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The Sky This Week: Venus and the Moon meet in Cancer

Catch some stunning solar system pairings and see if you can glimpse the zodiacal light from September 11 to 18.
RELATED TOPICS: OBSERVING
Zodiacal light over Iran
The zodiacal light
The zodiacal light thrusts upward from the horizon like a bright sword in this beautiful night-sky image. Starting this week, it’s the perfect time to search for this dusky glow in the early morning before sunrise.
Ariana Ahangary
Friday, September 11
The planet Neptune reaches opposition today at 4 P.M. EDT. At that time, Earth will sit between Neptune and the Sun, offering excellent views all night long of the farthest planet from our star. You can find the blue-tinged world in Aquarius the Water-bearer, rising around sunset and gaining altitude as darkness falls. Its disk spans 2.3" and shines at magnitude 7.9, visible in either binoculars or a telescope. The planet will be visible all night and into tomorrow morning, setting around sunrise.

Neptune is currently about 2° east of magnitude 4.2 Phi (φ) Aquarii, which provides us with a chance to press fast-forward on the life cycle of the Sun. Phi is roughly 265 times brighter than the Sun and its radius stretches out to nearly 39 times our own star’s, or half the size of Mercury’s orbit. It’s a red giant star that matches nearly exactly the predictions of astronomers for the condition of our own Sun once it reaches an age of 12 billion years, when it will no longer fuse hydrogen into helium in its core. Eventually, both will evolve into a beautiful planetary nebula, before fading out and leaving behind a tiny white dwarf behind.  

Sunrise*: 6:38 A.M.
Sunset: 7:15 P.M.
Moonrise: 12:04 A.M.
Moonset: 3:31 P.M.
Moon Phase: Waning crescent (37%)
*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, September 12
Jupiter will finally complete its retrograde (westward) motion against the background stars tonight, coming to a standstill at 8 P.M. EDT. After that, it will begin moving east, closing the gap between itself and nearby Saturn. Jupiter is currently 8.1° west of Saturn; it will track nearly 1° east to end the month 7.4° from the ringed planet.

The best time to view Jupiter and Saturn is in the evening after sunset, when they’re already relatively high in the south. Both are located in Sagittarius, northeast of the famous Teapot asterism. Jupiter appears 44" across and glows at magnitude –2.5. Saturn’s disk is 18" across, but its rings span 40" — nearly as wide as Jupiter — and it’s magnitude 0.4. Both are visible with binoculars or a telescope; you may even spot some of the planets’ largest moons.

Sunrise: 6:38 A.M.
Sunset: 7:13 P.M.
Moonrise: 12:55 A.M.
Moonset: 4:25 P.M.
Moon Phase: Waning crescent (27%)

Sunday, September 13
There’s a demon rising in the northwest this evening: the strange star Algol in Perseus the Hero. Algol, whose name translates to “ghoul” or “demon,” represents the gorgon Medusa’s head, which the Hero holds in his fight against Cetus the Whale (or, in Greek mythology, more appropriately a sea monster). Algol is a variable star whose magnitude swings between 2.1 and 3.4 every 2.867 days. Tonight, it will reach that minimum at 8:08 P.M. EDT; at that time, it will be low to the horizon for many observers, but still visible and rising with time. And if you wait just a few hours, letting it rise, its magnitude will quickly increase to its usual 2.1.

What’s going on here? Algol is not one variable star, but a system of two objects called an eclipsing binary. In this case, Algol consists of a bright B star and a dimmer K star. These stars are oriented so that we are looking at their orbital plane edge on, which means every 2.867 days, we see the dimmer star pass in front of, or eclipse, the brighter star, making the latter appear to quickly dim and, just as quickly, regain its brightness. The entire process takes about 10 hours, so if you observe Algol exactly at its minimum, it will be back at its maximum five hours later (around 1 A.M. EDT on the 14th).

Sunrise: 6:39 A.M.
Sunset: 7:11 P.M.
Moonrise: 1:54 A.M.
Moonset: 5:14 P.M.
Moon Phase: Waning crescent (18%)
Finder chart for Venus and the Moon, September 14, 2020
Venus buzzes the Beehive
Venus and the crescent Moon rendezvous before sunrise near the Beehive Cluster (M44) on September 14, offering early risers a morning treat.
Astronomy: Roen Kelly
Monday, September 14
The Moon passes 4° north of Venus at 1 A.M. EDT this morning; as an added bonus, the pair is located near the Beehive Cluster (M44) in Cancer the Crab. An hour before sunrise, you’ll find the celestial trio above the eastern horizon, with the bright star Regulus in Leo below them. Venus is magnitude –4.2 and 65 percent lit; its full disk spans 17". The planet is currently receding from Earth along its tighter orbit around the Sun, and will lose an arcsecond of width by the end of the month.

M44 is an open cluster consisting of young stars about 577 light-years away. It is sometimes called Praesepe or the Manger. With a magnitude of 3.1 and a size of 95', you may be able to spot M44 as a large fuzzy region under dark skies. It is that naked-eye magnitude that allowed ancient people to identify the cluster without optical aid; if you do turn binoculars or a telescope on M44, you can expect to see several of its glittering, blue-white stars.

