The Sky This Week: Planets galore in the late summer skies

Between August 21 and 28, Jupiter's moons go head to head in a race across the planet and our own Moon reaches First Quarter.
By | Published: August 21, 2020 | Last updated on May 18, 2023
Ganymede's shadow crosses the face of Jupiter
Ganymede gives Jupiter a black eye
Ganymede’s huge shadow often crosses Jupiter as the moon passes in front of the planet. This shot from the Hubble Space Telescope shows the moon’s large shadow blotting out part of Jupiter’s iconic Great Red Spot.
NASA, ESA, and A. Simon (Goddard Space Flight Center); Acknowledgment: C. Go and the Hubble Heritage Team (STScI/AURA)
Friday, August 21
The Moon reaches the closest point, called perigee, in its not-quite-circular orbit around Earth at 6:57 A.M. EDT. It will then sit 225,876 miles (363,512 kilometers) from our planet.

Tonight and overnight into tomorrow morning, Jupiter’s moons will put on a show you won’t want to miss. The gas giant shines at magnitude –2.6 in the southwestern sky, where it’s about 8.3° west of magnitude 0.3 Saturn.

Jupiter is already about 20° high at sunset and climbing. At that time, a small scope will reveal that the planet’s largest moon, Ganymede, is about 26″ from Jupiter’s eastern limb. Over the next few hours, Ganymede closes in; at 10:59 P.M. EDT it begins to transit the disk. Keep watching overnight into the early hours of the 22nd to follow Ganymede’s trek across the planet’s 45″-wide disk, and skip down to the next entry for more moon action as another Galilean moon prepares to make its mark.

Sunrise*: 6:18 A.M.
Sunset: 7:48 P.M.
Moonrise: 9:07 A.M.
Moonset: 9:44 P.M.
Moon Phase: Waxing crescent (10%)
*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 22
Whether you’ve been following Jupiter since late last night or are just tuning in for an early-morning view, train your scope on the gas giant in Sagittarius this morning as a double header between two Galilean moons gets underway.

August 22 opens with Ganymede already trekking across Jupiter’s disk. By about 1 A.M. EDT, the planet’s Great Red Spot should be roughly centered on the planet’s disk, while Ganymede is located to the Spot’s northwest. At 1:11 A.M. EDT, Io slips onto the eastern side of the planet as well. Io’s shadow is the next to make an appearance, becoming visible as a small black spot on the planet’s disk at 2:03 A.M. Less than 20 minutes later, Ganymede leaves the planet’s western edge at 2:19 A.M. It’s just over 10 minutes later that Ganymede’s large, dark shadow finally slides onto Jupiter’s face at 2:31 A.M. (all times EDT).

Ganymede wins this race — but Io will be back to challenge it in a rematch next week.

Sunrise: 6:19 A.M.
Sunset: 7:46 P.M.
Moonrise: 10:20 A.M.
Moonset: 10:14 P.M.
Moon Phase: Waxing crescent (18%)

Sunday, August 23
Picture the constellation Gemini as two stick figures, whose heads are the bright stars Castor and Pollux, with their bodies stretching out to the southwest. An hour before sunrise, the Twins are lying on their side in the east, with Pollux closer to the horizon than Castor. Halfway between Pollux’s head and toes (marked by Alhena, Gamma [γ] Geminorum), you’ll find bright Venus, blazing at magnitude –4.3. The planet sits about 0.5° due south of Mekbuda (Zeta [ζ] Geminorum), sometimes drawn as Pollux’s left knee.

Just past the Twins’ toes is the easily recognizable constellation of Orion the Hunter, whose belt is nearly perpendicular to the horizon. Orion’s right shoulder is marked by the famous red giant Betelgeuse, which recently underwent a strange dimming episode that caused it to fade noticeably. Astronomers now ascribe the episode to a “stellar sneeze,” during which it blew out a cloud of dust that blocked some of the star’s light from reaching Earth, making it appear to dim.

Sunrise: 6:20 A.M.
Sunset: 7:45 P.M.
Moonrise: 11:33 A.M.
Moonset: 10:45 P.M.
Moon Phase: Waxing crescent (27%)

The globular cluster Messier 15
A globular cluster in the Winged Horse
The ancient globular cluster Messier 15 sits just off the nose of Pegasus.
APAstroLab (Flickr)
Monday, August 24
Pegasus the Winged Horse is rising in the east after sunset, climbing higher in the sky as evening proceeds. This large constellation is home to the globular cluster M15 (NGC 7078), sometimes called the Great Pegasus Cluster. M15 is located just over 4° northwest of magnitude 2.4 Enif, the brightest star in Pegasus despite its designation of Epsilon (ε) Pegasi. You’ll find Enif west of the Great Square of Pegasus, roughly 20° west of Markab. Enif, which means “the nose” in Arabic, is typically drawn as the flying horse’s mouth or muzzle.

M15 itself is magnitude 6 and spans 12.3′. It lies about 33,600 light-years away and is one of our galaxy’s oldest globular clusters. M15 will look noticeably fuzzy in binoculars, especially compared to the 6th-magnitude star located just to its west.

