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30 Jul 2020

August Astronomy: The Month Offers Close Pairings of the Moon and the Brightest Planets—Plus Perseid Meteors

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August’s planetary scene features giants in tandem in the evening—and four very close pairings of the moon with the three brightest planets. The Perseid meteor shower, somewhat diminished by moonlight this year, peaks in predawn on Aug. 12.

Going camping? The best evenings for viewing the Milky Way from dark sites 90 minutes to three hours after sunset are Aug. 9-21.

Bright Jupiter in the southeast to south-southeast at dusk, and Saturn about 8 degrees to its east, are an attractive pair for evening telescopic observation. Views of Jupiter’s cloud belts and four bright satellites, and Saturn’s spectacular rings, always inspire. Jupiter will be near the moon; on Aug. 1, just before sunset, using binoculars, try to see Jupiter in the daytime, within 3.2 degrees to the upper right of the moon. An hour after sunset, they’re easy to see with the unaided eye, 3.6 degrees apart.

On the next evening, Aug. 2, Saturn and Jupiter appear 9 and 16 degrees to the upper right of the nearly full moon.

In August, Mars rises in the late evening, shortly before or after Jupiter reaches its high point in south. Mars’ tiny disk requires high magnification and steady air to reveal its surface details. I use my 6-inch reflecting telescope at 200-power with Mars high above the horizon, which occurs just before dawn this month. On the night of Aug. 2, Mars passes perihelion, its least amount of distance from sun. And on Sept. 1, the south pole of Mars is tipped its greatest angle toward the sun, initiating the planet’s summer in its southern hemisphere. These events combine to make Mars’ south polar cap, mostly of frozen carbon dioxide, shrink rapidly this month, so look soon for the best views. The south polar cap still appears as a small, bright white oval near the south end of Mars’ mostly rust-colored disk.

Also, the first permanent surface feature noted on Mars, by Christiaan Huygens in 1659, directly faces Earth for several consecutive mornings while Mars is high in our sky. The feature, named Syrtis Major, is an expanse of basaltic volcanic rock appearing as a dark triangle north of the center of the Martian disk. Look about 39 minutes later each day: Aug. 1 at 4:05 a.m.; Aug. 2 at 4:44 a.m.; and Aug. 3 at 5:23 a.m. For resources on observing Mars during 2020, visit www.abramsplanetarium.org/msta.

In the morning sky, Venus rises more than three hours before the sun and is well up in the eastern sky as dawn brightens. Against the star background, Venus shifts 30 degrees in 30 days, Aug. 1-31. On Aug. 2, it passes 1.8 degrees south of third-magnitude Zeta Tauri, tip of the bull’s southern horn. Crossing through northernmost Orion into Gemini, Venus ends August within 9 degrees south of first-magnitude Pollux, Gemini’s brighter twin.

Mercury begins the month 29 degrees to the lower left of Venus and within 7 degrees south of Pollux. Speeding toward superior conjunction on far side of the sun on Aug. 17, Mercury moves 36 degrees to the lower left of Venus by Aug. 8, when binoculars will be useful for spotting it low in bright twilight.

Also on Aug. 8, in morning twilight, find Mars high in the south, 10 degrees to the upper left of the waning gibbous moon, 79 percent full. In the late evening on Aug. 8, as Jupiter reaches due south just before 11 p.m., the moon and Mars will have just risen in the east, with the red planet only 1.7 degrees to the upper left of the 73 percent moon. The moon and Mars will appear closest, 1.4 degrees apart center-to-center, at 1:14 a.m. on Aug. 9. They’re still just 2 degrees apart at sunrise on Aug. 9, with Mars to the lower right of the 70 percent moon.

On the night of Tuesday and Wednesday, August 11-12, the annual Perseid meteor shower reaches its peak. Even before twilight ends on Tuesday evening, shortly after 9 p.m. locally, some “earth-grazing” meteors might be seen. They will be long-lasting, because the meteoroids, leftover particles from previous passages of Comet Swift-Tuttle, will be entering the atmosphere on paths nearly parallel to the horizon. They’ll appear to radiate from the constellation Perseus, then rising in the north-northeast below the familiar “W” of Cassiopeia.

