From the June 2018 issue

The Red Planet’s revival

By and | Published: June 27, 2018 | Last updated on May 18, 2023
Mars puts on its best show in 15 years this month. The Red Planet spans 24.3″ in late July, when telescopes should show an impressive array of surface features.
NASA/ESA/The Hubble Heritage Team (STScI/AURA)/J. Bell (ASU)/M. Wolff (SSI)
July features the finest views of Mars in 15 years. But that’s just one highlight in a month full of them. The five planets known since antiquity span the evening sky: Mercury and Venus appear in twilight, Jupiter and Saturn remain visible until after midnight, and Mars is a showpiece all night.

As if to underscore the Red Planet’s peak, a total eclipse of the Moon visible from most of the Eastern Hemisphere occurs near Mars on the night of July 27/28. Binoculars will show Uranus and Neptune as they climb high before dawn, and a telescope reveals Pluto sliding past a bright star. It’s a great month to watch the night sky, so let’s get started.

Evening twilight hosts both Mercury and Venus. Mercury crosses from Cancer into Leo this month, passing within 1° of the Beehive star cluster (M44) July 3 and 4. The innermost planet glows at magnitude 0.0 and should be easy to spot with your naked eye, though you’ll need binoculars and a transparent sky to see M44. The two stand 7° high 45 minutes after sunset.

Mercury dims and dips closer to the horizon as July progresses. It crosses into Leo on the 14th, when a slender crescent Moon passes 2° above it. Use binoculars to best view the pair. A telescope reveals Mercury’s 8″-diameter disk, which appears 38 percent lit. Evening twilight swallows the planet within another week.

As you search for Mercury, you can’t help but see Venus to its upper left. The second planet from the Sun shines at magnitude –4.1 in early July and brightens to magnitude –4.3 by month’s end. Venus spends July among the background stars of Leo, passing 1° north of 1st-magnitude Regulus on the 9th.

A waxing crescent Moon slides 2° to the planet’s right July 15. The stunning duo stands 15° high in the west 45 minutes after sundown. The conjunction provides a nice photo opportunity. Shoot the twilight scene before 9:30 p.m. local daylight time and you’ll also capture Regulus to the lower right and Mercury closer to the horizon. Point a telescope at Venus this evening and you’ll see an 18″-diameter disk that appears nearly two-thirds illuminated.

The Red Planet shines at magnitude –2.8 when it reaches opposition in late July among the dim stars of Capricornus.
All illustrations: Astronomy: Roen Kelly
As Venus and the Moon slip close to the horizon, shift your gaze to their left and you’ll encounter Jupiter, Saturn, and then Mars. Jupiter appears about 30° high in the southwest while Saturn stands 20° high in the south-southeast. And Mars pokes above the southeastern horizon around 9:30 p.m. local daylight time.

The solar system’s two gas giants are on view every clear evening. Jupiter lies in Libra, 2° northwest of Zubenelgenubi (Alpha [α] Librae). Saturn resides in northern Sagittarius, not far from several deep-sky gems. Jupiter shines at magnitude –2.2 in mid-July, nearly 10 times brighter than magnitude 0.1 Saturn. Both remain within 0.1 magnitude of these values all month.

Catch Jupiter first while it is still high in the sky. The giant planet’s equator spans 41″ in early July and 38″ late in the month. Despite its dwindling size, Jupiter remains a fine object through telescopes of all sizes. Any instrument shows its two dark equatorial belts, one on either side of a noticeably brighter zone.

Jupiter’s dynamic atmosphere can provide hours of enjoyment, but it’s also worth viewing the planet’s four bright moons. Io, Europa, Ganymede, and Callisto orbit Jupiter with periods ranging from 1.8 to 16.7 days, and their relative positions change noticeably hour by hour.

Scan east from Jupiter and you’ll find Saturn set against the rich Milky Way of Sagittarius. In early July, the ringed planet lies 4.7° due east of the Trifid Nebula (M20), with the even brighter Lagoon Nebula (M8) 1° south of the Trifid. Open cluster M25 resides 4.0° northeast of Saturn and globular cluster M22 sits 3.5° southeast of the ringed world. Saturn drifts slowly westward during July, closing in on the Trifid and Lagoon. By the 31st, the planet stands 2.7° east of M20.

Despite this wonderful backdrop, the best views of Saturn come through a telescope. The planet reached its peak at opposition in late June, but the view changes little in July. The gas giant sports a disk measuring 18″ across surrounded by a spectacular ring system that spans 41″. Structure in the rings shows up well because they tilt 26° to our line of sight.

