The Sky This Week: Double stars delight amid the Full Moon

Despite a bright Moon, the week of August 28 to September 4 holds planets, stars, and more for late summer observers.
By | Published: August 28, 2020 | Last updated on May 18, 2023
Deep image showing the Lagoon Nebula, Trifid Nebula, M23
Deep sky in Sagittarius
Although the bright Moon this week will wash out objects like the Lagoon and Trifid Nebulae (at the center of this image), star clusters like M23 (lower left) are great targets for moonlit nights.
Mark VandeWettering (Flickr)
Friday, August 28
A waxing gibbous Moon passes 1.4° south of Jupiter at 10 P.M. EDT tonight. At sunset, they’re 2.25° apart in the southeast. Jupiter is magnitude –2.6, easy to spot in the fading twilight. Just 8.3° east of the giant planet is Saturn; at magnitude 0.3, it won’t stand out quite as well but should still be easily visible, especially in the deepening twilight.

The bright Moon will affect most deep-sky viewing this week, but that doesn’t mean stargazers are out of luck. Even with our satellite so close to Jupiter and Saturn, a small (or large) scope will bring out these gas giants’ detailed atmospheres, particularly as they rise higher, climbing out of the thicker layers of Earth’s atmosphere that tend to distort objects closer to the horizon. Pay close attention to their moons: Saturn’s brightest moon, Titan, may be visible about 3′ west of the ringed planet. But Jupiter is where you’ll really want to focus your attention. At sunset, it’s flanked by its four largest satellites: Io and Ganymede to its east and Europa and Callisto to its west. As the hours tick by, Europa and Callisto pull away, but Io and Ganymede close in on the planet’s limb. If you’re willing to stay up overnight and into Saturday morning, Ganymede and Io will both transit the disk of Jupiter in a rematch of their race last week.

While you’re waiting, skim your scope east along a line connecting Jupiter and Saturn to find Neptune, shining at magnitude 7.8 in Aquarius. If you can find the outermost planet easily enough, slew about 18.5° south-southwest to spot the dwarf planet 1 Ceres, which reached opposition earlier today at 8 A.M. EDT. Search the region halfway between Fomalhaut in Piscis Austrinus and Skat in Aquarius to spot its magnitude 7.7 glow.

Sunrise*: 6:24 A.M.
Sunset: 7:37 P.M.
Moonrise: 5:08 P.M.
Moonset: 1:35 A.M.
Moon Phase: Waxing gibbous (80%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 12 P.M. local time from the same location.

Saturday, August 29
This morning, Io challenges Ganymede in a race across the face of Jupiter. The match starts when Ganymede crosses onto the planet’s disk at 2:31 A.M. EDT. Io slips onto the disk 28 minutes later. Unfortunately, Jupiter sets for observers in the eastern and midwestern U.S. before the race resolves; those in the western portion of the country will see Io overtake Ganymede to come out ahead. Io crosses off the disk at 5:15 A.M. EDT, while Ganymede follows 35 minutes later.

The Moon passes 1.2° south of Pluto, located roughly two-thirds of the way between Jupiter and Saturn, at 7 A.M. EDT. Our satellite then passes 2° south of Saturn at 1 P.M. EDT. By sunset tonight, the Moon is already 5.5° southeast of Saturn, having pulled away from Sagittarius and crossed into Capricornus. If you’re up for a challenge (and have a larger scope), try spotting M30, the magnitude 6.9 globular cluster that sits just over 3° east of Zeta (ζ) Capricorni. The 11′-wide cluster may be difficult to spot in the light of the Moon, so remember this spot for later in the month when our satellite has left the sky. Then, compare and contrast the view with and without the bright background.

Sunrise: 6:25 A.M.
Sunset: 7:36 P.M.
Moonrise: 5:57 P.M.
Moonset: 2:31 A.M.
Moon Phase: Waxing gibbous (88%)

Diagram showing Mu Bootis' three stars
Three for the price of one
The three stars of Mu Boötis are less than 2′ apart. This diagram shows their orientation, as well as each star’s magnitude and spectral class.
Wikimedia Commons
Sunday, August 30
If you’re up for an early evening challenge, see if you can spot Mercury today. It’s sinking fast in the west about 12° east of the Sun. The planet is located just off the back foot of Leo the Lion, although the stars of the constellation won’t be visible yet. Just 20 minutes after sunset, the tiny planet is a little less than 2° high, but it’s a bright magnitude –0.7. You’ll likely spot it with the naked eye, given a clear view of the horizon. With binoculars or a scope, you’ll see its 5″-wide disk is 93 percent illuminated.  

Once darkness falls, try for another nearby target: Mu (μ) Boötis, formally named Alkalurops. This is a triple-star system, with component A (the brightest) shining an easy magnitude 4.3. Look 1.8′ south of it to spot components B and C, a pair of Sun-like stars separated by just 1.5″. Their combined glow reaches magnitude 6.5. From a planet circling the primary star Alkalurops A, B and C would appear 100 times brighter than Venus appears from Earth, separated by roughly 1° in the sky.  

Sunrise: 6:26 A.M.
Sunset: 7:34 P.M.
Moonrise: 11:33 A.M.
Moonset: 10:45 P.M.
Moon Phase: Waxing gibbous (94%)

Monday, August 31
Mars is nearly 60° above the horizon by 4 A.M. local time. It stands high in the south, just over 5° northwest of Alrescha, Pisces’ magnitude 3.8 alpha star. The Red Planet is magnitude –1.8 and brightening by the day. It’s also growing larger (currently 19″) as it approaches its October opposition. Nearby in Aries lies Uranus, a much dimmer magnitude 5.7 that you’ll need at least binoculars to pick up. Follow the line from Mars to Uranus further northeast, and you’ll run right into the Pleiades. This bright open star cluster is easily visible even in a relatively bright sky, so it’s a perfect target as the Moon continues to grow. It will reach Full in just two days.

