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30 Jul 2018

August Astronomy: After This Month, Mars Won't Be This Close to Earth Until 2035

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Evenings in August feature a spectacular panorama of four planets, including Venus, Jupiter and Mars far outshining all nighttime stars, and Saturn, ranking sixth in brilliance, after only those three planets and the stars Arcturus and Vega.

Set up a telescope and share views of these four showpiece planets. For best results, view them in order from west to southeast at dusk: 1. Venus changes from gibbous to crescent phase, 57 percent full on Aug. 1, to 40 percent on the 31st. (Venus will be even more impressive in September, as it goes through thinner crescent phases, with the disk growing in apparent size as Venus approaches Earth.) 2. Jupiter shows its cloud belts and as many as all four of the bright moons discovered by Galileo. 3. Saturn displays its rings still tipped a generous 26 degrees into our view, with satellite Titan in a 16-day orbit. 4. Mars’ south polar cap of frozen carbon dioxide is shrinking with the advance of its spring season—look soon! View other surface features, such as Syrtis Major and Hellas Basin, provided that Martian dust storms don’t obscure our view.

Catch Mercury low in the east-northeast to east during morning twilight in late August and early September.

August is a prime month for early evening viewing of the Milky Way. The best dates—at the end of evening twilight, with no moonlight—are Aug. 1-13, and Aug. 30-Sept. 11. From a dark location, follow the Milky Way band from the “cloud of steam” (the Greater Sagittarius Star Cloud) just above the spout of the Teapot, through the Cygnus Star Cloud along the neck of the Swan within the Summer Triangle, and beyond. Viewed through binoculars, the Cygnus Star Cloud easily resolves into stars.

Aug. 1 at dusk: Four planets—in order from west to southeast, Venus-Jupiter-Saturn-Mars—span 127 degrees. A line from Venus to Jupiter, 49 degrees long, extended its own length past Jupiter locates Saturn. Extend another 30 degrees past Saturn to find Mars. Telescopes now show Venus as a gibbous disk 57 percent lit. Mars, just two nights past closest approach, still shines at magnitude -2.8 and shows a disk large enough for a magnification of 80-power to make it look as large as the moon with the unaided eye!

Aug. 4, dawn: It’s the last quarter (half) moon.

Aug. 5, dusk: Venus and Jupiter are 45 degrees apart; Saturn and Mars are 27 degrees apart through Sept. 5.

Aug. 6, predawn: The crescent moon, Hyades and Aldebaran look spectacular in binoculars! While you’re out with your binoculars, view the Pleiades “Seven Sisters” cluster about 14 degrees above Aldebaran.

Aug. 6, dusk: Earth’s current orbital velocity around the sun is 18 miles per second, toward the constellation Aries in the morning sky, and directly away from Jupiter and the constellation Libra in the evening sky. An hour after sunset, when the sun is below the west-northwest horizon, try to visualize the motions of all the planets visible at the time. An observer viewing from north, or “above,” the solar system, from the direction of the constellation Draco, would observe all the planets revolving counterclockwise around the sun. Venus, moving faster, is catching up with Earth and will overtake us in late October. In the last three months, we have overtaken all three bright outer planets—Jupiter in early May, Saturn in late June, and Mars in late July (on dates of their oppositions). We are now headed directly away from Jupiter, and are leaving all three of those slower-moving planets behind.

Aug. 7, dusk: Venus, Jupiter, Saturn and Mars span 120 degrees.

Aug. 8-10, dawn: Follow the moon’s last days of the lunar cycle in the morning sky. On Aug. 9, Pollux is 9 degrees to the left of the moon (a 6 percent crescent) and slightly higher. Castor is 4-1/2 degrees to the upper left of Pollux. On Aug. 10, the moon (only 1 percent) rises in twilight 14 degrees below and slightly to the right of Pollux. Begin watching below Orion for the first appearance of Sirius in the east-southeast.

Aug. 9, dusk: Venus and Saturn are 90 degrees apart.

Aug. 10, dusk: Venus and Spica are 20 degrees apart. Venus is now going east by a degree per day.

Aug. 12, 30 minutes after sunset: Find the young moon, a 4 percent crescent, 41 hours after new, 23 degrees to the lower right of Venus. Later, on the same night: Perseid meteors peak overnight, best from late in the evening on Aug. 12 until the first light of dawn on Aug. 13, increasing in numbers as the radiant rises higher, from 20 degrees up in the north-northeast as evening twilight ends, to 60 degrees up at start of morning twilight. Meteors can appear anywhere in the sky, but members of this shower, if their tracks are extended backward before their light-up points, would all originate from a common radiant below the “W” of Cassiopeia.

