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30 Aug 2018

September Astronomy: As Autumn Arrives, We Can Still Enjoy Fantastic Views of Five Planets

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Four planets are still visible at dusk, until Venus departs in early October. Venus is very low in the west-southwest, with Jupiter in the southwest, and Mars in the south-southeast, standing out in brilliance. As Earth recedes from Mars, the red planet slips to third place in brightness, after Venus and Jupiter. Saturn, in the south, ranks sixth, a little fainter than the stars Arcturus in the west, and Vega just north of overhead.

Other bright stars: Spica is 1.3 degrees to the upper right of Venus on Sept. 1 and sinks into bright twilight to the lower right of Venus by midmonth. Use binoculars to keep seeing Spica for a while longer. Antares is below a line joining Jupiter and Saturn. Altair and Deneb complete the Summer Triangle with Vega.

Follow the moon from a thin crescent very low in the west on Sept. 10, to full, low south of east, on Sept. 24. Watch the moon pass four planets and two first-magnitude stars Sept. 11-20.

If you have access to a telescope, set it up, and share views of these showpiece planets! For the best results, begin your observations no later than a half-hour after sunset, to catch Venus low in west-southwest before it drops even lower, and conclude with Mars, climbing in the south-southeast. Observe the planets in order from west to east, allowing you time to savor close-ups of each:

1. See Venus as a crescent, 40 percent full on Sept. 1, narrowing to 17 percent on the 30th. Venus reaches peak brilliance late in the month, and the crescent becomes ever more impressive: On Sept. 1, the disk is 30 arcseconds across—1/120 of a degree, big enough for a 32-power telescope to make it appear as large as the moon with the unaided eye—and grows in apparent size by more than 50 percent by month’s end, as Venus decreases its distance from Earth by more than a third. To reduce the planet’s glare against a darkened sky, look in daylight, or very soon after sunset, and even ordinary 7- or 8-power binoculars will reveal the crescent!

2. Just 23 to 14 degrees to the upper left of Venus, find Jupiter. A telescope may show up to four of its bright moons, discovered by Galileo.

3. Next, 45 to 41 degrees to the upper left of Jupiter, find Saturn. A telescope shows its rings now tipped nearly 27 degrees into our view—this year’s best view, and the best until 2032—and Saturn’s largest satellite, Titan, in a 16-day orbit nine times farther out than the rings’ outer edge, in the same plane.

4. Finally, 27 to 33 degrees to the lower left or left of Saturn, find Mars. After nightfall, binoculars show an attractive, compact, 2-by-1 degree, kite-shaped asterism of four stars, the “Territory of Dogs,” just west of Mars, in the same field. Through a telescope, the red planet may still show a tiny remnant of the south polar cap of frozen carbon dioxide, greatly shrunken in its late spring season, and dark surface features, such as Syrtis Major, provided that Martian dust storms don’t obscure our view. Visit the Sky Calendar extra content page at abramsplanetarium.org/msta to read the Chinese legend about the Territory of Dogs, and for details on observing Mars close up, explanations of the graphs of planet rising/setting times, the evening and morning twilight charts, January’s total lunar eclipse, and more.

Autumn begins on Saturday, Sept. 22, at 6:54 p.m., as the sun passes directly over Earth’s equator. On Monday, Sept. 24, the full harvest moon rises just 4 degrees south of due east at 6:52 p.m., within a quarter-hour after sunset. The moon rises in twilight on next two nights in Palm Springs, at 7:24 p.m. on Sept. 25, and at 7:57 p.m. on Sept. 26. Rising later and farther north each evening, by Sept. 29, the moon rises in the east-northeast at 9:53 p.m., and two nights later, on Oct. 1, the northernmost moonrise occurs at 11:35 p.m.

The best dates for the early-evening viewing of the Milky Way at the end of evening twilight, with little or no moonlight, are through Sept. 11, and Sept. 28-Oct. 11. From a dark location, follow the Milky Way band from the “cloud of steam” (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, part of our own spiral arm, easily resolves into stars.

September in morning mid-twilight: Early this month, bright Mercury is still visible low in the east to east-northeast, getting lower each morning as it heads toward the far side of the sun, while faint Regulus, heart of Leo, emerges higher daily, owing to Earth’s revolution around the sun. Mercury and Regulus appear closest, 1.2 degrees apart, on Sept. 6. Binoculars will come in handy for seeing the pair in same field low in twilight for a few mornings around that date. The old crescent moon appears near Regulus on Sept. 8.

Stars: Beginning with the brightest star, Sirius, in the southeast, go clockwise around the “Winter Hexagon” to Procyon, Pollux, Castor, Capella, Aldebaran, Rigel and back to Sirius. (If you count the Twin stars Pollux and Castor as one vertex of the polygon, then it’s a Hexagon, with Betelgeuse inside.) Regulus in Leo is chasing the Hexagon across the sky. The only other bright star up in morning twilight, in the northwest, is Deneb, the last Summer Triangle star to disappear.

Follow the waning moon in the morning sky Sept. 1-8. Watch it leapfrog over the Hyades cluster and Aldebaran Sept. 2-3, and pass 10 degrees south (to the lower right) of Pollux on Sept. 6, and 1.4 degrees north (to the upper left) of Regulus on Sept. 8. Bright Mercury is 4.5 degrees to the lower left of the moon-Regulus pair that morning. The moon begins another pass through the morning sky on Sept. 24, with the not-quite-full moon setting just south of west. On the morning of Sept. 30, the 70 percent waning gibbous moon is 3 degrees east of Aldebaran. Binoculars will easily show the bright star in the same field, and some stars of the Hyades cluster a little farther from the moon.

The website of the Astronomical Society of the Desert at www.astrorx.org has a listing of our evening star parties at two locations. Sawmill Trailhead (SMT), our high-altitude site (elevation 4,000 feet), will have star parties starting at dusk on Saturday, Sept. 8 and Oct. 6. Monthly star parties resume on Saturday, Oct. 13, at our more accessible site, the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument, on Highway 74, within 4 miles south of Highway 111 in Palm Desert.

Abrams Planetarium publishes a monthly Sky Calendar with an evening sky map Subscriptions are $12 per year, for three printed issues mailed quarterly. The current set includes the 50th anniversary issue, for October 2018. Subscribe or view a sample back issue at www.abramsplanetarium.org/skycalendar.

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. Dr. Jeffrey L. Hunt, a retired planetarium director now living in the Chicago area, made the rising/setting sky charts below; he has taught astronomy and sky watching to all ages. He studied astronomy education at Abrams Planetarium at Michigan State University. Jeff writes an astronomy blog at jeffreylhunt.wordpress.com and can be followed on Twitter at @jeff_hunt.

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