As the night progresses, our part of the Earth’s surface is turned ever more broadside to the incoming meteors, increasing the numbers seen, while the radiant (direction of the source) climbs higher in the northeastern sky. Throughout the darkness hours, meteors might light up anywhere in the sky, but the actual members of the shower, if their tracks are extended backward before the point where they lit up, will intersect at their common radiant, in upper Perseus. (The paths are actually parallel in space, but appear to converge in the distance.) By 11 p.m., Jupiter and Saturn will be 8 degrees apart in the southern sky, and Mars will have risen in the east. Soon after midnight, the count will suffer a setback with the rising of a fat crescent moon—but will increase again for the rest of the night as the radiant continues its ascent. A noted expert on meteor showers predicts that Earth will pass through a dense trail of comet particles around 3 a.m. If so, more meteors than usual will then be seen.

At first light of dawn soon after 4:30 a.m. on Aug. 12, the Pleiades star cluster, beautiful in binoculars, will appear 7 degrees to the upper left of the moon, and red-orange Aldebaran, follower of the Pleiades and eye of Taurus, the Bull, will be 11 degrees to the moon’s lower left. Mars will then be approaching its high point in the south, while Venus will be gleaming in the east. Venus stands at greatest elongation, 46 degrees from the sun, and shows as a “half moon” through a telescope.

On the following morning, Aug. 13, the moon will pass 4 degrees north of Aldebaran. On the 14th, look for Venus 12 degrees to the moon’s lower left; and red Betelgeuse, Orion’s shoulder, 16 degrees to the moon’s lower right. Farther right and a little lower is blue-white Rigel, Orion’s foot. Midway between these colorful stars is the Hunter’s 3-star belt. Follow its line downward to Sirius, the Dog Star, just risen in east-southeast. Close to the east and about the same distance above the horizon, look for Procyon, forming the nearly equilateral Winter Triangle with Sirius and Betelgeuse. If you catch Sirius soon after it rises, turn around to see the entire Summer Triangle of Vega, Deneb and Altair in the northwest to west, before Altair sets, if there’s no mountain blocking your view.

In mid-August, Mars is predicted to equal and then surpass Sirius in brightness, but it’s hard to make a fair comparison now, since Mars is high in the sky while Sirius is low. As Earth closes in until October, Mars nearly doubles its brightness each month in August and September.

Venus near the moon: The waning moon continues eastward through the zodiac toward the sun, and on the morning of Aug. 15, an hour before sunrise, the 16 percent crescent appears within 4 degrees to the north (upper left) of Venus. It’s a pretty sight, with earthshine on the moon’s non-sunlit side! Keep track until sunrise, and you can get a daytime sighting of Venus with the naked-eye or binoculars.

On Aug. 16, the 9-percent crescent moon appears in the east-northeast, 5 to 9 degrees to the lower right of Pollux and Castor, the Gemini twins. The last easy old crescent moon occurs on Aug. 17, as a thin 3 percent sliver, low in the east-northeast, 12 degrees below Pollux.

The invisible new moon occurs as it passes the sun on Aug. 18 at 7:42 p.m. Grab binoculars on the evening of Aug. 19, about 30 minutes after sunset. From a site with an unobstructed view midway between the west and west-northwest, you’ll get a chance to view a young crescent moon, 2 percent full, only 4 degrees up, and just more than 24 hours old.

Follow the waxing moon nightly one hour after sunset, Aug. 20 through Sept. 2. On Aug. 22, the 22 percent crescent moon passes within 6 degrees north of Spica in the west-southwest, in Virgo. On Aug. 25, the 54 percent moon, just past first quarter phase (when it’s half full and 90 degrees east of the sun), passes within 6 degrees to the north of the red supergiant star Antares, heart of the Scorpion.

Jupiter near the moon, reprise: On Aug. 28, just before sunset, use binoculars to try for a daytime sighting of Jupiter, within 2.3 degrees to the upper left of the gibbous moon in the southeast. One hour after sunset, Jupiter will be 2.4 degrees almost directly above the 84 percent moon in the south-southeast. Saturn will be nearly 8 degrees to the left of the moon. That night, Jupiter and Saturn are 8.3 degrees apart, and the gap between them begins to close until their rare and spectacular conjunction just 0.1 degrees apart, at dusk on Dec. 21, 2020.

On Aug. 29, an hour after sunset, find Saturn and Jupiter 6 to 14 degrees to the 91 percent moon’s upper right.

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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