Wrapping up at the end of a night of seeing Perseids, I enjoy watching for the rising of Procyon, the “before the Dog” star in the east, and Sirius “the Dog Star” in east-southeast in morning twilight. (Follow Orion’s belt downward to Sirius.) Completing the Winter Triangle with Orion’s shoulder, Betelgeuse, both Dog stars are visible by an hour before sunrise, when the Summer Triangle of Altair, Vega and Deneb is low in the west to northwest. Can you see the Summer and Winter Triangles simultaneously? You must face different directions to see all six stars!

Aug. 13 at dusk: Venus is growing in apparent size as it approaches Earth, while Mars is shrinking as we recede from it. They now appear the same size, but strikingly different: Brilliant, cloud-covered Venus is half-illuminated, while Mars is 98 percent full, with its shrinking polar cap tipped nearly 10 degrees toward Earth. Watch the moon pass Venus and Jupiter through the 17th.

Aug. 13-Sept. 10 at nightfall: For next four weeks, Mars moves very slowly against the stars, and binoculars will show a striking, compact kite-shaped grouping of four stars of magnitude 4.5 to 4.8 in the same field, within a few degrees west of Mars. The long diagonal of the kite is just more than 2 degrees long, and the short diagonal about 1 degree. The gathering, a Chinese asterism called Dog Nation or Territory of Dogs, consists of the stars Omega, 59, 60 and 62 Sagittarii. For star tales about Sagittarius, including the fascinating Chinese fable about the Territory of Dogs, visit http://www.ianridpath.com/startales/sagittarius.htm and http://www.lcsd.gov.hk/CE/Museum/Space/archive/StarShine/Starlore/e_starshine_starlore14.htm.

Aug. 15 at dusk: The moon is in the west-southwest, 7 degrees to the upper right of Spica. Venus is 15 degrees to the lower right of Spica. Watch Venus close in on Spica for the rest of month..

Aug. 16 at dusk: Jupiter passes 0.6 degrees north of Alpha in Libra, or Zubenelgenubi, the third-magnitude star marking the southern claw of a larger, former version of the Scorpion. Binoculars give a good view.

Aug. 17 at dusk: Venus reaches greatest elongation, 46 degrees from the sun in the afternoon and evening sky. Note Jupiter to the moon’s lower right. The moon is at first quarter (half full) overnight, within an hour after moonset. Mars close up: Syrtis Major passes just north of the center of Mars’ disk at 8:52 p.m.

Aug. 18 at dusk: Jupiter is 25 degrees west of Antares. Watch the moon skip over Antares Aug. 18-19, and pass Saturn and Mars Aug. 20-23. Mars close up: Syrtis Major passes just north of the center of Mars’ disk at 9:29 p.m. (37 minutes later each day).

Aug. 19 at dawn: Mercury ends retrograde, but on the near side of its orbit and backlighted; at magnitude +1.6 this morning, it is a difficult target in bright twilight. It gets easier: Mercury brightens to magnitude +1.0 by Aug. 21, to 0.0 by Aug. 25, to -0.5 by Aug. 28, and to -1.0 by Sept. 3.

Aug. 20 at dusk: Venus and Spica are 10 degrees apart. Mars close up: Syrtis Major passes just north of the center of Mars’ disk at 10:43 p.m. Mars is highest in Palm Springs at 10:58 p.m.—the closest match!

Aug. 23 at dusk: Venus and Jupiter are now within 30 degrees. Later in the night, Mars is at 10:45 p.m., while Syrtis Major passes just north of the center of Mars’ disk at 12:34 a.m. on Aug. 24.

Aug. 24-26, at dawn: Mercury forms a nearly equilateral triangle with Pollux and Procyon, about 23 degrees on a side. Compare to larger Winter Triangle of Sirius, Procyon and Betelgeuse.

Aug. 26: The full moon is at 4:56 a.m. At dawn, Mercury is very low in the east-northeast, at greatest elongation, 18 degrees from the sun. At dusk, for a dozen evenings, through Sept. 6: Venus and Spica are within 5 degrees, and appear closest, 1.3 degrees apart, on Aug. 31. Binoculars give good views of the pair low in twilight.

Aug. 27 at dusk: Mars ends retrograde in far southeast Sagittarius. Binoculars show Chinese asterism, “Dog Nation” or “Territory of Dogs,” closely west of Mars for at least two more weeks.

August 28 at dusk: Venus and Spica are 3.1 degrees apart. The moon rises very nearly as Venus sets.