This dense globular cluster is also the first globular discovered to contain a planetary nebula, which is the shell of gas blown out by a dying star and subsequently lit up by that star as it fades. The nebula, Pease 1, is challenging to spot even with a large (15 inches or greater) telescope and requires dark skies and patience to pick out from the busy background of the packed cluster. If you’re an experienced observer or photographer looking for a challenge, push the magnification to 300x or greater and look about 1′ northeast of the cluster’s core for the tiny, bubblelike nebula.

Sunrise: 6:21 A.M.
Sunset: 7:43 P.M.
Moonrise: 12:46 P.M.
Moonset: 11:30 P.M.
Moon Phase: Waxing crescent (38%)

Tuesday, August 25
First Quarter Moon occurs at 1:58 P.M. EDT. Because it rises at essentially the same time, our satellite is visible all evening, already in the sky at sunset and sinking below the horizon just shy of midnight. It’s a great time to settle back for some easy lunar observing. The terminator, which divides night from day, is moving relatively fast; between sunset and moonset, you may be able to detect its motion if you come back to the Moon several times.

Several of the Moon’s large seas are on display for Northern Hemisphere observers: Frigoris, Serenitatis, Tranquillitatis, Nectaris, Crisium, and Fecunditatis. The large crater at Fecunditatis’ eastern edge is Langrenus. From directly above, the 81-mile-wide (130 km) crater is a near-perfect circle; but from our perspective here on Earth, it appears more as an oblong shape, thanks to the curvature of the lunar surface.

Sunrise: 6:21 A.M.
Sunset: 7:42 P.M.
Moonrise: 1:57 P.M.
Moonset: 11: 59 P.M.
Moon Phase: First Quarter (50%)

Star chart showing the location of Cygnus X-1
Deceptively docile
Cygnus X-1 lies close to Eta (η) Cygni. Let this chart guide you to its visible component, the blue giant star HDE 226868.
Astronomy: Roen Kelly
Wednesday, August 26
Already high overhead at sunset, Cygnus the Swan is your target tonight. Lurking within this graceful grouping of stars is the sky’s surest black hole, one of two components of the binary system called Cygnus X-1. Although the black hole itself is invisible, its companion star — a hot, blue-white giant with the inauspicious designation HDE 226868 — is not. HDE 226868 (also designated HIP 98298, among a few other names) is an 8th-magnitude star less than 0.5° east of Eta (η) Cygni. It’s detectable with binoculars and more easily with a telescope.

HDE 226868 orbits its black hole companion every 5.6 days, flung around by the compact massive object’s immense gravity. Astronomers suspect the pair is so close that the black hole is actively feeding on HDE 226868 as well, pulling material off the star and onto a disk of material that ultimately swirls into the black hole like water flowing down a drain.

Sunrise: 6:22 A.M.
Sunset: 7:40 P.M.
Moonrise: 3:06 P.M.
Moon Phase: Waxing gibbous (61%)

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Thursday, August 27
With a brilliant opposition coming up October 13, we’re now in peak Mars observing season. The best time to observe it is a few hours before sunrise, when the Red Planet is high above the southern horizon, shining at magnitude –1.7 in southeastern Pisces. Its disk is currently 90 percent lit and spans 18″. By the end of the month, it will span 19″ and grow to 92 percent lit.

Summertime is approaching in the Red Planet’s southern hemisphere; if you observe the planet with a telescope, you’ll see that its southern polar cap, tilted toward Earth, is relatively small. Because of its brilliance, the bright Hellas basin is often confused for the polar cap. The large basin is visible around 1 A.M. EDT and rotates off the disk within about three hours. Come back tomorrow night at the same time for a bit more time to search it out — thanks to the Red Planet’s rotation rate of 24 hours 37 minutes, Mars’ features appear to move slightly backward when viewed at the same time on successive nights.

Sunrise: 6:23 A.M.
Sunset: 7:39 P.M.
Moonrise: 4:10 P.M.
Moonset: 12:43 A.M.
Moon Phase: Waxing gibbous (71%)

Friday, August 28
Dwarf planet 1 Ceres reaches opposition at 8 A.M. EDT. It’s glowing at magnitude 7.7 — within reach of binoculars or a small scope — and visible in Aquarius both early this morning and late this evening. The tiny world is currently 186 million miles (299 million km) from Earth. You’ll find it this morning about halfway on a line drawn between the bright stars Fomalhaut in Piscis Austrinus and Skat in Aquarius. It’s moving slowly west but is still roughly between the same stars by evening.

While you’re in the area, look about 13° northeast of Skat to find Neptune. For a closer signpost, the planet is less than 3° east of 4th-magnitude Phi (φ) Aquarii. At magnitude 7.8, our solar system’s outermost planet is roughly the same brightness as Ceres.

Asteroid 20 Massalia reaches opposition at 5 P.M. EDT, when it is nearly 147 million miles (236 million km) from Earth. It, too, is in Aquarius, about 14° due west of Neptune but glowing a much fainter magnitude 9.4.

The Moon passes 1.4° south of Jupiter at 10 P.M. EDT. The pair, along with Saturn just 8.3° east of Jupiter, is in Sagittarius. The constellation is southwest of where you’ll find Ceres, Massalia, and Neptune in Aquarius.

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%)

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