of the early autumnal sky.  Look for the Moon about three degrees east of the golden glimmer of Saturn in the early morning hours of June 1st.  

Luna’s glow washes out the faint stars of the late spring sky through the weekend, but by the close of the week she rises late enough to enable you to get a couple of hours of viewing to enjoy the rising stars of summer.  By 11:00 pm you’ll find the bright springtime star Arcturus crossing the meridian, and in the east you’ll see the three bright stars of the Summer Triangle rising above the eastern horizon haze.  The brightest of these three is Vega, which stands highest above the horizon in the northeast.  Vega is the second-brightest star in the northern sky after Arcturus, and it was the first star other than the Sun to be photographed way back in 1850.  It was also the first star to have its spectrum photographed (in 1872) and it has probably been the most extensively studied star after Old Sol.  It was long used as a “standard candle” to set the logarithmic magnitude scale, which used Vega as a benchmark to designate “zero magnitude” across all visible wavelengths of light.  More recent observations show that the star is slightly variable, and improvements in both space- and ground-based instrumentation have revealed that Vega is not quite the “standard” star it was once thought to be.  Infrared observations made by our first orbiting infrared satellite in 1983 revealed a large disc of dust surrounding the star, and high-precision ground-based interferometers have detected its exceptionally high rotation speed of nearly 240 kilometers per second.  This results in Vega showing a pronounced equatorial bulge that’s some 20% greater than its polar diameter.  At 25 light-years distant, Vega is one of the closer stars to the solar system, and was one of the first to have its distance measured by its annual parallax.  It is the brightest star in the small constellation of Lyra, the Harp, leading a small parallelogram of third-magnitude stars that make up the constellation.  Two favorite targets of amateur astronomers may be found in Lyra.  The easiest of these to spot from urban areas is the quadruple star system of Epsilon Lyrae, popularly known as the “Double-Double”.  Harder to spot from suburban skies is Messier 57, the Ring Nebula, a glowing cloud of tenuous gas illuminated by a very hot central star.

Venus continues to keep apace of the Sun in the evening twilight, relentlessly moving eastward along the ecliptic.  This week she closes in on the Twin Stars of Gemini, Castor and Pollux and sets shortly after 11:00 pm.  Next week she will form a straight line with the Twin Stars as she moves into the faint constellation of Cancer, the Crab. 

Jupiter now dominates the evening hours, appearing in the southeast during the fading of evening twilight and arcing across the southern horizon during the course of the night.  You should be able to start viewing him with a telescope as soon as he appears, and you’ll have lots of time to enjoy the view until the wee hours of the morning.  You should have a fine view of the planet’s famed Great Red Spot on the evenings of June 1st and 3rd, and on the evening of May 30th you can watch his innermost large moon Io drag its shadow across the planet’s disc between 11:07 pm and 1:17 am EDT.  Jupiter spends the week slowly passing north of the second-magnitude star Zubenelgenubi, brightest star of the constellation of Libra, the Scales.

Saturn now rises at around 10:00 pm, appearing low in the southeast along with the stars of the “teapot” asterism of Sagittarius.  The ringed planet transits the meridian at around 2:30 am, which will be the best time to view him this week.  Saturn will reach opposition in another month, when he’ll be visible in the sky all night long.

Mars has entered the obscure constellation of Capricornus, where he’ll spend much of the coming opposition season.  His headlong rush to the east is slowing as he approaches the first stationary point in his path which will occur on June 28th.  His disc is rapidly growing as Earth catches up to the more distant red planet.  The best time to view him is still during the morning twilight hours, but the view in a modest telescope should be rewarding. 

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