Have you seen our interstellar visitor?

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

Friday, July 18
Mars remains close to Sigma (σ) Leonis, Leo the Lion’s back foot, in the evening sky for several days. Shining at magnitude 1.5, the Red Planet is just over 1° southwest of the 4th-magnitude star tonight. 

An hour after sunset, Mars is still 15° high in the west. Through a telescope, its disk appears just 5” across and will continue to grow smaller still in the coming months as Earth pulls away on its inside track around the Sun. Mars will be in conjunction with the Sun early next year, disappearing from our view by mid-November after meeting twice with Mercury in the autumn, so it’s going out with a bang! 

For now, Mars will appear to remain about the same height above the horizon night by night, sliding eastward across the sky. Tonight it is southwest of Sigma Leo; tomorrow it will sit less than 1° due south of the star, and on the 20th the planet will be slightly southeast of it. Check Mars’ position tonight and for the next few days to catch its motion as the Red Planet makes its way toward Virgo, entering the Maiden’s domain before the end of the month.

Sunrise: 5:47 A.M.
Sunset: 8:25 P.M.
Moonrise: 12:08 A.M.
Moonset: 2:25 P.M.
Moon Phase: Waning crescent (42%)
*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, July 19
Seventh-magnitude 4 Vesta is an easy catch from any location and with any equipment: binoculars or a telescope. Tonight, it’s just 5’ from a magnitude 7.5 field star, creating a temporary artificial double star to enjoy. 

You’ll find Vesta in the northwestern corner of Libra, near that constellation’s border with Virgo. It now sits about 7.5° north-northwest of 3rd-magnitude Zubenelgenubi, the Balance’s alpha star. (Note that this star is a double visible to many without any optical aid at all, with a 5th-magnitude companion visible 4’ to the primary’s northwest. Even if you can’t split it by eye, it will certainly appear in binoculars or any scope.) 

Once you land on Vesta’s position, you’ll see two points of light. The asteroid is the slightly brighter point to the northwest of the fainter field star (cataloged as HIP 71417 or HD 128275).

Despite the fact that it was the fourth asteroid discovered, Vesta is the second-largest body in the main belt, after dwarf planet 1 Ceres.

Sunrise: 5:48 A.M.
Sunset: 8:25 P.M.
Moonrise: 12:38 A.M.
Moonset: 3:42 P.M.
Moon Phase: Waning crescent (31%)

Sunday, July 20
The morning sky is where the action is at today, as the Moon passes in front of several stars in the Pleiades cluster in an occultation, best seen from North and Central America. Both the view and its timing are heavily location dependent — you can check this International Occultation Timing Association’s page to find when Electra (17 Tauri), the first star to vanish behind the lunar limb, will vanish from your observing site. 

In the Midwest, the Moon rises in the northeast around 1:15 A.M. CDT (note this time is also location dependent) and 4th-magnitude Electra disappears around 3:30 A.M. CDT. It reappears about an hour later — the reappearance will be easier to catch than the disappearance, as it occurs at the darker, shadowed limb of the Moon. This means Electra will pop into view from darkness rather than disappearing behind the bright limb, where poor contrast will make the proceedings a bit more difficult to watch.

The Moon continues through the Pleiades as dawn breaks, so which stars you will see disappear or reappear depends on your location and when the sky grows too light to follow the event. Observers along the U.S. West Coast may be able to catch the reappearance of 3rd-magnitude Alcyone (Eta [η] Tau) in the eastern portion of the cluster. (The IOTA timing page for this star is here.)

Uranus is currently located some 4.5° south-southeast of the Pleiades — you can hop down to observe the 6th-magnitude ice giant with binoculars or a telescope in between tracking the Moon’s progress through the cluster. The Moon will pass 5° due north of Uranus at 9 A.M. EDT.

The Moon also reaches perigee, the closest point to Earth in its orbit, at 9:55 A.M. EDT this morning. At that time, our satellite will sit just 228,690 miles (368,041 km) away.