Aug. 31 at dusk: Venus, Jupiter, Saturn and Mars, span 95 degrees. Venus passes within 1.3 degrees south of Spica (closest tonight). Telescopes show Venus as crescent, 40 percent full, and Mars 94 percent full.


Extras

As mentioned above: Mars this month shows its south polar cap of frozen carbon dioxide, shrinking with the advance of southern spring, as well as surface features, such as Syrtis Major and Hellas Basin best on the nights of Aug. 17-24, provided that Martian dust storms don’t block the view.

As August opens, Mars, just a day past closest approach, still shines at magnitude -2.8 and shows a disk 24.3 arcseconds across. It is mid-spring in Mars’ southern hemisphere, and the bright south polar cap is shrinking noticeably with the approach of Mars to its perihelion in mid-September, and to its southern summer solstice in mid-October.

Syrtis Major, a prominent dark marking first noticed in 1659, will be in good position for observing on the nights of August 17-24 as it passes near the center of Mars’ disk within 2 1/2 hours of when the planet is highest in our local night sky. But Mars is farthest south in mid-August, so it is less than 30 degrees up when due south for residents of the Coachella Valley. Be patient and wait for good seeing. The best times to look for Syrtis Major, which resembles a dark triangular, northward-pointing India passing north of the disk center, occur about 37 minutes later each night, starting on Aug. 17 at 8:52 p.m., and ending on Aug. 25 at 1:11 a.m. At all these same times, look for light-colored Hellas Basin crossing south of disk the center, between Syrtis Major and the polar cap. Martian dust storms might render these surface features difficult or impossible to observe. Let’s hope for clear skies—on Mars as well as on Earth!

The next “windows” for best viewing of Syrtis Major and Hellas occur 36 days later, in late September. Mars will then reach its high point in south about two hours earlier in evening. Between now and then, watch the face of Mars shift by 10 degrees in longitude per day, if you look when Mars passes due south. For a map of Mars for any date and time, visit Sky and Telescope Mars Profiler: https://is.gd/marsprofiler.

See also these detailed descriptions of Mars’ 2018 apparition:

http://www.alpo-astronomy.org/jbeish/2018_MARS.htm

http://www.nakedeyeplanets.com/mars.htm

Asteroid Vesta in August 2018: For much of the month, asteroid Vesta is an easy target for binoculars, and the 3.3-magnitude star Theta Ophiuchi is the starting point and key to locating it. Find Theta Oph 12 degrees east of Antares, nearly halfway toward 2.8-magnitude Lambda Sagittarii, top of the Teapot. Theta Oph is plotted on Sky Calendar diagrams for Aug. 7 and Aug. 17-23. On Aug. 1, Vesta, of magnitude 6.3, ends retrograde 2 1/4 degrees north-northeast of Theta Oph while moving 4 arcminutes southward each day. Note these two stars in the same binocular field to the upper left of Theta: 44 Ophiuchi, of magnitude 4.2 and 1.3 degrees northeast of Theta; and 51 Oph, of magnitude 4.8 and nearly 1.2 degrees east-northeast of Theta. After Aug. 1, Vesta curves eastward. On Aug. 16, a line from Theta to 44 Oph, 1.3 degrees long, extended nearly half its length past 44 Oph, locates Vesta, of magnitude 6.7. The Moon appears near this field on the evenings of Aug. 19 and 20. On the evenings of Aug. 20 and 21, Vesta, of magnitude 6.8 and moving east-southeast by 9 arcminutes per day, appears very close to 4.8-magnitude 51 Oph. The asteroid passes 7 arcminutes south of the star during the day on Aug. 21. For a Vesta finder chart, visit http://www.skyandtelescope.com/observing/vesta-2018-opposition/.

Here’s a link to article on 2018 apparitions of Uranus and Neptune with finder charts:

https://is.gd/urnep

http://www.skyandtelescope.com/observing/ice-giants-neptune-and-uranus/

For suggestions for observing projects during August through October 2018, go to http://abramsplanetarium.org/msta/.

The website of the Astronomical Society of the Desert has a listing of our evening star parties, Sawmill Trailhead, our high-altitude site (elevation 4,000 feet), will have a star party starting at dusk on Saturday, Aug. 11.

Abrams Planetarium publishes a monthly Sky Calendar with an evening sky map. Subscriptions are $12 per year, for three printed issues mailed quarterly. Subscribe or view a sample back issue at www.abramsplanetarium.org/skycalendar.

Wishing you clear skies!

Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing sky watching opportunities for a variety of groups in the Coachella Valley. Robert D. Miller did graduate work in planetarium science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.

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