The Sky This Week from Aug. 20 to Sept. 5: Saturn’s time to shine

Sky This Week is brought to you in part by Celestron.

Friday, August 29
The waxing Moon lies in southern Libra tonight. Our satellite is quickly approaching Scorpius, where it will meet the Scorpion’s brightest star in just a few days. 

High in the southeast in the few hours after sunset is the small constellation Sagitta the Arrow. It lies above the larger, more easily recognizable constellation Aquila the Eagle, which is marked by the bright star Altair (part of the Summer Triangle). 

Tonight, we’re looking for the globular cluster M71, an 8th-magnitude ball of ancient stars that sits roughly halfway between Delta (δ) and Gamma (γ) Sagittae. Around 9:30 P.M. local daylight time, magnitude 3.8 Delta lies some 9.7° above magnitude 0.8 Altair. Magnitude 3.5 Gamma Sge lies just to Delta’s upper left, about 3° away. M71 sits midway between them, covering about 7’ on the sky. The cluster’s bright central core is easy to spot with binoculars or a small scope, while higher magnification (or larger scopes) will reveal an increasing number of stars. M71 is a relatively loosely packed cluster, so note how its appearance differs from denser objects, such as M13 in Hercules. 

M71 lies about 13,000 light-years away, and its bright core covers a true diameter in space of about 24 light-years.

Sunrise: 6:25 A.M.
Sunset: 7:36 P.M.
Moonrise: 12:53 P.M.
Moonset: 10:23 P.M.
Moon Phase: Waxing crescent (36%)
*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 30
Saturn’s moon Iapetus reaches greatest western elongation today, now located 9.5’ west of the ringed planet. Iapetus has two vastly different hemispheres, one light and one dark. As the moon orbits Saturn, it slowly rotates so that one and then the other side faces us, with its brightness varying vastly depending on which region is pointed at Earth. At western elongation the icy, brighter side of Iapetus is turned toward us, bringing the moon’s brightness up to 10th magnitude, in line with some of Saturn’s other moons (Rhea, Tethys, and Dione). Titan, at mid-8th-magnitude, is still brightest — we’ll make sure to locate it tonight, too. 

Saturn is rising in the east about an hour after sunset. Give it a little time to climb out of the horizon haze and then point your scope at it late this evening or overnight into the early-morning hours. Around 11 P.M. local daylight time, it’s reached 25° in altitude in the east, hanging below the Circlet of Pisces and shining as the brightest point of light in that region of the sky. 

Through a telescope, you’ll spot Titan some 3’ east of Saturn; again, it’s the brightest of the planet’s moons. Then look far — a full 9.5’! — west of Saturn to spot 10th-magnitude Iapetus. Much closer in, around midnight EST you’ll also see Tethys just off the eastern end of the rings, with Rhea just north of the rings also to the planet’s east. Dione lies west of the western edge of the rings, with much fainter (12th magnitude) Enceladus between it and the edge of the rings. You may not be able to spot Enceladus, so don’t be surprised if you can’t make it out. 

There’s more going on around the ringed planet this week — and all next month, as it approaches opposition September 21 — so stay tuned. We’ll be back in this region of the sky soon! 

Sunrise: 6:26 A.M.
Sunset: 7:34 P.M.
Moonrise: 1:56 P.M.
Moonset: 10:58 P.M.
Moon Phase: Waxing crescent (45%)

Sunday, August 31
Bright Venus hangs near the lovely Beehive Cluster (M44) in Cancer this morning. Both rise more than two hours before the Sun and reach nearly 20° high in the east an hour before sunrise. Venus shines at magnitude –3.9, a blazing beacon just 1.4° southwest (to the upper right) of the Beehive. 

At magnitude 3.1, M44 is visible to the naked eye, although viewing it this way while it’s low in the sky and twilight is approaching will be difficult. Instead, try for the bright, scattered stars of the Beehive with binoculars or your telescope’s finder scope. A small scope with a wide field of view will also reveal several of its suns, and even catch Venus within the view.

Through a telescope, Venus shows off a 12”-wide gibbous disk that is 84 percent lit. What you’re seeing is sunlight reflecting off the venusian cloud tops, as the planet is swathed in a thick blanket of carbon dioxide that both hides the surface from view and creates a powerful greenhouse effect that heats the planet to some 870 degrees Fahrenheit (465 degrees Celsius).

