Key Takeaways:
- On September 19th, a close conjunction of the Moon, Venus, and Regulus will be observable in the eastern sky before sunrise. The Andromeda Galaxy (M31), along with its companion galaxies M32 and NGC 205, will be visible in the evening sky.
- September 20th features a rare transit of Saturn's moon Titan and its shadow across the planet's disk, observable after midnight EDT. Saturn reaches opposition on this date.
- The autumnal equinox occurs on September 21st, and Neptune reaches opposition on September 23rd, both offering opportunities for observation. A partial solar eclipse will also occur on September 21st.
- September 24th presents a brief evening viewing opportunity of the Moon, Mars, and Spica low on the western horizon. On September 25th, asteroid 2 Pallas will be stationary in Aquila, and the constellation Triangulum, along with the galaxy M33, becomes visible in the evening.
Sky This Week is brought to you in part by Celestron.
Friday, September 19
The Moon passes 0.8° north of Venus and 1.3° north of the magnitude 1.4 star Regulus at 8 A.M. EDT, while Venus passes 0.5° north of Regulus at 9 A.M. EDT. You can catch the trio together in the eastern sky about 10° high an hour before sunrise.
This evening, the moonless sky offers a great opportunity to view one of the top sky objects on most observers’ lists: the Andromeda Galaxy (M31). Located about 1.3° west of magnitude 4.5 Nu (ν) Andromedae, this fuzzy patch of light is some 3° long and 1° wide. It’s visible to the naked eye from a clear, dark site, glowing at an impressive magnitude 3.4. That’s because Andromeda is the closest large galaxy to the Milky Way, roughly 2.5 million light-years away.
Regardless of its naked-eye visibility from your location, you can enjoy M31 with binoculars or any telescope. The former will give you a broader view, while the latter will allow you to zoom in more on specific regions. Note the galaxy’s brighter center — its nucleus — as well as the wispier spiral arms. Look for darker streaks that signify dust lanes, where thick dust blocks our view of its starlight.
Also visible are its two brightest companion galaxies, 8th-magnitude M32 and NGC 205. M32 is just 25’ south of Andromeda’s nucleus, while NGC 205 lies 36’ northwest of the larger galaxy’s center.
Note that NGC 205 often appears cataloged as M110 This was done by amateur astronomers in the 1960s, and Astronomy magazine does not officially recognize this object as M110.
Sunrise: 6:45 A.M.
Sunset: 7:02 P.M.
Moonrise: 4:24 A.M.
Moonset: 6:10 P.M.
Moon Phase: Waning crescent (5%)
*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 20
Shortly after September 20 begins in the Eastern time zone, Saturn’s moon Titan transits the ringed world alongside its shadow — the second-to-last shadow transit we will see for more than a decade.
The event starts around 12:30 A.M. EDT on September 20 — note this is early Saturday for those on Eastern time, but late Friday night for U.S. time zones farther west. At that time, Saturn stands some 40° high in the southeast, located to the lower left of the Circlet in Pisces. The planet is the brightest point of light in that region of the sky, shining at magnitude 0.6. (We’ll be visiting this part of the sky plenty this week, so you’ll become quite familiar with it!)
Zoom in on Saturn with a telescope and you’ll see mid-8th-magnitude Titan approaching the planet’s northeastern limb shortly before 12:30 A.M. EDT. It crosses in front of the disk starting around 12:33 A.M. EDT, taking several minutes to fully cross the limb. Five minutes later, around 12:37 A.M. EDT, the moon’s shadow begins to appear, slightly farther north on the limb.
The two cross the disk of Saturn together, reaching roughly halfway in their journey around 2:30 AM EDT (now the 20th in all time zones but Pacific). About 90 minutes later, the shadow slips off the disk first, disappearing by 4:04 A.M. EDT. Titan’s transit ends half an hour later, at 4:37 A.M. EDT.
Several other moons are visible during the transit as well, including 10th-magnitude Tethys, Rhea, and Dione. Check the chart above for their locations. Enceladus is transiting with its shadow around the same time as Titan, but closer to the rings, just north of them. This moon is much smaller and fainter than Titan, and will likely be lost in the glare.
You’ll get one more chance to view a Titan shadow transit in early October before this series of shadow transits ends until the geometry of the solar system brings it back in 15 years.
