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Sky this month: Venus meets Mercury

A look at the skies in May 2020.

A brilliant Venus shines to the right of an overexposed crescent Moon during twilight at Lake Wendouree in southern Australia. A barely perceptible Mercury hovers below and slightly to the right of Venus.
Whether you prefer evening or morning observing — or both — May has you covered. Venus and Mercury appear in the night sky this month, offering fine views during evening twilight. Meanwhile, the morning sky holds the magnificent trio of Jupiter, Saturn, and Mars. All three planets are improving as they approach their respective oppositions later this year, which results in larger disks when viewed with a telescope. But first, let’s begin with the inner planets in the evening sky.

Venus is the brilliant beacon hanging in the western sky soon after sunset. Shining at magnitude –4.7, it dazzles near the northern horn of Taurus the Bull. During the first week of May, the Hyades and Pleiades sit low near the horizon before disappearing in twilight. Look west an hour after sunset to catch these open clusters before low altitude fades them.

Venus sits 37° east of the Sun on May 1, and stands 23° above the western horizon an hour after sunset. Its beautiful, 24-percent-lit crescent disk spans 39" when viewed through a telescope. The world maintains a reasonable altitude through the middle of May as its separation from the Sun diminishes. And on May 16, it stands 11° high an hour after sunset. By then, the disk of Venus has grown to 50" wide, but it’s even more slender, just a 10-percent-lit crescent. As the month presses on, Venus rapidly descends deeper into twilight and its altitude falls as it nears June’s inferior conjunction.
However, there’s one more event that observers may want to catch: a conjunction with Mercury, which is moving on the far side of the Sun in the opposite direction of Venus. Mercury passes through superior conjunction May 4 and reaches conjunction with Venus on May 21/22. Their closest approach occurs during the early morning of May 22, so the best time for U.S. observers to target the pair is on the previous evening, May 21, when they stand slightly more than 1° apart one hour after sunset. At this point, Mercury shines at magnitude –0.6 and Venus at magnitude –4.4.

Telescopic views of Mercury reveal a tiny 6"-wide disk that’s 69 percent lit. This vividly contrasts with the Venusian crescent, which spans 53" as a 6-percent-lit crescent. Within days, Venus descends out of view. But at the same time, Mercury climbs higher along the ecliptic, making it relatively easy to spot the rest of the month. Look to the west-northwest in a clear sky May 31 to find the innermost planet hovering 8° high an hour after the Sun goes down, shining at magnitude 0.1.
The innermost planets share the evening sky
The innermost planets stand only about 1° apart above the west-northwestern horizon shortly after sunset May 21.
All illustrations: Astronomy: Roen Kelly
Jupiter rises close to 2 a.m. across mid-northern latitudes May 1, and by May 31, it is up by midnight. Look for the gas giant rising above the southeastern horizon as Altair in the Summer Triangle reaches an altitude of 20°. Saturn rises 20 minutes later, standing less than 5° from Jupiter.

Jupiter begins the month at magnitude –2.3 and brightens to –2.6 by May 31. Saturn glows at magnitude 0.6 in early May before brightening by 0.2 magnitude by May 31. Both planets slow their eastern track against the background stars as the month goes on, with Saturn reaching its stationary point May 11 and Jupiter May 14.

The beautiful pair remain less than 5° apart all month, straddling the border of Sagittarius and Capricornus. Check out the field of view with binoculars — can you spot the dim (magnitude 9.5) globular cluster M75 forming an isosceles triangle with the planetary duo? It lies less than 2° south of a line between the two planets. A waning gibbous Moon stands 3° south of Jupiter on the morning of May 12.

Thanks to Jupiter’s southerly declination, it remains at a relatively low altitude for the rest of the year. It reaches its highest elevation of about 30° above the southern horizon (depending on your latitude) during morning twilight, when it’s located in eastern Sagittarius. Jupiter is just two months away from opposition, and a telescope will reveal its growing apparent size. During May, it expands from 41" to 45", just 5 percent shy of its opposition peak. 

The dramatic display of atmospheric belts and zones are Jupiter’s main attraction. Most obviously, a pair of dark equatorial belts straddle Jupiter’s equator. Other features are more delicate, however, and the dazzle of the planet can drown them out for casual observers. Let your eye become accustomed to its brilliance for a minute before seeking out finer details. The view of Jupiter is exquisite when our turbulent earthly atmosphere calms for a second or two and enables a beautiful spacelike view of the solar system’s giant planet.
Morning planets
A trio of planets — Mars, Jupiter, and Saturn — will grace the southeastern sky a few hours before sunrise this month.
In addition to the appeal of the cloud belts, Jupiter’s four Galilean moons offer an ever-varying display, including occultations, eclipses, and transits. Individual events occur throughout May, and occasionally dual events occur within minutes of each other. One such back-to-back display occurs the morning of May 3, when Callisto casts its large shadow onto Jupiter’s northern temperate cloud tops as Ganymede begins to disappear behind the same limb, south of the southern equatorial belt. As Jupiter rises, Callisto’s shadow is already on the jovian disk while Callisto itself lies to the east of the planet. Ganymede lies west of Jupiter, and during a couple of hours, Callisto’s shadow nears the western limb of Jupiter while Ganymede approaches. Both events occur almost simultaneously at 4:49 a.m. CDT (in the eastern time zone, it’s already twilight, though the event is still observable). Ganymede’s disappearance takes just seven minutes, while Callisto’s shadow ambles off the disk over the course of 20 minutes. This is because Callisto is in a wider, slower-moving orbit — illustrating the laws of planetary motion.

