Sky This Month: September 2021

Six major planets are in view before midnight during September, offering a full range of binocular and telescopic sights. Mercury and Venus hug the western horizon soon after sunset, while Jupiter and Saturn provide a dazzling spectacle in the southeast. Both planets are well placed all evening. Uranus and Neptune are best viewed in binoculars or a telescope.

Elusive Mercury tries to hide from us, but dedicated observers should successfully find it in evening twilight. Mercury is not particularly well placed for Northern Hemisphere observers as it reaches its greatest elongation east of the Sun (27°) Sept. 13. This is because the ecliptic forms a very shallow angle to the horizon. The planet shines at magnitude 0.2 on the 13th and then slowly dims, reaching 0.7 by the 27th.

Look for Mercury 30 minutes after local sunset. On the 1st, it stands 3.5° high in the western sky. By Sept. 7, Mercury stands slightly south of due west. A one-day-old Moon lies closer to due west but sets within 50 minutes of the Sun, so a clear western horizon and transparent skies are needed to see it. Try the more favorable evening of the 8th, when a fatter crescent Moon stands 5° nearly due north of Mercury 30 minutes after sunset. Look for the Moon in binoculars, then scan downward to find Mercury. The planet outshines Spica (magnitude 1), which appears between Venus, the brightest planet in the sky, and Mercury.

By Sept. 9, the waxing crescent Moon stands just under 4° north of Venus, a stunning pair in the celestial blue of twilight. Look with binoculars about 5° directly below the Moon to find Spica. Use the Moon and Venus to aid in finding Mercury, still close to the horizon and 13° to the lower right of the Moon. Thirty minutes after sunset, Mercury is less than 3.5° high.

Mercury and Spica stand 1.5° apart on Sept. 21, with Mercury about a half magnitude brighter than its stellar companion. Begin searching shortly after sunset, because you know by now that Mercury sets quickly. After this date, the planet dips even lower, becoming harder to find.

Venus is easier to spot, even though it’s low in the sky. This is due to its brilliance: It shines at magnitude –4. Early in the month, Venus sets half an hour after Mercury and stands 11° high in the southwest 30 minutes after sunset. Spy Venus through binoculars on Sept. 4; as the sky darkens, see if you can spot Spica less than 2° due south of the dazzling planet.

As we’ve mentioned, the Moon and Venus stand close on the 9th, while Venus coasts across the backdrop of southern Virgo. By the end of September, Venus sets nearly two hours after the Sun, although it hugs the horizon all month. With a telescope, you can watch the planet inflate from 15" to 19" in apparent diameter and change from 73 percent to 62 percent lit throughout the month.

Across the sky, two bright planets are well placed in the southeast for viewing. Saturn is first, shining at magnitude 0.3 in western Capricornus. It stands 14° high at sunset. Jupiter follows Saturn across the sky, located 18° farther east in Capricornus. At magnitude –2.9, the giant planet clearly outshines Saturn. Both planets are past opposition, which they reached in August, and are best viewed from 10 P.M. local time onward, when they stand high in the southern sky.

Saturn is a stunning object in any telescope, revealing its magnificent ring system encircling the 18"-wide planetary disk. The rings are tilted 19° to our line of sight, with their northern face on view. Over the next few years, the rings will become narrower, revealing more of the planet’s southern hemisphere. The outer dusky Ring A is separated from the brighter Ring B by the Cassini division. If you can easily make out the division, your seeing conditions are good. The planet shines with a yellowish hue and rarely reveals significant features unless a storm erupts, visible as an obvious white spot.

Titan is Saturn’s brightest moon, shining at magnitude 8.5. It orbits Saturn every couple of weeks and appears north of the planet Sept. 3 and 19, and south of the planet Sept. 11 and 27.

Rarely, a field star of similar magnitude enters the field of view, as on Sept. 12 and 13. On the 12th, the two are near each other southwest of Saturn; on the 13th, Titan remains southwest and the slightly brighter field star is southeast of the planet at a similar distance.

Magnitude 12 Enceladus tours around Saturn a few arcseconds from the bright edge of the rings, which make it hard to spot. A collection of brighter 10th-magnitude moons — Tethys, Dione, and Rhea — orbit a bit farther out with periods ranging from two to five days.