If you prefer evening observing, our solar system’s other ice giant will be on display: Neptune is above the horizon when darkness falls and visible with binoculars at magnitude 7.8. Tonight, it lies just 2.5° from 4th magnitude Phi (φ) Aquarii in the east.

Sunrise: 6:27 A.M.
Sunset: 7:32 P.M.
Moonrise: 7:14 P.M.
Moonset: 4:35 A.M.
Moon Phase: Waxing gibbous (98%)

Tuesday, September 1
Venus makes a brilliant predawn target, nearly 30° high in the east an hour before sunrise. It blazes at magnitude –4.3 in Gemini. The planet is about 8.5° from Pollux this morning; it will pass 9° south of the star at 1 P.M. EDT. Golden Pollux is the brightest star in the Twins, shining at magnitude 1.2. Nearby Castor, which is a hotter blue-white, is nonetheless fainter at magnitude 1.6.

Once you’ve spied Venus and the Twins, look for several other easy-to-identify constellations nearby: Ursa Major (and its Big Dipper asterism) to the northeast, Canis Major (with the bright star Sirius) to the south, and Orion the Hunter with his familiar belt, sword, and bow to the southwest. See how many of these familiar patterns you can follow into the brightening dawn.

Sunrise: 6:28 A.M.
Sunset: 7:31 P.M.
Moonrise: 7:44 P.M.
Moonset: 5:38 A.M.
Moon Phase: Waxing gibbous (99%)

Finder chart for R and T Lyrae in the summer sky
Take a look at Lyra
This star chart shows the locations of both R and T Lyrae, which lie about 6° northeast and 2° southwest of Vega (Alpha [α] Lyrae), respectively.
Astronomy: Roen Kelly
Wednesday, September 2
Full Moon occurs at 1:22 A.M. EDT this morning. Later today, the Moon will pass 4° south of Neptune at 5 P.M. EDT. Asteroid 2 Pallas is stationary at 9 A.M. EDT; it’s been swinging in a wide southwestern arc against the background stars of Hercules that will now turn toward the east.

Already high overhead at sunset, focus your attention on Lyra tonight. Its bright star Vega (magnitude 0.03) is easy to spot, making up one of the three points of the famous Summer Triangle asterism. The other two points are formed by Deneb in Cygnus and Altair in Aquila.

Within Lyra is the stunningly red star T Lyrae. It’s a red giant rich in the element carbon, which causes the star’s atmosphere to preferentially absorb blue light, pumping up its ruby color. T Lyrae is one of the reddest stars in the sky; it’s also a variable star, swinging between magnitude 7.5 and 9.3. It’s located 2° southwest of Vega.

If you’d like an easier target, turn instead to brighter R Lyrae, located 6° northeast of Vega. Also a red giant, it goes from magnitude 3.9 to 5.0 every 46 days, never fading from naked-eye visibility. Turn binoculars or a scope on this star to fully enjoy its warm, orange glow.

We’ll come back to the same region of sky tomorrow for a look at some of Lyra’s other treasures.

Sunrise: 6:29 A.M.
Sunset: 7:29 P.M.
Moonrise: 8:11 P.M.
Moonset: 6:40 A.M.
Moon Phase: Full (100%)

The Double-Double pair of double stars in Lyra
Epsilon Lyrae is a “double double,” containing two pairs of double stars. (There’s actually a fifth star in the system, not visible here.)
Nikolay Nikolov/Wikimedia Commons
Thursday, September 3
Return to Lyra this evening for Epsilon (ε) Lyrae, often called the Double-Double. That’s because it contains two pairs of double stars each, for a total of four stars. A mere 1.7° northeast of bright Vega, Epsilon’s two pairs are separated by 209″. One pair, magnitudes 4.7 and 5.8, lie 2.4″ apart. The other, magnitudes 5.3 and 5.5, are 2.3″ apart.

The optics available to you will determine whether you see two or four stars. Binoculars will show just two stars 209″ apart, but a telescope will resolve each of the pairs into two, revealing its “double double” nature.

Another brilliant double lies nearby in Cygnus the Swan: Albireo (Beta [β] Cygni) is 15° southeast of Epsilon Lyrae, offering an easier-to-split pair of stars with gorgeous contrasting colors of orange-yellow and blue-white. Separated by 34″, the stars are magnitude 3.1 (yellow) and 5.1 (blue). They can be separated using binoculars, no telescope needed.

Sunrise: 6:30 A.M.
Sunset: 7:28 P.M.
Moonrise: 8:35 P.M.
Moonset: 7:40 A.M.
Moon Phase: Waning gibbous (98%)

Friday, September 4
As the Moon rises in the east this evening, turn your gaze to the west. 9.5° northwest of Kaus Borealis, the top of the Sagittarius Teapot’s lid, is M23 (NGC 6494). This open cluster of stars is easy to spot with binoculars or a small scope, spanning 27′ and shining a collective magnitude 5.5. It contains at least 170 stars that, while young at 220 million years, make them quite old as far as open clusters go. In binoculars, M23 will likely appear only as a smudge with a few individual stars visible. You’ll need at least a small telescope to begin resolving most of its stars. The particularly bright star in the cluster’s northwest is not actually a member, but a magnitude 6.5 foreground star called HIP 87782.

Sunrise: 6:31 A.M.
Sunset: 7:26 P.M.
Moonrise: 8:59 P.M.
Moonset: 8:38 A.M.
Moon Phase: Waning gibbous (94%)

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