On May 6 and May 22, Io and its shadow traverse the jovian disk. And a notable trifecta of events occurs the morning of May 21, when Ganymede itself and Europa’s shadow are both visible soon after 2 a.m. CDT before being joined by a transiting Europa at 3:23 a.m. CDT.

Before moving on to Saturn, it’s worth pointing out that Pluto sits just 2.1° west of Jupiter throughout May. But at magnitude 14.7, you’ll need a large scope and ideal viewing conditions to spot it without photographic equipment.

Jupiter’s moons
On May 3, Callisto’s shadow traverses Jupiter’s northern cloud tops as Ganymede begins to disappear behind the gas giant.
Saturn sits just 5° east of Jupiter every evening in May. The planet’s disk spans 18" and its expansive rings stretch 40" wide by the end of May. Any telescope will reveal Saturn’s rings. The minor axis of the rings is just 14", narrower than the planet itself, so you’ll notice the northern pole of Saturn arcs above the far ring edge.

It’s also worth observing Saturn’s moons, including its largest and brightest, Titan. This intriguing world orbits Saturn once every 16 days, and stands due south of the planet May 5 and 21, and due north May 13 and 29. At magnitude 8.6, it will remain the brightest object near Saturn unless a rare field star joins in. Shining between magnitude 10 and 11, the moons Tethys, Dione, and Rhea orbit closer to Saturn and with shorter periods. Their relatively quickly changing positions are easy to follow with modest telescopes.

May is also a great month to spot Iapetus. This odd saturnian moon changes brilliance between eastern and western elongations depending on whether its bright or dark hemisphere faces us. Iapetus shifts between magnitude 10 and 12 and reaches the middle of this range May 11 when it’s at inferior conjunction (48" due south of Saturn). Iapetus is brightest at western elongations, like the one it reaches May 31, when it shines at about magnitude 10.5 and stands about 9' due west of the planet.

Mars is best viewed in the hour before dawn during May. On the 1st, it stands 15° above the southeastern horizon by 4:45 a.m. local daylight time, and it climbs to 26° high at the same time May 31. Still far from its October opposition, Mars’ apparent size continues to grow during May, expanding from 8" to 9". This is still too tiny for small scopes to resolve much detail, but the better resolution of 10-inch scopes and larger will reveal fine details. This is a good time to begin practicing video imaging, as it will enhance details not visible to the eye.

Mars continues to climb to higher declinations as it crosses into Aquarius. It begins the month at 15° south in northeastern Capricornus, which puts it 2.7° from 3rd-magnitude Deneb Algiedi. The Red Planet then shines at magnitude 0.4. It crosses into Aquarius on May 9, passing less than a Moon’s-width from Iota (ι) Aquarii May 11/12. By the end of May, the planet shines at magnitude 0.0 and is at a declination of –9°, putting it 2° south-southeast of Lambda (λ) Aquarii.

Neptune returns to the night sky by late May. On May 31, it sits 8.6° east-northeast of Mars and 3° east of 4th-magnitude Phi (ϕ) Aquarii. To spot Neptune with binoculars, use Mars as a guide. Neptune shines at magnitude 7.9, but at just 15° high an hour before twilight, its low altitude makes it more difficult to spot than later in the year.

Meanwhile, Uranus rises with the onset of twilight and remains difficult to spot throughout May. So your best bet is to hold off until next month before trying to pick out its blue-green glow from the starry background.

Rising Moon: Cracks, craters, and domes

Today’s sea of tranquilLity was once quite untranquil. It began 4 billion years ago, when a giant impact excavated a large basin. Millions of years later, lava erupted through fissures in the basin floor. The cracks tended to be oriented along stress lines that were created by other large impacts. During a second round of upwelling lava, the terrain unevenly heaved from below, causing the formation of a scarp or fracture. In other places, lava tubes collapsed into rilles. Nearby, some volcanoes made it to the surface, but the failed ones created domes. Mini-asteroids that slammed into the surface millennia later added the final touch of texture.