Sunrise: 5:48 A.M.
Sunset: 8:24 P.M.
Moonrise: 1:15 A.M.
Moonset: 4:59 P.M.
Moon Phase: Waning crescent (21%)

Monday, July 21
Saturn’s tidally locked, two-toned moon Iapetus reaches eastern elongation today, standing a full 9′ east of Saturn. The moon appears faintest at eastern elongation, when its darker hemisphere is turned toward us and it glows a faint 12th magnitude. However, without our Moon lighting the early-morning sky, you might try to spot faint Iapetus — along with several other members of Saturn’s brood. 

Try around 4 A.M. local daylight time, as Saturn is heading toward its highest point in the sky but the background remains dark. At this time, Saturn is some 40° high in the south, near the Circlet of Pisces. The planet is the brightest point of light in the sky there, making it easy to find and center in your scope. Through the eyepiece, you’ll see that Saturn’s disk spans 18.5”, while its rings stretch nearly 43” from end to end. 

About 2.5’ to Saturn’s west, you’ll see its brightest moon, mid-8th-magnitude Titan. Iapetus sits more than three times this distance to Saturn’s east, and glows some four magnitudes fainter. Closer to the planet are 10th-magnitude Rhea, Tethys, and Dione; Rhea is just east of the edge of the rings, while Tethys and Dione are west of them, with the former closer to the planet than the latter.

The waning crescent Moon now passes 7° north of Venus at 3 P.M. EDT in the daytime sky. Although it is sometimes possible to catch Venus in daylight with a telescope, this is an undertaking for advanced observers only, as it requires extreme care when pointing and tracking the telescope to prevent permanently damaging your eyes. Warmer daytime air is also more turbulent, blurring the image even for those who are able to track the planet. Astronomy Associate Editor Michael Bakich recommends that if you’re trying to see Venus in daylight, opt for a yellow, orange, or red filter to reduce the glare of the blue sky. 

For a much easier (and safer) observation, you can catch the pair in the early-morning sky tomorrow, with the 8-percent-illuminated Moon now to the lower left of bright Venus in the east before dawn. 

Sunrise: 5:49 A.M.
Sunset: 8:23 P.M.
Moonrise: 2:02 A.M.
Moonset: 6:11 P.M.
Moon Phase: Waning crescent (12%)

Tuesday, July 22
Can you find Lacerta the Lizard? This small, dim constellation is visible in the northeast after sunset, sitting below the flying form of Cygnus and to the lower right of house-shaped Cepheus. 

Lacerta’s alpha star shines at magnitude 3.8; you can find it about 19° east of bright Deneb in Cygnus. This luminary may seem unimpressive, but it’s a great jumping-off point to reach open cluster NGC 7243, also cataloged as Caldwell 16. The cluster lies just 2.6° west of Alpha Lac. 

Visible to the naked eye as a dim, fuzzy patch of light, NGC 7243 shines at magnitude 6.4 and has a diameter of 15’ on the sky. It’s a great binocular target for beginners. The cluster’s brightest star, shining at 8th magnitude, lies in the center, presenting a picturesque view once you’ve added some magnification. A small telescope will show nearly 40 stars, while larger scopes will continue adding suns to that count. 

NGC 7243 is a relatively young cluster, as astronomers estimate it is about 100 million years old. 

The Moon passes 5° north of Jupiter at midnight EDT — we’ll catch the pair tomorrow in the predawn sky. 

Sunrise: 5:50 A.M.
Sunset: 8:23 P.M.
Moonrise: 2:59 A.M.
Moonset: 7:13 P.M.
Moon Phase: Waning crescent (6%)

Wednesday, July 23
Now let’s view the Moon and Jupiter in the morning sky as they rise together in Gemini the Twins. The pair is about 3° high in the northeast an hour and a half before sunrise, forming an elongated triangle with 3rd-magnitude Epsilon (ε) Geminorum between and slightly above them. 