Iapetus’ brightness varies by a factor of five as it orbits the ringed planet. A good time to catch it is when it passes due north of the gas giant July 1.
Saturn also hosts a large family of moons. Any telescope reveals 8th-magnitude Titan, the planet’s brightest satellite. It circles the giant world once every 16 days.

A 4-inch scope brings in four more moons. Tethys, Dione, and Rhea all glow at 10th magnitude and orbit closer to Saturn than Titan. Iapetus is trickier to locate. It orbits well beyond Titan and shines five times brighter (magnitude 10.2) when farthest west of the planet than when it’s farthest east (magnitude 11.9). Your best chance to spot Iapetus comes July 1, when it glows at 11th magnitude some 1.7′ due north of Saturn.

You can find Pluto 15° east of Saturn. Everyone’s favorite dwarf planet reaches opposition July 12, but its appearance doesn’t change during the month. It glows at 14th magnitude, so you’ll need an 8-inch or larger telescope to see it. Pluto passes a mere 35″ west of the 6th-magnitude star 50 Sagittarii on the evening of July 3. The finder chart (opposite page, top) will help you find Pluto on other nights.

On the night of July 26/27, Mars reaches its most favorable opposition since 2003. And its elliptical orbit brings it closest to Earth (35.8 million miles away) four nights later. This is the month to view the Red Planet — it won’t be as good again until 2035.

Mars spends July among the background stars of Capricornus, but it shines more than 100 times brighter than any of the Sea Goat’s stars. In early July, the Red Planet rises around 10:30 p.m. local daylight time and climbs highest around 3 a.m. It shines brilliantly at magnitude –2.2. But Mars doesn’t stand pat. It brightens to magnitude –2.8 in late July, far brighter than any other nighttime object except for the Moon and Venus (which sets around 10 p.m.). The ruddy world then rises around sunset and peaks in the south shortly before 1 a.m.

Spying this 14th-magnitude dwarf planet is a bit easier than normal when it skims just north of 6th-magnitude 50 Sagittarii during July’s first week.
When viewing detail on Mars’ surface through a telescope, it all comes down to angular size. It’s a small planet to begin with, so the weeks surrounding opposition are special. Mars spans 20.9″ on July 1 and grows to 24.3″ by month’s end.

A day on Mars lasts 37 minutes longer than on Earth. This means if you observe at the same time each night, markings appear to shift westward 9.1° per day, so you can view the entire planet during the course of the month. Features appear best when they lie near the central meridian — the line joining Mars’ north and south poles that passes through the disk’s center — though they remain visible for a couple of hours on either side.

Both Syrtis Major, a prominent dark feature with a triangular shape, and the bright Hellas Basin cross the central meridian around 2 a.m. EDT on July 12 and 13. Don’t confuse Hellas with the brighter south polar cap on the planet’s limb. The bright Elysium plain takes center stage at 2 a.m. EDT on July 21. At the same time on the night of opposition, Mare Sirenum appears on the central meridian.

Less than 24 hours after Mars reaches opposition, the Full Moon plunges through Earth’s shadow. Observers across most of Europe, Asia, Africa, and Australia will see a total lunar eclipse, with the eclipsed Moon hanging 7° north of Mars. Totality runs from 19h30m to 21h13m UT on July 27 (before dawn on the 28th for people in eastern Asia and Australia). The 103 minutes of totality makes this the longest total lunar eclipse since 2000.

After the thrilling sight of Mars, July’s final two planets might be a letdown. But Neptune and Uranus deserve a few moments of your time. Neptune lurks in eastern Aquarius, rising around midnight on the 1st and two hours earlier by month’s end. The distant world glows at magnitude 7.8, so you’ll need binoculars or a telescope to spot it. Wait until it climbs high in the south before dawn and then zero in on 4th-magnitude Phi (ϕ) Aquarii. Neptune lies west-southwest of Phi all month; it is 0.9° from the star on the 1st and 1.4° away on the 31st. A telescope reveals the planet’s blue-gray disk, which measures 2.3″ across.

Plan to target Uranus shortly before twilight begins. The magnitude 5.8 planet then lies reasonably high in the east and should be easy to find through binoculars. It lies in southwestern Aries some 12° south of 2nd-magnitude Alpha Arietis. A few 6th-magnitude stars lie in the planet’s vicinity, so you’ll need a telescope to confirm a Uranus sighting. Only the planet shows a distinct blue-green color and a 3.5″-diameter disk.

