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Skylights featured nine times on Earth Science Picture of the Day: 1 2 3 4 5 6 7 8 9


Photo of the Week. Look what's coming!

Astronomy news for the two weeks starting Friday, July 3, 2015.

Phone: (217) 333-8789
Prepared by Jim Kaler.

Clear skies and thanks to Skylights' blogger visitor reader.

Go to STARS for previous stars of the week. Last week's Skylights is still available. Access Skylights' Archive and photo gallery. From the Sun to the Stars: the OLLI Lectures provides a linked, illustrated introduction to astronomy.
The Constellations has a linked list with locations and brightest stars. Constellation Maps show the locations of the constellations. The 170 Brightest Stars lists them through magnitude 3.00. For more on stars and constellations, visit Stellar Stories.
Tour the Milky Way. Watch a total eclipse of the Moon and an annular eclipse of the Sun. Moon Light presents scenic photos of the Moon. Go to MoonScapes for labelled telescopic images of the Moon and other lunar information.
See the Moon move and pass just below Nu Virginis. Watch planets move against the background stars. See a classic proof of the curvature of the Earth with a "hull down" series. Visit Measuring the Sky to learn about the celestial sphere.
Admire sunsets, rainbows, and other sky phenomena in Sunlight. Read the illustrated Day Into Night on the phenomena of the sky See the The Aurora and the Midnight Sun. See and understand the ocean tides.
Enjoy Our Complex Universe: A Human Understanding through Art, with 12 illustrations. Advances in Astronomy, 1989-2011. Take a ride aboard Asteroid 17851 Kaler (1998 JK). Look for Books about the sky and stars.


ASPSupport science literacy by joining the Astronomical Society of the Pacific, an international organization that is among the world's premier providers of astro education. Get Mercury and a variety of other benefits.

Presenting three audio courses with 70 to 100-page study guides, narrated and written by Jim Kaler.
Heavens Above: Stars, Constellations, and the Sky from Recorded Books. Astronomy: Earth, Sky, and Planets, is available from Recorded Books. Astronomy: Stars, Galaxies, and the Universe, is also now available from Recorded Books.
Astronomy: Earth, Sky, and Planets is published as Vault of the Heavens: Exploring the Solar System's Place in the Universe by Barnes and Noble.

Enjoy Our Complex Universe:A Human Understanding through Art, with 12 illustrations.

Read "Heaven's Touch: From Killer Stars to Seeds of Life, How We Are Connected to the Universe," Princeton University Press.

SSTo learn about stellar spectra, read STARS AND THEIR SPECTRA: An Introduction to the Spectral Sequence, Second Ed., with two new chapters and 140 new illustrations, Cambridge University Press (UK or North America), 2011.

Read From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.

SSNEWEST! FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. Read the interview with Jim Kaler.

NEW! Read Bearly North in Stellar Stories.

The next skylights will appear July 17, 2015. Happy Independence Day. And Bastille Day too.

We begin as usual with the Moon, which starts our fortnight in its waning gibbous phase as it heads toward third quarter on Wednesday, July 8. It then takes a week to go through waning crescent, which ends at new Moon on Wednesday the 15th. The morning of Saturday the 11th finds the Moon to the right of the Pleiades of Taurus. By the following morning, the crescent will lie at the western edge of the Hyades cluster with Aldebaran down and to the left, the Pleiades now on top. Your last look at the thin rising crescent will be morning of Tuesday the 14th. With a good horizon you then might spot the waxing crescent in the west the twilit evening of Friday the 17th, Jupiter and Venus shining up and to the left. (Reserve the evening of Saturday the 18th for a spectacular gathering of the crescent Moon, Venus, Jupiter, and the star Regulus.) The Moon goes through perigee, where it is closest to the Earth, on Sunday the 5th.

A day after lunar perigee, at 2;41 PM CDT on Monday, July 6, the Earth goes through aphelion, where it is farthest from the Sun, a distance of 94.507 million miles (152.093 million kilometers), 1.7 percent farther than average. That aphelion occurs during the high heat of northern summer tells that distance from the Sun has little to do with the seasons, which are caused by the 23.4 degree tilt of the Earth's rotation axis against its orbital axis. All things equal, the eccentricity of the Earth's orbit should produce wider seasonal swings in the southern hemisphere than in the northern, but the effect is largely absorbed by the oceans, which dominate the southern hemisphere.

