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31 Aug 2020

September Astronomy: Mars, Brightening and Rising Earlier in the Evening, Takes the No. 2 Spot From Jupiter

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Venus, the brightest planet, and ruddy Mars dominate September’s predawn mornings. Bright Jupiter and nearby Saturn float in the southern sky at nightfall, while creeping slightly closer together. Mars doubles in brilliance for the second consecutive month—and will outshine Jupiter by late September—while the red planet’s rising time shifts two hours earlier, into evening twilight.

Do you enjoy watching moonrises? During Sept. 1-7, the moon rises no more than 30 minutes later on each successive evening. Notice the moon’s reddened color and flattened shape at each moonrise; the moon’s decreasing phase from one day to the next; and the northward shift of its rising place from day to day, from the full moon’s rise at 7:23 p.m. on Sept. 1, through the 70-percent full moon’s rise at 10:13 p.m. on Sept. 7. Note bright Mars less than a degree above the rising moon on evening of Sept. 5. They’re still 3-4 degrees apart, high in the southwest in the hour before sunrise, on Sunday, Sept. 6.

Another chance to witness a string of evening moonrises will occur Oct. 1-7, beginning with the harvest moon of Oct. 1.

September at dusk: Jupiter is the bright “star” in south-southeast to south, with Saturn 8.3 to 7.4 degrees to its east (left) in the course of the month. Blue-white Vega is nearly overhead, with nearby Altair and Deneb completing the Summer Triangle. Golden Arcturus is still well up in the west, getting lower as the month progresses. Reddish Antares, heart of the Scorpion, is in the south-southwest to southwest. Early in the month, it’s still easy to see Spica very low in the west-southwest. On Sept. 14, can you spot brighter Mercury 10 degrees to the lower right of Spica? Binoculars and an unobstructed view are needed to follow this pair through their approach low in bright twilight, within 0.6 degrees apart on Sept. 21 and 0.8 degrees apart on Sept. 22. Fomalhaut, mouth of the Southern Fish, appears in the southeast by late in month.

Mars rises in the early evening some 10 degrees north of east within a quarter-hour after Jupiter reaches due south in early and mid-September—or at nearly the same time as Jupiter passes south late in month. Mars surpasses Jupiter’s brightness in late September. To compare them fairly, wait until they’re at equal altitudes later in evening, four hours after sunset around Sept. 9, and three hours after sunset around Sept. 30.

September at dawn: In the first days of September, Venus, shining at magnitude -4.3 to -4.1, stands at its highest in the eastern sky for this apparition. Moving just more than a degree per day against background stars, Venus passes within 9 degrees south of Pollux on Sept. 1. On Sept. 5, the “Twin” stars Castor and Pollux form a straight line with Venus, 15 degrees long. Don’t miss the close pairings of Venus and Regulus on Oct. 2 (0.5 degrees apart) and Oct. 3 (0.7 degrees apart). Venus then goes 1.2 degrees per day eastward against the stars. Following Venus and Regulus daily Sept. 28-Oct. 7 an hour before sunrise should be quite engaging, as day-to-day changes will be easy to notice. With daylight saving time still in effect, sunrise isn’t unreasonably early, so predawn sky watching can be done with little disruption.

Mars, doubling in brilliance from magnitude -1.8 to -2.5 in September, now clearly outshines the brightest star, Sirius. Find the red planet in the southwest to west at dawn, getting lower as month progresses. Bright stars: The entire Winter Hexagon is now in fine view. Begin with its brightest member, Sirius, the Dog Star, in the southeast to south-southeast. Then, in clockwise order, find Procyon, Pollux and Castor, Capella nearly overhead, Aldebaran, Rigel and back to Sirius. Orion’s shoulder, Betelgeuse, is inside the hexagon. It forms the nearly equilateral Winter Triangle with the big and little dog stars, Sirius and Procyon. By the end of the first week of September, Regulus, heart of Leo, emerges low, north of east, and climbs higher daily in the morning twilight glow. By Sept. 30, Venus pulls within 3 degrees to the upper right of Regulus. Deneb, last star of the Summer Triangle to set, is departing in the north-northwest.

Follow the moon, waning from full to a thin crescent, on mornings of Sept. 2-16. Look about one hour before sunrise to catch the moon about 3 degrees to the upper left of Mars on Sept. 6; near Aldebaran, eye of Taurus, on Sept. 9; near Twins Pollux and Castor on Sept. 12 and 13; about 6 degrees to the lower left of Venus on Sept. 14; and near Regulus on Sept. 15. On Sept. 16, look 45 minutes before sunrise to catch the last old crescent moon, 3 degrees up and 12 degrees north of east.

Follow the moon, waxing from thin crescent to full, on the evenings of Sept. 18-Oct. 1. On Sept. 18, look 40 minutes after sunset to catch the thin 4 percent crescent, very low in the west to west-southwest. With binoculars, try for Mercury 5 degrees to the moon’s lower left, and fainter Spica 4.5 degrees to Mercury’s upper left. At the same stage of twilight on Sept. 19, find a thicker 10-percent crescent moon low in the west-southwest, with Mercury 14 degrees to its lower right, and Spica 3 degrees to Mercury’s upper left. For binoculars, Mercury and Spica appear closest to each other, 0.6 degrees apart, very low in bright twilight on Sept. 21. Spica is getting lower each evening (it will be on far side of the sun in middle of next month), while Mercury in this poor apparition edges only marginally higher for another week.

On Sept. 21 and 22, an hour after sunset, look for Antares 8-9 degrees from a fat crescent moon. The moon reaches first quarter phase, half full and 90 degrees east of the sun, on Sept. 23. On Sept. 24, one hour after sunset, Jupiter appears about 3 degrees to the moon’s upper left, with Saturn 7.7 degrees east of Jupiter. Check again some four hours later when they’ve nearly set, and you’ll find Jupiter only about 2 degrees to the upper right of the moon. On Sept. 25, an hour after sunset, the moon will be 4 degrees to the lower left of Saturn and 11 degrees from Jupiter. On Oct. 1, watch for the full moon rising due east within half an hour after sunset. Another half-hour later, watch for Mars rising within 13 degrees to the moon’s lower left.

On night of Oct. 2, shortly after 8 p.m., the moon will be 1 1/4 days past full, and its center will pass only 1.4 degrees south (to the lower right) of Mars. The moon and Mars gradually spread apart for rest of that night, until sunrise on Oct. 3, when you’ll find Mars about 4 degrees to the lower right of the moon. In early October, Mars will present its closest and brightest (magnitude -2.6) approach to Earth until 2035.

The Abrams Planetarium’s monthly Sky Calendar now has more than 3,000 subscribers nationwide. I originated Sky Calendar in October 1968 and produced the October 2020 issue, featuring Mars at its brightest. For more information about Sky Calendar and a sample, visit www.abramsplanetarium.org/skycalendar.

To check for eventual resumption of star parties hosted by the Astronomical Society of the Desert, visit the club’s website at www.astrorx.org.

Wishing you clear skies!

Robert Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing informal sky watching opportunities for folks in and around Palm Springs. 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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