Iapetus is at inferior conjunction with Saturn on the last day of August. As September opens, it lies 1.5' southwest of the planet and glows near 11th magnitude. The moon brightens throughout the month, reaching a peak around magnitude 10 at western elongation Sept. 20, when it stands 9' due west of Saturn. The change occurs as its brighter trailing hemisphere becomes more visible from Earth at western elongation.

Jupiter’s retrograde loop carries it deeper into western Capricornus, passing 1.5° north of 3rd-magnitude Deneb Algedi on Sept. 12. The planet is due south at roughly 35° altitude at local midnight (depending on your local latitude). This is a few degrees better than last fall. Higher elevation means less interference from our own turbulent atmosphere, producing a brighter planet for those capturing video.

Observing Jupiter is always a thrill. The planet exhibits two thick dark equatorial belts on a broad 48"-wide disk. Brighter zones to either side of these dark belts are replete with both subtle and not-so-subtle cloud features, which the planet carries along with its rapid rotation period of less than 10 hours. Occasionally, the Great Red Spot makes an appearance. Such features move visibly within 10 minutes, making the planet a hit with all observers.

Jupiter is joined by four bright moons: Io, Europa, Ganymede, and Callisto. Collectively, these Galilean moons, so called because Galileo first reported them, change relative positions as they orbit with periods ranging from less than two days to 14 days.

The moons regularly undergo eclipses, transits, and shadow transits, sometimes two at a time. On Sept. 5, East Coast observers see Europa and its shadow crossing the planet’s face just as Ganymede begins a transit at 8:59 P.M. EDT. Ganymede begins at the eastern limb, while Europa leaves the western limb one minute later. Europa’s shadow leaves Jupiter 51 minutes after that, at 9:51 P.M. EDT. As darkness falls across the midwestern U.S., Ganymede’s transit is well underway. At 10:45 P.M. EDT, Ganymede’s huge shadow falls on the jovian cloud tops.

Illustrating the wonderful synchronous orbital periods of these two moons, this event repeats itself on Sept. 12/13. Europa begins a transit across Jupiter at 8:26 P.M. EDT. Its shadow transit starts 9:37 P.M. EDT. Look east of Jupiter to find Io and Ganymede moving in opposite directions. They are closest — less than 6" apart — at 10:20 P.M. EDT. Ganymede heads to a 12:20 A.M. EDT (on Sept. 13) transit, followed by its shadow at 2:47 A.M. EDT. The huge, jet-black spot is clearly visible in any telescope.

There are many other transits and occultations of individual moons with Jupiter. On Sept. 3, Io and Europa disappear behind Jupiter’s western limb 72 minutes apart, with Io vanishing at 10:30 P.M. EDT. A good place to check for other events is the Royal Astronomical Society of Canada’s 2021 Observer’s Handbook.

Neptune reaches opposition Sept. 14 and remains visible all night. At its distance of nearly 2.7 billion miles, Neptune musters a span of 2" and a magnitude of 7.7, rendering it visible in binoculars. High magnification in a telescope and steady seeing is needed to see its bluish-green disk. Neptune stands 6° north of the Sept. 19 Full Moon.

You can track Neptune’s motion against the background of stars in eastern Aquarius using binoculars. The planet is located nearly 5° east of 4th-magnitude Phi (ϕ) Aquarii, and the gap reduces to less than 4° as the month progresses. Look for a triangle of 6th-magnitude field stars roughly 6.5° south of the Circlet in Pisces — Neptune is near this grouping all month. In fact, on Sept. 23, the planet makes a particularly close pass (1.5') of the westernmost star in the triangle. You’ll need a telescope to see this close appulse.

Uranus rises around 10 P.M. local time on Sept. 1, and two hours earlier by Sept. 30. At midmonth, it is 20° high in the eastern sky by 11 P.M. local time. The magnitude 5.7 planet is an easy target for binoculars, although it remains challenging to spot in a dim part of southern Aries. A telescope shows a delightful pale bluish disk spanning 4".

To find Uranus, begin by searching for a triangle of stars formed by Omicron (ο), Sigma (σ), and Pi (≠) Arietis. This trio of 5th-magnitude stars stands 6° due north of Mu (μ) Ceti. Uranus starts the month in the lower middle of this triangle, equidistant from Omicron and Sigma. It then wanders westward toward Omicron as the month progresses. By Sept. 30, Uranus stands within 25' of this star. Note the gibbous Moon in the vicinity Sept. 24, with Uranus 5.5° northeast of our natural satellite shortly after rising.