Venus and Jupiter remain close together in western evening twilight, Jupiter the fainter, the two making a memorable pairing. With Mars and Mercury pretty much out of sight, all that's left is Saturn, which by the time the sky darkens is already past the meridian low to the south to the northwest of Antares in Scorpius.

OH, but there is Pluto, which goes through opposition to the Sun on Monday the 6th against the densely-packed stars of the Milky Way in Sagittarius. The New Horizons spacecraft is due to make its close flyby on Tuesday, July 14 when some of the veil of mystery is lifted from the distant body.

With the Moon out of the way and the sky dark, it's a fine time to admire the Milky Way, which plunges southward from Cygnus (upside down, the Northern Cross) through Aquila, to Sagittarius (which holds the Galaxy's center), and Scorpius.

STAR OF THE WEEK: MU CYG (Mu Cygni). "A lovely double star" say Smythe and Chambers, writing to us from the nineteenth century. Discovered by William Herschel, currently separated by 1.7 seconds of arc, over a long period of time one can watch the two stars partially orbit each other. Made of fifth and sixth magnitude (4.75 and 6.18) F6 and G2 dwarfs (with a combined magnitude for the naked eye of 4.49), Mu Cyg is tucked away in far southeastern Cygnus far away from the Swan's classical figure, just over the border with Pegasus, 3.1 degrees north of Kappa Pegasi. The naming is a bit unusual, as the two carry separate Bright Star and Henry Draper catalogue numbers, Mu A and B (brighter and fainter) also respectively called Mu-1 and Mu-2 Cyg. Old-time astronomers appropriately called the brighter "white," while visual effects made the fainter seem "blue," which it isn't; if anything it's more yellow-white.

Mu Cyg The components of Mu Cygni, the "lovely double star," mutually orbit every 789 years (the scales on the axes in seconds of arc), averaging a distance between them of 118 Astronomical Units, which does not mean all that much since the high eccentricity (0.66) takes them between 40 and 196 AU apart. While the two actually go around a common center of mass that lies between them, it's mathematically simpler to graph the fainter member, Mu-2 Cygni, as going around the brighter, Mu-1, which lies at the cross. The major axis of the true orbital ellipse, given by the dot-dash line, is offset from that of the observed ellipse because of the 76-degree orbital tilt and the orientation to the plane of the sky. The direction of motion is shown at the lower right corner. Note that the most recent modern observations (in blue) are much more accurate than the earlier ones obtained by traditional means. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

At a distance from us of 72.5 light years (give or take just 0.7), the two orbit with a period of 789 years at an average separation of 118 Astronomical Units. A rather high eccentricity carries them between 40 and 196 AU apart. While they were last closest in 1958, the orbital tilt (76 degrees) and orientation made them seem closest, just over half a second of arc apart, in 1937. Kepler's Laws give them a total mass (A plus B) of 2.64 times that of the Sun. The classes are not all that well known, opinions about Mu-1 (A) ranging from F4 to F7, Mu-2 (B) from F3 to G2 (F6 and G2 generally adopted). The temperatures are respectively 6325 and (from the sunlike G2 class) 5780 Kelvin. With little infrared or ultraviolet correction, the distance then yields luminosities of 4.7 and 1.4 times that of the Sun and radii of 1.8 and 1.2 times solar. A projected equatorial rotation speed for Mu-1 of 18 kilometers per second yields a rotation period under 5 days. Theory then suggests respective masses of 1.4 and 1.1 Suns, which add to 2.5 solar, in fine agreement with that found from the orbit. If you could travel there, from each the other would look like unresolved points hundreds of times brighter than the full Moon. So far as we know, however, you would find no planet to stand on. Hovering around the pair is a veritable cluster of stars that range from twelfth magnitude Mu Cyg C 77 or so seconds of arc away through fourteenth magnitude Mu Cyg G at 90 seconds. Alas, their rapid movements relative to the AB pair show them all to be line of sight coincidences. Oddly, the DE pair, 13 seconds of arc from each other, might possibly be a real couple. The seventh magnitude A5 primary ("D") should (from its expected absolute brightness) be about 260 light years away, which makes the thirteenth magnitude secondary ("E") a red dwarf. With a separation of at least 1000 AU and given that an A5 dwarf carries a mass of say 1.7 Suns, they would take at least 24,000 years to orbit each other. They are probably not related, but they do make a nice conclusion to the story of the AB pair, one of the nicer double stars that sky has to offer.

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