First Quarter Moon occurs at 2:25 A.M. EDT this morning. The Moon then passes 0.7° south of Antares at 7 A.M. EDT, although neither is visible at that time. You can catch the pairing in the southern sky this evening, with the waxing Moon now sitting about 7.5° to the left (southeast) of ruddy Antares in the darkening twilight. They remain above the horizon for a few hours, setting around 11:30 P.M. local time. 

Sunrise: 6:27 A.M.
Sunset: 7:33 P.M.
Moonrise: 2:57 P.M.
Moonset: 11:41 P.M.
Moon Phase: Waxing gibbous (54%)

Monday, September 1
Cepheid variables are some of the most well-known variable stars in the sky, responsible for helping astronomers accurately measure cosmic distances and famously clueing Edwin Hubble in to the fact that the Andromeda Galaxy was far beyond the Milky Way. So, tonight let’s begin September by finding the Cepheid variable that started it all: Delta Cephei, the archetype Cepheid variable. 

Related: The star that changed the cosmos

If you know how stellar names work, you’ll already know where to find this star — it’s in the constellation Cepheus. The King lies high in the north after dark at this time of year, his house-shaped outline appearing upside-down early in the evening, then slowly rotating onto its side as the hours progress. Cepheus is a circumpolar constellation, meaning it appears to circle the North Celestial Pole (and the North Star, Polaris) rather than rise in the east and set in the west. 

Two hours after sunset, Cepheus is 50° high in the north, standing on the top of its peaked roof, marked by magnitude 3.2 Gamma Cep. The two stars marking the eaves of the house (at the top of its square shape when rightside-up) are above Gamma at this time — they are magnitude 3.2 Beta (β) and magnitude 3.5 Iota (ι) Cep. Above Beta is magnitude 2.5 Alpha (α) Cep, and above Iota is magnitude 3.4 Zeta (ζ) Cep. It is near Zeta that we’ll finally find our target, Delta Cep, which lies about 2.5° to Zeta’s northeast, or that star’s lower right early this evening. 

Delta Cep varies in brightness between magnitudes 3.6 to 4.3 over a period of 5.366 days. So, you can use Zeta as a comparison once you find the star — is Delta close in brightness to Zeta, or is it notably fainter? Once you’ve noted the relative magnitude, make sure to come back every day for the next several nights and watch whether it brightens or fades. If you’re experienced in taking astrophotos, try taking a photo each night to chart the star’s changes; its cycle may be easier to follow on a series of photos than with your memory and your eyes. 

Sunrise: 6:28 A.M.
Sunset: 7:31 P.M.
Moonrise: 3:55 P.M.
Moonset: —
Moon Phase: Waxing gibbous (64%)

Tuesday, September 2
Mercury passes 1.2° north of Regulus at 6 A.M. EDT; the pair is visible in the east just before sunrise, but it might be a challenging observation. You’ll want to get to a location above the surrounding terrain if possible, with an eastern horizon free of tall trees or buildings. 

Rising around 5:30 A.M. local daylight time, Mercury reaches an altitude of 4° roughly half an hour before sunrise. It’s now magnitude –1.3, which should help it stand out against the brightening sky, although binoculars can also help you locate it to the far lower left of blazing Venus, which stands some 20° high at this time. Magnitude 1.4 Regulus will be more challenging and may only be visible with binoculars or a telescope, just over a degree to the lower right of Mercury in the sky as they are rising. The star should be just within the same field of view as the planet with a telescope, and definitely within a single binocular field. 

If you’re using a telescope, take a moment to focus on Mercury as well. The tiny planet appears 5” wide — nearly 2.5 times smaller than Venus’ disk, currently — and is 90 percent lit, slightly more than Venus’ 85 percent. The latter, brighter planet hangs  2° directly below M44 this morning, so if you’d like to catch a binocular view of the two, you can look for them a little earlier, while the sky is still dark — try some 60 to 90 minutes before sunrise. 

Note that you should always put away any binoculars or telescope at least a few minutes before sunrise occurs from your location, which may differ slightly from the times listed below. 

Sunrise: 6:29 A.M.
Sunset: 7:30 P.M.
Moonrise: 4:44 P.M.
Moonset: 12:33 A.M. 
Moon Phase: Waxing gibbous (73%)

Wednesday, September 3
With a waxing Moon lighting the sky for most of the night, we’re focusing on brighter stars this evening. Our target for tonight is a famous asterism that is currently visible all night long. You can catch it already above the horizon in the east an hour after the Sun disappears, then follow its progress as it arcs through the sky overnight. 