Sunrise: 6:40 A.M.
Sunset: 7:10 P.M.
Moonrise: 3:01 A.M.
Moonset: 5:58 P.M.
Moon Phase: Waning crescent (11%)
Jupiter and its moons Io and Ganymede, September 15, 2020
A close pass
Io and Ganymede pass each other while moving in opposite directions on September 15. Europa and Callisto are also visible, farther to the west.
Astronomy: Roen Kelly
Tuesday, September 15
Jupiter’s moons Io and Ganymede make a close pass tonight off the western limb of the planet. The buildup to their meeting starts at sunset; for observers in the eastern half of the U.S., Io and its shadow are crossing the face of Jupiter as darkness falls. Io slips off the disk at 9:51 P.M. EDT heading west, while Ganymede is moving east. The two moons are closest at about 10:30 P.M. EDT, when they sit 10" apart off Jupiter’s western edge. Io’s shadow is still a small dark circle on the planet’s face. About half an hour later, Io’s shadow leaves the planet at 11:03 P.M. EDT. Finally, Ganymede slips into Jupiter’s shadow, disappearing from view behind the planet at 11:43 P.M. EDT.

Io and Ganymede are just two of Jupiter’s four largest moons, also called the Galilean moons. The three innermost moons — Io, Europa, and Ganymede — are locked in resonance. (The fourth Galilean moon, Callisto, is not in resonance with the rest.) For every four orbits Io makes, Europa completes two and Ganymede completes one. It is this relationship that makes Io the most volcanic body in the solar system, as it is regularly stretched and squeezed by the gravity of Jupiter, Europa, and Ganymede.

Sunrise: 6:41 A.M.
Sunset: 7:08 P.M.
Moonrise: 4:13 A.M.
Moonset: 6:35 P.M.
Moon Phase: Waning crescent (5%)

Wednesday, September 16
High overhead tonight is Cepheus the King, Cassiopeia’s husband and Andromeda’s father. This house-shaped constellation is home to several excellent binocular objects, and the Moon’s late phase and early set time are ideal for setting your sights on this celestial king.

Get your evening off to a stellar start by targeting Mu (μ) Cephei, one of the largest, most luminous supergiant stars in the Milky Way. Just southwest of Mu is IC 1396, an open cluster of stars that requires dark, steady skies to spot. Most of its stars are magnitude 9 or fainter, so you’ll need higher-powered binoculars to see them. Low-powered binoculars will reveal some of the cluster’s scattered brighter stars. The cluster itself sits within an emission nebula, but the gas’ deep red color makes it difficult to spot. If you have a telescope and a narrowband nebula filter, however, you may have some luck.

About 5.5° north-northeast of IC 1396 is NGC 7160, another open cluster with a few brighter stars of magnitude 7 and 8. This small cluster was discovered in 1789 by William Herschel.

Sunrise: 6:42 A.M.
Sunset: 7:06 P.M.
Moonrise: 5:28 A.M.
Moonset: 7:10 P.M.
Moon Phase: Waning crescent (1%)

Thursday, September 17
New Moon occurs at 7 A.M. EDT. That means dark skies for observers and the perfect opportunity to try to catch the zodiacal light — sunlight scattered off dust in the solar system. You can look for this light throughout the rest of the month when moonless conditions occur. The zodiacal light typically appears as a conelike glow centered on the ecliptic, which is the plane of the planets in the solar system. It appears in late summer and early fall before sunrise, earning it the name “false dawn.” (In late winter, the zodiacal light appears after sunset and is sometimes called false dusk, instead.)  

For a double feature you won’t want to miss, step outside early this morning around 5 A.M. to also look for Mars, which appears high in the southwest, nestled in Pisces the Fish. The Red Planet, magnitude –2.2, is a little over 5° north-northwest of Alrescha — although the star makes a poor signpost, since it’s a much fainter magnitude 3.8. Mars observing will only get better in the coming weeks, as its October opposition is now right around the corner.

Sunrise: 6:43 A.M.
Sunset: 7:05 P.M.
Moonrise: 6:43 A.M.
Moonset: 7:41 P.M.
Moon Phase: New (0%)

Friday, September 18
The Moon reaches perigee, when it is closest to Earth in its orbit, at 9:48 A.M. EDT this morning. At that time, it will be 223,123 miles (359,082 kilometers) from Earth. It’s also a mere three-percent-lit crescent, visible largely during daylight hours and setting only an hour after the Sun.

At Sunset, the young Moon is about 5° north of Mercury; both are low in the west, sinking with the not-yet-visible stars of Virgo. Half an hour after sunset, the magnitude 1 star Spica may appear, a scant 4° above the horizon. Mercury is even lower, but a bright magnitude –0.1. If you manage to catch the planet with a telescope, you’ll see its 6"-wide disk is 78 percent lit.

You’ll have slightly more time to spend on the Moon — look for earthshine, which occurs when sunlight reflecting off Earth illuminates the portion of our satellite in shadow. The delicate crescent will sink lower in the deepening twilight, leaving a dark sky overnight for deep-sky observers to celebrate. Some possible targets you may want to try for tonight include the Andromeda Galaxy (M31), the Triangulum Galaxy (M33), and the Ring Nebula (M57). Early in the evening, you may even catch 9th-magnitude Comet 88P/Howell, about 3.5° west of globular cluster M80 in Scorpius tonight.

Sunrise: 6:44 A.M.
Sunset: 7:03 P.M.
Moonrise: 7:59 A.M.
Moonset: 8:12 P.M.
Moon Phase: Waxing crescent (3%)
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