These elongated craters stand out near Luna’s southwestern limb on the July 23 waxing gibbous Moon.
Consolidated Lunar Atlas/UA/LPL; inset: NASA/GSFC/ASU

A pair of oval impact scars

The striking form of Schiller stands out in the deep southwest of a waxing gibbous Moon. This shoeprint-shaped impact crater lights up July 23, when its mostly flat floor and exceptionally elliptical shape catch the eye. Of course, even circular craters located close to the Moon’s limb appear oval, an effect called foreshortening. But Schiller seems squashed twice as much as any other impact feature in the area. The shadows appear quite long the evening of the 23rd, which helps exaggerate the terrain’s height. 

Early lunar observers were left scratching their heads trying to explain Schiller’s shape. A few decades ago, however, scientists studying high-velocity impacts in laboratories on Earth showed that single projectiles striking a surface at a grazing angle can produce some unusual craters, including ones that look like Schiller. (Messier A in the Moon’s southeast is the poster child for such low-angle impacts.) Long after the impact gouged out Schiller, lava welled up through crustal fractures and created its smooth floor.

Scan to the crater’s northeast and you can’t miss the elliptical crater Hainzel. A close look reveals this feature to be the result of three separate impacts. The roughly circular crater on the northwestern side shows a classic central peak. The southeastern structure formed later — the timing becomes clear when you notice that the floor of the overlapping area matches its texture and albedo. The southern component was first on the scene. Not only do the other two craters overlay it, but its rim is also softer, a result of wear from long-term bombardment.

Fighting off a Full Moon fever

Although the Southern Delta Aquariid meteor shower doesn’t reach the heights of its more famous cousins, it stands out for its longevity. The shower lasts from mid-July to mid-August, and that’s a good thing this year because it peaks July 30, just three days after Full Moon. Your best bet is to watch under the dark skies before dawn prior to July 25.

Or, if you live in the Eastern Hemisphere, keep an eye out for meteors during the total lunar eclipse the night of July 27/28. The eclipse brings nearly two hours of darkness and a grand stage for viewing totality, Mars, and a handful of “shooting stars.”

This periodic comet returns to the inner solar system every 6.6 years. You can find it in mid-July as it treks from Cygnus into Cepheus.
The return of a periodic visitor

It’s been more than a year since a short-period comet put on a decent show. But this autumn promises no fewer than four that should crest brighter than 10th magnitude, and one of these — 46P/Wirtanen — could reach naked-eye visibility.

The first up is 21P/Giacobini-Zinner. French astronomer Michel Giacobini first spotted this comet in 1900, and German astronomer Ernst Zinner recovered it two orbits later, in 1913. In September 1985, it became the first comet to be visited by a spacecraft when the International Cometary Explorer flew past.

Giacobini-Zinner could reach 6th magnitude at its September peak. This month, it should glow around 10th magnitude. Plan to look for it under Moon-free skies during July’s second and third weeks. It then appears against the photogenic northern Milky Way, passing from northern Cygnus into southern Cepheus. Although this region remains visible all night, it climbs highest before dawn. The comet likely won’t be visible at low power through a 4-inch scope, however. Use an eyepiece that provides a magnification of around 100x, dark adapt, and try averted vision to pick up the faint cotton ball.

The sky’s brightest asteroid should be an easy target as it wanders southwest against the backdrop of Ophiuchus the Serpent-bearer.
Spotting the easiest asteroid of all

The second-largest object in the asteroid belt, 4 Vesta, is the brightest. Not only does Vesta orbit closer to the Sun (and thus Earth) than dwarf planet Ceres, but its surface reflects more sunlight. Vesta should be easy for beginners to track through binoculars from the suburbs, and a challenge for seasoned observers to follow with their naked eyes from a dark site.

Vesta fades from magnitude 5.6 to magnitude 6.3 this month as it crosses one of the legs of Ophiuchus the Serpent-bearer. With binoculars, start at magnitude 2.4 Eta (η) Ophiuchi and then drop south one field to magnitude 4.4 Xi (ξ) Oph. Our target lies a little to the left (east) of Xi. Before you head out, make a sketch of the brighter anchor stars using the chart below. Once you’re under the stars, add the ones near Vesta’s position. Return a couple of nights later to see which one has moved.

Experienced skywatchers are familiar with the Prancing Horse dark nebula just west of our galaxy’s nucleus. During the New Moon weekend of July 14, Vesta crosses the dusky steed’s front shoulder. The contrast between asteroid and cloud should help veteran observers spot Vesta without optical aid.