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The Sky This Week, 2018 May 22 - 29

Bright Moon washing out the sky? No problem...
Observing the Moon with USNO's 1895-vintage
30.5-cm (12-inch) f/15 Clark/Saegmüller refractor

The Moon waxes in the evening sky this week, diving southward along the ecliptic as her phase increases.  Full Moon occurs on the 29th at 10:19 am Eastern Daylight Time.  May’s Full Moon has popular names that incorporate the pleasant climate of late spring.  These include the Milk Moon, Flower Moon, and Corn Planting Moon.  Luna passes less than a degree north of the second-magnitude star Porrima in the constellation of Virgo on the evening of the 24th.  Porrima is a closely-spaced double star whose components are now slowly separating as they orbit their center of mass.  This is one of the few double stars where a significant change in separation and position angle of the components can be easily observed over a ten-year period, and it’s one that I have followed for about 30 years.  The components are almost equal in brightness and are currently about 3 arcseconds apart, so they should be visible inn a 4-inch telescope.  The stars were closest in 2005, when I could not resolve them in my 9.25-inch scope.  One night after Luna passes by Porrima she will be placed well north of Spica, the brightest star in Virgo.  On the 27th she sits just to the east of the bright planet Jupiter.

The Moon washes out many of the fainter stars of the springtime sky, but one bright star stands out in the evening no matter how much light the Moon throws at it.  Arcturus shines with the glow of a shimmering topaz high in the eastern sky as twilight deepens.  Arcturus is the brightest star in the northern hemisphere sky and the fourth brightest of all, only exceeded by Sirius and the southern hemisphere stars Canopus and Alpha Centauri.  It is relatively near to the solar system with a measured distance of just under 37 light years.  Its mass is thought to be slightly greater than that of the Sun, but its luminosity is some 170 times more than that of Old Sol.  Its age is estimated to be about 7 billion years, so Arcturus may be a good model to study the future fate of the Sun.  Arcturus is an “evolved” star that has depleted the supply of hydrogen in its core; it is now fusing hydrogen into helium in a shell around the core.  This causes the star to swell in size to about 25 times the Sun’s diameter.  As its surface expands it cools, giving the star its characteristic warm color.  It has the most rapid proper motion of all of the first-magnitude stars except the nearest, Alpha Centauri, moving about two arcseconds per year.  This means that it moves the apparent diameter of the Moon’s disc in 900 years.  Arcturus’ proper motion was discovered in 1718 by astronomer Edmond Halley, who noticed that it was far from the position measured by the Greek astronomer Hipparchus some 1800 years earlier.  The name “Arcturus” derives from ancient Greek and means “Guardian of the Bear” for its position in the sky, following the stars that form the constellation Ursa Major.  It is the lead star in the constellation of Boötes, the Herdsman, which in my mind looks more like a kite or an ice-cream cone that the ancients’ interpretation of a man with a staff holding two dogs on a leash!

As Arcturus climbs in the east, Ursa Major crosses the meridian high overhead.  The constellation’s signature asterism, the seven stars that form the “Big Dipper”, should be easy to find despite the increasing glow of light scattered from the Moon.

Bright Venus still dazzles in the western sky as evening twilight falls.  She is now coursing her way through the stars of Gemini, the Twins, and reaches her most northerly declination in this year’s evening apparition on the 22nd.  She will continue her eastward trek across the stars and will start to slowly move southward over the horizon over the course of the next few months.

Jupiter becomes visible in the southeast shortly after sunset, and he spends the evening wheeling across the sky, crossing the meridian at around 11:00 pm.  The giant planet is at his best for telescopic viewing, and you should be able to enjoy him in the telescope all night long.  In addition to looking for the planet’s signature equatorial cloud belts and Great Red Spot, watch the changes in the configurations of his four bright Galilean moons.  On the evening of the 23rd watch his innermost large moon Io drag its shadow across the planet’s disc between 9:13 and 11:23 PM EDT.

Look for Saturn low in the southeast during the late evening hours.  Saturn spends most of the next several months in the vicinity of the top of the “Teapot” asterism of Sagittarius and is near the most southerly declination in his 29-year orbit.  Wait for a night with steady air to get a good glimpse of the ringed planet.

Mars is beginning to slow his eastward progress among the faint stars of Capricornus as he nears the first stationary point of this year’s apparition, which he’ll reach next month.  Like Saturn, he will spend this year’s apparition at a very southerly declination, so nights of steady air will give you the best views of his growing, ruddy disc.  Your best view of him this week continues to be in the hours before dawn.

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