Look east an hour after sunset and you’ll spot a large diamond of stars — this is the Great Square of Pegasus, which depicts the body of the Winged Horse. It is bounded by four stars; lowest early in the evening is magnitude 2.8 Gamma Pegasi. Moving counterclockwise, some 16.5° to its upper right (west) is magnitude 2.5 Markab (Alpha Peg). The next star is Scheat (Beta Peg), which shines at magnitude 2.5 and sits roughly 13° north (to the upper left) of Markab. As the Great Square is Rising, Scheat appears at the top of the diamond, directly above Gamma. 

Finishing off the Great Square is a star that isn’t actually in Pegasus — it’s Alpheratz, Andromeda’s magnitude 2.1 alpha star, which lies just over 14° east (to the lower left) of Scheat. 

Although the sky is bright with moonlight tonight, taking the time to learn the Great Square will help guide your way to many fainter objects when the nights grow darker again. 

Sunrise: 6:30 A.M.
Sunset: 7:28 P.M.
Moonrise: 5:27 P.M.
Moonset: 1:34 A.M. 
Moon Phase: Waxing gibbous (82%)

Thursday, September 4
We’re back at Saturn early this morning (or late on the 3rd in the western half of the U.S.) to catch a transit of Titan together with its shadow across the ringed world. 

The shadow transits first, beginning at 1 A.M. EDT (again, early on the 4th in the eastern half of the U.S. but late on the 3rd in the Mountain and Pacific time zones). At this time, Saturn is high in the southeast in the eastern U.S., while appreciably lower and in the east on the West Coast. 

Watch through a telescope as Titan’s shadow slowly appears at the northeastern limb of the planet, taking some 25 minutes to fully emerge. Titan is still far from the limb as it casts its shadow at this time, as Saturn is still more than two weeks from opposition. The moon’s shadow takes about two hours to reach midway across the disk (around 3 A.M. EDT), as Titan slowly closes in. Titan finally begins its transit, which will carry it only partially across Saturn’s northern polar region, around 5 A.M. EDT — not long after the shadow has begun its half-hour-long disappearing act over at the northwestern limb. Titan’s shadow is fully gone from the disk by 5:15 A.M. EDT, and Titan’s partial transit ends 15 minutes later. 

Tenth-magnitude Tethys is also transiting this morning, crossing Saturn between about 4:30 A.M. EDT and 6:25 A.M. CDT (note the time change, as sunrise occurs on the East Coast during the transit). Planetary imagers might be able to pick up the small moon, while it may be invisible to visual observers. Its shadow will likely be indistinguishable as well. 

The Moon passes 0.06° south of Pluto at 10 A.M. EDT.

Sunrise: 6:31 A.M.
Sunset: 7:26 P.M.
Moonrise: 6:03 P.M.
Moonset: 2:40 A.M. 
Moon Phase: Waxing gibbous (89%)

Friday, September 5
Rising before midnight and visible all morning long until dawn, the distant planet Uranus now resides within the domain of Taurus the Bull. Its current location makes it easier than usual to find, just 4.5° south-southeast of the Pleiades (M45). By 4 A.M. local daylight time, the region is some 60° high in the east, above the easy-to-find three-star belt of Orion. 

At that time, you’ll find Uranus to the lower right of the Pleiades. With binoculars, scan 4.5° due south of Alcyone (Eta [η] Tauri) to land on a pair of 6th-magnitude stars in an east-west line. The easternmost star is 14 Tauri, and Uranus is just under 3° east of this star. 

The ice giant is currently 1.8 billion miles (2.9 billion kilometers) from Earth and spans 4” on the sky — small but decidedly nonstellar through a telescope. You may notice it appears “flatter” than the pinprick stars nearby, and also shows off a grayish hue.

Uranus is creeping westward through the sky relative to the background stars, ever-so-slowly closing in on 14 Tau (and its eastern partner, 13 Tau), passing south of this pair in December.

Sunrise: 6:32 A.M.
Sunset: 7:25 P.M.
Moonrise: 6:34 P.M.
Moonset: 3:52 A.M. 
Moon Phase: Waxing gibbous (95%)