The Moon is a delicate crescent that is just 3 percent illuminated, with Jupiter standing to its right and shining at magnitude –1.9. Farther to the Moon’s left is magnitude 1.6 Castor, Gemini’s alpha star. 

Zoom in on Jupiter with a telescope to see some — or all, depending on your location — of its Galilean moons. East Coast observers may be able to catch Europa alone to the planet’s west, while Io (closest), Ganymede, and Callisto (farthest) lie to Jupiter’s east. But around 5:15 A.M. EDT, just as Jupiter is peeking over the horizon in the Midwest, Europa vanishes while still 10” from the northwestern limb as it passes into the planet’s dark shadow and out of view. The moon will remain hidden for the rest of the morning, as it doesn’t reappear until just after sunrise in the Pacific time zone. 

Sunrise: 5:51 A.M.
Sunset: 8:22 P.M.
Moonrise: 4:07 A.M.
Moonset: 8:04 P.M.
Moon Phase: Waning crescent (2%)

Thursday, July 24
New Moon occurs at 3:11 P.M. EDT, leaving the sky dark and perfect for observing faint objects such as 3I/ATLAS, our newest interstellar interloper. Currently around 17th magnitude and gliding through Ophiuchus, you’ll want a scope that’s on the larger side (at least 6 inches, preferably more) to nab your own photograph of this ancient comet, possibly the oldest we’ve ever seen. 

3I will rise around 5 P.M. local daylight time and is highest around 10 P.M., when it appears 30° high in the south. Tonight it is conveniently close to 2nd-magnitude Sabik (Eta Ophiuchi). Once you center Sabik in your scope, you’ll find 3I about one field of view away, just 2.3° south of the star. It’s moving through a rich region near the plane of the Milky Way — which you’ll certainly notice given the huge swath of background stars, making it harder to spot the comet in a single exposure. However, take several images to see which faint point of light is moving, and that’s your sign you’ve nabbed a visitor from another star system in your shot. 

3I will stand near this star for several more nights, so you can continue to use it as a guide for finding it for at least a few more days. The comet is moving west with respect to the background stars, so it will appear progressively farther southwest of Sabik on subsequent nights. 

Sunrise: 5:52 A.M.
Sunset: 8:21P.M.
Moonrise: 5:20 A.M.
Moonset: 8:44 P.M.
Moon Phase: New

Friday, July 25
The tiny, distant dwarf planet Pluto reaches opposition at 3 A.M. EDT. We’ll focus on observing this faraway world next week, so stay tuned!

Today, let’s first focus on a famous nebula along the plane of the Milky Way: the Eagle Nebula (M16). This gorgeous star-forming region consists of an open cluster (NGC 6611) nestled within an emission nebula (IC 4703), located within Serpens Cauda. It stands highest around 11 P.M. local daylight time tonight, when it is some 35° above the southern horizon. 

Situated above the Sagittarius’ Teapot’s spout, M16 lies about 10° east-northeast of magnitude 3.5 Xi (ξ) Serpentis. For a closer signpost, look for it instead 2.6° west-northwest of magnitude 4.7 Gamma (γ) Scuti.

The nebula is broad, spreading over some 7’ of sky, and shines at magnitude 5.7. A small telescope will show numerous stars in the cluster, embedded within the larger gaseous clouds of the emission nebula, which spreads toward the south. At the heart of the nebula’s glow is a set of dark fingers of cold gas, inside which new stars are being born. The formation at the tip of this region is known as the Pillars of Creation, made famous through the Hubble Space Telescope’s close-up view of them. If you want to see them for yourself, opt for at least a 5-inch scope (preferably larger!) and a very dark observing site, plus an OIII or UHC filter if you’ve got one. 

The Pillars are a favorite astrophotography target as well, and a camera can often capture what the eye cannot. Give it a try! 

Sunrise: 5:53 A.M.
Sunset: 8:20 P.M.
Moonrise: 6:34 A.M.
Moonset: 9:16 P.M.
Moon Phase: Waxing crescent (2%)