On May 26, roughly four days after New Moon, the Sun rises on the fascinating region of crater Cauchy, just north of the terminator’s midpoint. The terminator is the line that divides lunar day and night, but at such a young age, the dark face of our sister Luna might be modestly lit by the gray glow of earthshine, which is sunlight reflected from Earth’s dayside.

Crater Cauchy itself is a modestly small, 7.5-mile-wide, simple impact feature with nice, sharp edges that reveal its relative youth. Immediately to Cauchy’s north is a prominent rille named Rima Cauchy, where years of tiny impacts have softened its edges. Within a couple of Earth days, the higher angle of the Sun shines light directly into the rille, wiping out the shadows and making it all but invisible. South of Cauchy is a prominent fault scarp called Rupes Cauchy, which lies parallel to the rille. Well known to lunar aficionados, the fault is second only to the famous Straight Wall. When the Sun rises over the region, the scarp casts a sharp shadow westward, but the dark line disappears a couple of Earth evenings later.

As an added bonus for this region, a pair of lava domes sit at the edge of Mare Tranquillitatis. Look for the small, light-dark pairing caused by these hills protruding up in the sunlight. The eastern dome, cataloged as Omega, is likely a shield volcano, while the western one, Tau, appears to have been formed by uplifting from below.
Eta Aquariid meteor shower
Partly due to the low altitude of the Eta Aquariids’ radiant when viewed from northern latitudes, the shower is not considered one of the year’s best. But with Mars hovering in the nearby sky this year, it’s worth taking the time to view it.

Meteor watch: Comet Halley returns to Earth’s sky

May’s main Meteor shower is heavily affected by a nearly Full Moon that remains visible most of the night. The Eta Aquariids is one of two showers associated with Halley’s Comet, which has spent eons littering its path with debris that results in the yearly shower (the Orionids in October is the other).

The Aquariids sport a maximum observable rate of about 10 meteors per hour under perfectly dark skies. However, with a bright Moon present, you’ll be lucky to spot five streaks per hour in most urban locations. The first few days of May after the Moon sets (4 a.m. on May 2) is a good time to catch the few early shower members streaking through the sky. A New Moon on May 22 offers dark skies for viewing sporadic meteors, as well as the occasional straggler of the Eta Aquariid shower.

Comet PanSTARRS (C/2017 T2)
It takes roughly 15 minutes for light from the comet to reach us, but about 12 million years for light from the Cigar Galaxy to get here.

Comet search: Peaking near a cigar

Highlight of the year: Comet PanSTARRS (C/2017 T2) shares a low-power field with the Cigar Galaxy (M82)! As ideal as modest-telescope comets get, PanSTARRS hits a peak brightness of 8th or 9th magnitude while sailing high in the northern sky. The comet makes its closest approach to the Sun, or reaches perihelion, May 4, at a distance of 149 million miles. On the plus side, PanSTARRS crests near Polaris, making it accessible to northern observers all night.

New Moon occurs May 22, which is perfect timing because PanSTARRS lies only 3/4° from M82 the following weekend. PanSTARRS is just barely visible with binoculars; it will show up better in a 4-inch scope, and will likely sport some green when viewed through a 12-inch scope.

From May 24 to May 26, make sure to look for a double-spike. The comet’s short, fan-shaped tail flattens to edge-on when Earth passes through the orbital plane, so as long as our visitor from the Oort Cloud produces a decent amount of both gas and dust, we can expect to see a green or blue streak on the leading edge and an “anti-tail” knife blade of white on the other. If you can, take a peek every other night to catch the whole transformation. For the rest of the apparition we get more and more of a top-side view.
23 Thalia shares the sky with 40 Harmonia
The two 10th-magnitude asteroids will take some effort to pick out from a backdrop of similarly bright stars.

Locating asteroids: Double feature

When it comes to asteroids, luck works both ways. Rather than one bright asteroid all by itself, May brings two fainter ones almost side by side. During midevening from the Northern Hemisphere, the blue-white luminary Spica shines in the southeast. If you then look about 15° east (1 hour of right ascension), you’ll spot 4th-magnitude Iota (ι) Virginis — the anchor star for this month’s pair of space rocks. Both 23 Thalia and 40 Harmonia sport a diameter of about 67 miles and lie beyond the orbit of Mars in the inner asteroid belt.

Correctly identifying both asteroids will take a bit more care and patience than usual, as they stand in front of many background stars glowing at 10th magnitude. Despite being far from the Milky Way’s bulge, the lack of dust clouds here lets light from many distant stars shine through. If you want to risk the weather, you can sketch the star field May 15 or 16 with 40 Harmonia on the southern edge of the field, then return May 29 through 31 to pinpoint the extra dot on the northern edge that is 23 Thalia.

During the month, the two asteroids are never closer than about 2°, but that is actually an easier shift than you might think. If your telescope uses a diagonal, take a picture of the chart and flip it left to right to match the orientation in the eyepiece.


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