Sunrise: 6:46 A.M.
Sunset: 7:00 P.M.
Moonrise: 5:30 A.M.
Moonset: 6:32 P.M.
Moon Phase: Waning crescent (1%)
Sunday, September 21
Saturn reaches opposition at 2 A.M. EDT, visible all night and offering stunning views of its rings and moons. The ringed planet now rises around sunset and is highest around local midnight, when it stands around 45° high in the south. But you can easily observe beginning around an hour or two after sunset and until an hour or two before sunrise.
Saturn shines at magnitude 0.6 in Pisces, just southeast of the Circlet asterism. Through a telescope, the planet’s disk stretches 19” and its rings reach 44” from end to end. They are tilted at just 1.7° to our line of sight and look nearly edge-on, their shadow on the planet’s cloud tops all but invisible.
Saturn’s largest, brightest moon is Titan, which tonight lies about 2’ west of the planet after crossing the disk yesterday morning. Smaller moons, such as 10th-magnitude Tethys, Rhea, and Dione, are also generally visible. Tonight, Rhea is alone some 1’ east of Saturn, while Dione is nearly the same distance to the planet’s west. Tethys is closing in on Saturn’s southwestern limb from the west early in the evening, then disappears behind the planet in an occultation around 2:45 A.M. EDT on the 22nd (still late on the 21st for the West Coast).
Some 2.6° from Saturn is distant Neptune, which we’ll visit in just a few days when the ice giant reaches opposition on the 23rd.
New Moon occurs at 3:54 P.M. EDT, bringing a partial solar eclipse following September 7’s total lunar eclipse. This solar eclipse is visible from a thin region along Australia’s eastern coast, as well as New Zealand, portions of Antarctica, and the south Atlantic Ocean. At maximum eclipse, which occurs over the ocean, the Sun will be some 85.5 percent obscured.
You can get more details about the eclipse’s path and timing at timeanddate.com and eclipsewise.com.
Sunrise: 6:47 A.M.
Sunset: 6:58 P.M.
Moonrise: 6:33 A.M.
Moonset: 6:52 P.M.
Moon Phase: New
Monday, September 22
The autumnal equinox occurs at 2:19 P.M. EDT. On this date, the Sun appears directly above the equator and the Northern and Southern hemispheres receive the same amount of sunlight. The autumnal equinox also officially brings the Northern Hemisphere summer to an end and begins astronomical autumn, while bringing the end of winter in the Southern Hemisphere and beginning astronomical spring.
One of autumn’s showpiece objects is the Double Cluster in Perseus. This two-for-one deal comprises open clusters NGC 869 and NGC 884, which lie about half a degree apart in northwestern Perseus. To find them, look about 4.5° west-northwest of 4th-magnitude Eta (η) Persei. This region is 40° high in the northeast around 10 P.M. local daylight time.
Both clusters glow around 4th magnitude and cover half a degree each. You can view them together at low power, such as through binoculars or a finder scope, or take your time with each individually at higher magnification. NGC 869 is the westernmost cluster, while NGC 884 lies due east of NGC 869. NGC 869 is slightly richer, with more stars visible than its counterpart at the same magnification. But NGC 884 is home to RS Persei, a red supergiant star near the center of the cluster whose brightness varies between 8th and 10th magnitude.
Sunrise: 6:46 A.M.
Sunset: 6:56 P.M.
Moonrise: 7:35 A.M.
Moonset: 7:12 P.M.
Moon Phase: Waxing crescent (1%)
Tuesday, September 23
Now it’s Neptune’s turn to reach opposition; the solar system’s most distant planet is at opposition this morning at 9 A.M. EDT. Like Saturn, it is visible all night, passing highest in the hours around local midnight.
The easiest way to locate Neptune is to find Saturn, the brightest point of light in the southeastern sky around 10 or 11 P.M. local daylight time. At magnitude 7.7, Neptune is not visible to the naked eye but can be located with optical aid just 2.7° northeast of Saturn. The pair is visible together in binoculars, or you can zoom in on the individual worlds one at a time with a telescope.
Neptune’s disk now appears 2” wide, thanks to the planet’s huge distance from Earth of 2.68 billion miles (4.32 billion kilometers). Through a telescope, it should look like a “flat,” blue-gray star. There are no stars of similar magnitude close nearby, so the planet should be relatively easy to pick out, especially given its proximity to Saturn.