Mars is too close to the Sun to observe it during September. The Red Planet returns to the morning sky in December.

Sept. 22 marks the autumnal equinox (3:21 P.M. EDT), the time of year when the Sun appears on the celestial equator, moving southward.

Rising Moon: Snaking across the Sea of Serenity

One of the most striking sights on Mare Serenitatis is the wrinkle ridge Dorsa Smirnov, but the time must be right. When the Sun is low in the lunar sky, the modestly lifted terrain casts shadows across the hardened plains of lava. The prominently paired dark and sunlit faces disappear under a higher Sun.

Since most amateurs are evening observers, we normally focus on features just after lunar sunrise, when the Moon is waxing from crescent to Full. Thankfully, the Harvest Moon effect allows us to observe the waning gibbous Moon before midnight. On Sept. 24th and 25th, the Sun begins to set over the Sea of Serenity in what some call reverse lighting. Classic lighting of the Serpentine ridge occurs during the Moon’s evening crescent phase, on the 11th of this month and next.

Also called the Serpentine Ridge, Dorsa Smirnov’s 200-mile-long complex of wrinkles formed when the regions just under the plains of lava slowly pushed into each other, causing the surface to buckle gently upward. The shapes can remind us of the delicate strands of the Veil Nebula, although the source of those compression forces was a supernova explosion, unlike the much weaker tectonic activity responsible for Luna’s features.

Don’t miss the fantastic crater Posidonius just to the northeast — it’s a joy getting lost in the wonderful detail here. For starters, there are small craters, bumps, rilles, and a second wall. Farther south, the trio of Theophilus, Cyrillus, and Catharina are simply fantastic.

Meteor Watch: Catch the false dawn

After the flurry of major showers in August, meteor rates die down significantly in September. The sporadic or background rate of meteors reaches an average of seven meteors per hour, although the occasional fireball not associated with any shower can illuminate the night sky, so watch out for those events.

Meanwhile, in early September’s moonless predawn sky, look out for a delta-shaped brightening stretching from Cancer into Gemini along the steeply inclined ecliptic before the onset of twilight. It’s the zodiacal light, best observed from very dark locations far from any streetlights. The glow is from trillions of meteoritic dust particles left over from eons of comets cruising through the inner solar system, dumping fine material as the Sun heats their surface.

Comet Search: Rubbernecking encouraged

Seven years ago, the Rosetta spacecraft surveyed Comet 67P/Churyumov–Gerasimenko by dropping the Philae probe onto the surface, later finishing the adventure with its own soft landing.

The comet rises before midnight along with the Pleiades in the northeast, but you’ll want to wait until the wee hours of the morning for it to get above the thickest part of our atmosphere. Our two-week Moon-free window closes midmonth. Glowing feebly at magnitude 10 to 11, Churyumov–Gerasimenko requires an 8- to 10-inch scope from a dark site.

At high power, the rubber duck-shaped nucleus of a couple of miles across will be cloaked in a literal flurry of ice and dust — there’s even a movie of that! The east (Sun-facing) flank is well defined, the solar wind pushing the dust into a diffuse, stubby fan to the west. Prep your scope and eyepieces to ward off the formation of dew, which is common in September.

Identified in 1969, Churyumov–Gerasimenko is a modestly recent discovery. It returns every 6.4 years to a spot just inside Mars’ orbit before looping out to Jupiter’s vicinity. In the distant future, giant Jove’s gravity will fling Churyumov–Gerasimenko off on a new adventure.

Locating Asteroids: Backward motion

Speedier Earth overtakes the main-belt asteroid 89 Julia, causing it to go retrograde, or appear to move westward, through the constellation Aquarius. You’ll find it straight south of Pegasus’ nose star, magnitude 2.4 Enif (Epsilon [ε] Pegasi).

The bright globular cluster M2, visible in a 4-inch scope from the suburbs as an unresolved cotton ball, serves as a jumping-off point to magnitude 9 Julia. Avoid the nights leading up to the Full Moon, when there is extra light in the sky. Hiding in plain sight, Julia’s gradual shift from night to night gives it away. Keep a sketch of the three or four brightest stars in the field where the chart indicates, and you’ll pick out the one that moved. That’s how Édouard Stephan found it in 1866, by meticulously comparing the real sky against charts. Later, through recording the variations in brightness as Julia spins, astronomers deduced the asteroid is a 90-mile-wide crumpled-looking ball.