SKYLIGHTS

Skylights featured on Astronomy Picture of the Day

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

Mars/Saturn

Photo of the Week. Mars (upper right) and Saturn (lower left) make a fine pair, as seen in early June of 2016. The three-star head of Scorpius lies between them, while the red supergiant Antares is near the bottom edge. The fuzzy patch up and to the right of Antares is the globular cluster Messier 4. See full resolution.)


Astronomy news for the two weeks starting Friday, July 1, 2016.

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. 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.

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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, now in Chinese translation.

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 The Queen in Stellar Stories.



The next skylights will appear July 15.

As we begin, the Moon is in its waning crescent phase, which is terminated at new Moon, when it celebrates the Fourth of July. With a clear eastern horizon you might catch a glimpse of the ultrathin crescent the morning of Sunday, July 3. The Moon then switches sides, appearing as a very thin waxing crescent in twilight the evening of Tuesday the 5th, or at least by the following evening. Watch as it climbs toward Jupiter, gliding first south of the star Regulus in Leo the evening of Thursday the 7th then appearing just to the west of the giant planet the following night and well to the east of it a night still later. The crescent actually occults Jupiter as seen from portions of the eastern hemisphere. The Moon next takes a bead on two more planets, passing above Mars the night of Thursday the 14th then above Saturn the following night. After zipping through first quarter the night of Monday the 11th, the Moon goes into its waxing gibbous phase, full Moon not reached until July 19. The Moon passes apogee, farthest from Earth, the night of Tuesday the 12th. Regulus, Jupiter, Spica in Virgo, Mars, then finally Saturn make a semblance of a dotted line that falls along the ecliptic, the apparent path of the Sun in late summer and autumn.

The three planets are putting on quite a show, starting with Jupiter in the west as evening falls. As twilight draws to a close, Mars shines already to the west of the celestial meridian. Saturn follows to the east with the star Antares in Scorpius below. The trio of stars that makes the Scorpion's head lies between the two planets. Jupiter sets shortly before midnight, Mars around 2 AM, Saturn an hour later. Near the two extremes of the Solar System, Mercury, quite out of sight, goes through superior conjunction with the Sun on Wednesday the 6th, while just a day later Pluto does the opposite, going through opposition with the Sun on Thursday the 7th.

On the Fourth of July, only five hours after new Moon, the Earth goes through aphelion, farthest from the Sun at a distance of 94.51 miles (152 million kilometers), just three percent farther than its distance at perihelion in January, clearly showing that the seasons have little to do with solar distance but instead are caused by the 23.4 degree tilt of the rotation axis against the orbital axis. Launched five years ago, the spacecraft Juno also celebrates the Fourth of July by entering orbit around Jupiter, where it will observe the giant planet for the next year and a half, giving us glorious views.

North of Mars, Saturn, and Scorpius a dark early-evening sky will reveal the large distorted pentagon made by Ophiuchus, the Serpent Bearer, the Serpent itself presented in two parts, the Head, Serpens Caput, to the west of the Bearer, the Tail, Serpens Cauda, to the east. Climbing the eastern sky is the Summer Triangle made of Vega in Lyra at the northwestern apex, Deneb in Cygnus at the northeastern, Altair in Aquila at the southern. High, nearly overhead, the Big Dipper descends to the northwest.

STAR OF THE WEEK: 36 OPH (36 Ophiuchi). Because of their inherent faintness, class K dwarfs are not common in the naked-eye sky. The best-known is 61 Cygni, the first parallax star and the 13th closest star system, which is made of a pair of them. One-upping it is 36 Ophiuchi (known by its Flamsteed number). In far southern Ophiuchus (the Serpent Bearer) just 3.5 degrees north of the border with Scorpius and 2 degrees southwest of Theta Oph, it's triple-K, all dwarfs. At a distance of 19.40 light years (give or take 0.05), fourth magnitude (4.32) 36 Oph consists of a double K star (from RECONS K1.5 and just K, with identical magnitudes of 5.12) now 5 seconds of arc apart, plus a distant sixth magnitude (6.32) K5 dwarf 732 seconds of arc (a fifth of a degree) away. Together make the 76th closest system, just after Eta Cassiopeiae.

36 Oph The apparent orbit of 36 Ophiuchi B about brighter 36 Oph A (at the cross) is tilted almost into the line of sight. The actual major axis of the true orbit (dash-dot line) is way down at the bottom, with the cross at the focus of the true orbital ellipse. Given a bit more tilt and the stars could eclipse each other once apiece during the 470-or-so year orbit. Adding to the odd effect is a high eccentricity that takes the two from as far as nearly 160 AU apart to as close as 7. They will make closest approach (at "periastron") around the year 2150. In reality of course the stars orbit a common center of mass, which here remains undefined for lack of sufficient data. The orbital direction of "B" about "A" is clockwise with north presented down as is traditional in visual observations (since telescopes invert the image). The first measurement, made a couple hundred years ago, is obviously terrible. The data then quite noticeably improve to the right as time proceeds. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

A partial orbit from observations made over the past two centuries show that the inner pair (36 Oph A and B) go around each other with a period of 471 years at a mean distance of 82.3 Astronomical Units, about twice Pluto's distance from the Sun, a very high eccentricity of 0.92 running them between 7 and 157 AU apart. They were last most distant from each other in 1913 and will be at their closest in 2148. Be sure to look. Tilted by only 10 degrees, the orbital plane is not far from the line of sight. Kepler's Laws give them a combined mass of 2.5 times that of the Sun, each then 1.25 solar masses, which is much too high for class K dwarfs, so the orbit clearly needs a longer baseline of observations. What's a couple more centuries anyway? With respective temperatures of 5135 and 5100 Kelvin, to account for a bit of infrared radiation, the A and B have luminosities a third that of the Sun and radii of three-quarters solar. Masses are estimated at 0.85 Suns, giving a sum of 1.7 Suns, much lower than the orbital value, as expected. An adjustment to a mean distance of 72 AU would take care of the discrepancy, though the period could well need adjusting too. Both stars seem to have coronae like the one that surrounds the Sun, with temperatures that run between three and four million Kelvin. Metal contents seem to be about 60 percent solar. The distant third member, 36 Oph C, with a temperature of maybe 4500 Kelvin, has a luminosity just 14 percent solar and a radius of 0.63 times that of the Sun. With an estimated mass of 0.71 Suns, it's at least 4400 AU distant from the inner pair and must take at least 180,000 years to orbit. Another, eighth magnitude 36 Oph D, is moving too fast and is clearly just a line of sight coincidence. But twelfth magnitude (12.3) 36 Ophiuchi E may well be real. If so it would be a cool M7 or so red dwarf with a mass of maybe 12 percent solar orbiting at least 230 AU away over a period of at least 2500 years. There is really not all that much unusual here. Of the 100 closest star systems, 26 are double, while 6 (including this one) are triple, and one each quadruple and quintuple, leaving 66 as single. Of the 145 closest stars, about half are in double or multiple systems. Three of the single stars have planets, and who knows how many more there may be. (Thanks to RECONS, the Research Consortium on Nearby Stars for stellar data and statistics.)


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