The Moon passes 1.1° south of Spica at 6 P.M. EDT. We will visit our satellite in the evening sky tomorrow, after it also passes Mars.
Sunrise: 6:49 A.M.
Sunset: 6:55 P.M.
Moonrise: 8:36 A.M.
Moonset: 7:33 P.M.
Moon Phase: Waxing crescent (3%)
Wednesday, September 24
The Moon passes 4° south of Mars in Virgo at 11 A.M. EDT. The pair is briefly visible — along with Virgo’s brightest star, Spica — for a short time after sunset this evening.
You’ll need to be fast, though, as they are sinking quickly toward the horizon. About half an hour after sunset, 1st-magnitude Spica is just 1.5° high and may require binoculars to spot in the horizon haze. You can use the Moon and Mars — a little higher, some 6° above the western horizon — to guide you.
The Moon is now a 9-percent-lit waxing crescent, the easiest of the three to spot. Magnitude 1.6 Mars is 7° to the Moon’s upper right, roughly one binocular field away (make sure you wait until at least several minutes after sunset from your location to pull out any optics). Spica, a bit brighter than Mars, is 13° to the Moon’s lower right.
Spica sets first, by about 7:30 P.M. local daylight time. The Moon and Mars remain visible about half an hour longer, reaching the horizon by 8 P.M. local daylight time. For the best views, opt for an observing site with a clear western horizon — no tall trees or buildings — and try to get to a hill or elevated region above your local surroundings.
Sunrise: 6:50 A.M.
Sunset: 6:53 P.M.
Moonrise: 9:39 A.M.
Moonset: 7:56 P.M.
Moon Phase: Waxing crescent (8%)
Thursday, September 25
Asteroid 2 Pallas is stationary at 7 A.M. EDT in the constellation Aquila. At 10th magnitude, this space rock is a bit of a challenge, but should be visible through amateur scopes.
Aquila itself is hard to miss — its brightest star, magnitude 0.8 Altair, anchors one point of the huge Summer Triangle asterism and dominates northeastern Aquila. Pallas is not far from this star, about 6.5° (one binocular field) to its southeast.
Note that the main-belt world is just 5’ from a field star of similar brightness. If you spot the two points of light together in your optics, Pallas is the easternmost object of the two. Previously moving westward, or retrograde, against the background stars, Pallas will now make a turn back toward the east, resuming prograde motion. If you come back over the next few nights, you’ll see Pallas pull away from the background star, moving south and slightly east as it makes a slow, shallow turnaround in the sky.
Sunrise: 6:51 A.M.
Sunset: 6:51 P.M.
Moonrise: 10:41 A.M.
Moonset: 8:23 P.M.
Moon Phase: Waxing crescent (14%)
Friday, September 26
The Moon reaches apogee, the farthest point from Earth in its orbit, at 5:46 A.M. EDT. At that time, our satellite will stand 251,996 miles (405,548 km) away.
A seemingly small, unassuming constellation is rising in the east after sunset: Triangulum. One of the smallest constellations (ranking 78th out of 88), this simple star pattern is nonetheless relatively easy to locate. As its name implies, its outline consists of three stars: Alpha (α), Beta (β), and Gamma (γ) Trianguli. Beta is actually the brightest, at magnitude 3.0; Alpha is next (magnitude 3.4), and Gamma is faintest (magnitude 4.0). You can find these three stars to the right of Perseus and below Cassiopeia and Andromeda in the sky as the region rises.
Though small, Triangulum hosts several deep-sky treasures, including the galaxy M33 near its western border with Pisces. By 9:30 P.M. local daylight time, it is just over 30° high in the east, glowing at magnitude 5.7. You can find it by either looking just over 4° west-northwest of Alpha Tri, or by moving 7° southeast from brighter magnitude 2.1 Mirach (Beta Andromedae). M33 covers more than a degree of sky, and is best viewed with a telescope. Small scopes and even binoculars will spot it, while larger instruments will start to bring out some of its spiral structure.
Sunrise: 6:52 A.M.
Sunset: 6:50 P.M.
Moonrise: 11:44 A.M.
Moonset: 8:56 P.M.
Moon Phase: Waxing crescent (21%)
