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Sky This Month: May 2021

The morning Moon turns red.
RELATED TOPICS: OBSERVING
A red eclipsed Moon
On May 26, the Moon will take on a brilliant orange-red hue during an early-morning lunar eclipse.
Stephen Rahn

Planetary action is picking up again, with three rocky planets — Mercury, Venus, and Mars — easily visible in the evening sky. Jupiter and Saturn feature in the predawn sky. And the western half of the U.S. enjoys a fine total lunar eclipse on May 26.

We begin our tour of the spring skies with Mercury and Venus, both in Taurus the Bull. They are visible soon after sunset. With the addition of Mars higher above the horizon in Gemini, the trio offers a range of observational treats.

Mercury puts on its best evening show of the year this month. It appears soon after sunset on May 1, less than 5° below the Pleiades (M45) star cluster. Mercury is easy to find, shining at magnitude –1.2. The same day, Venus is just over 5° directly below Mercury and, although brighter, is a tricky target in bright twilight. See if you can spot Venus 30 minutes after sunset — it stands 2° high and shines at magnitude –3.9. The planet quickly sets, so you’ll need a clear western horizon to see it.

During the first week of May, both planets’ visibility improves dramatically. On May 2, Mercury lies 3° south of the Pleiades — a gorgeous sight in 7x50 binoculars. Its faster pace means it is pulling away from Venus, now 6° lower. Through a telescope, Mercury's tiny disk is 77 percent lit, spanning 6".

The Moon and Venus are stunningly close after sunset on May 12, just one day after New Moon. Venus stands 1.1° northwest of our satellite. Hold your pinky finger at arm’s length and it almost fills the gap between them. (Note: Venus is occulted by the Moon the morning of May 13 for observers in some parts of New Zealand).

Grab binoculars to see the Hyades star cluster 4° left of the Moon. In twilight, Aldebaran (not part of the cluster) pops out first, followed by the dimmer stars in the Hyades. The region sets an hour after sunset. Find a clear western horizon to view it.

The following evening, the Moon has moved up to join Mercury — they’re 3.1° apart and the planet has faded to magnitude 0.1. They remain above the horizon an hour longer than Venus, so the darker sky background enhances the view — particularly of earthshine on the Moon.

Mercury reaches its greatest eastern elongation on May 17, when it stands 22° from our star. It’s the steepness of the ecliptic with respect to the evening horizon that makes this Mercury’s best evening apparition for the year. A telescope will reveal an 8"-diameter disk that is 35 percent lit.

Chart showing Mercury, Venus, and Mars on May 28, 2021
Twilight trio
Mercury and Venus spend May together in Taurus the Bull. They sit closest together on the 28th, while Mars stands above them in Gemini.
All illustrations: Astronomy: Roen Kelly

Both inner planets continue their race across Taurus. For most of the month, Mercury stays ahead, but its easterly trek slows dramatically in the second half of May. On May 20, Venus stands 7° below Mercury. By May 23, when Mercury reaches a location between the horns of the Bull (Elnath and Alheka), they’re 5° apart. The two planets are closest five days later, on May 28, when they stand 24' apart and set just over an hour after the Sun. Mercury is much dimmer at magnitude 1.9 and best spotted using binoculars in the bright twilight.

A telescope that can fit a Full Moon within its field of view will comfortably capture both planets at once. Mercury’s disk spans 11" and is 12 percent lit. Venus shows an almost full phase but, due to its greater distance across the solar system, spans only 10" — although the planet is physically much larger than Mercury.

Following their close conjunction, Venus continues to climb higher, while Mercury sinks back toward the Sun for a June inferior conjunction.

Mars crosses the constellation Gemini during May. It begins the month 2.3° due north of 3rd-magnitude Mu (μ) Geminorum, which shares the northwest corner of Gemini with Eta (η) Geminorum, just 2° farther west.

Mars glows at magnitude 1.6 and spans about 5" — so small that surface details are difficult to see visually, rendering it a disappointing telescopic object. It spends May 8 to 10 within 1° of 3rd-magnitude Mebsuta, (Epsilon [ε] Geminorum). When the Red Planet occulted this star in April 1976, just three months before the famed Viking landings, major telescopes observed the event to study the martian atmosphere.

Mars ends the month 5° south of Pollux, Gemini’s second-brightest star. By then, the planet’s increasing distance from Earth has dimmed it by another 0.1 magnitude.

Saturn is a bit fainter than 1st magnitude and rises around 2:30 A.M. local time on May 1. It stands in Capricornus, 1° from Theta (θ) Capricorni. In the last full week of May, Saturn reaches its stationary point 0.6° shy of Theta, then begins retrograde motion. On May 31, Saturn — shining at magnitude 0.4 — rises shortly before 1 A.M. local time. This places the ringed planet 30° high at the onset of twilight, a fine location for telescopic views. It’s been many months since good views of Saturn have been available, so check out the disk for any erupting white spots indicative of new storm systems.

Saturn’s disk spans 17" at its equator and is 1.5" smaller from pole to pole. Its rings add dramatic width, spanning roughly 40" across the long axis and 11" across the minor axis. The rings’ narrower span compared with their appearance last year means the planet’s southern polar regions are becoming visible for the first time in eight years. Saturn stands nearly 7° northeast of a Last Quarter Moon on May 3 and 6° northwest of a waning gibbous Moon on May 31.

A few of Saturn’s many moons are visible in amateur scopes. Titan is the easiest to see at magnitude 8.4. On May 3, it lies 2.8' due east of the planet. It is south of the planet May 7 and 23, and north of the planet May 15 and 31.

Iapetus is at superior conjunction with Saturn May 6, on its way to eastern elongation and fading as it progresses. The moon has a dark hemisphere that is turning our way, taking it from 11th magnitude near conjunction to 12th magnitude at greatest eastern elongation May 25, when it reaches 8.3' east of Saturn. Tenth-magnitude Tethys, Dione, and Rhea congregate near Saturn, orbiting every few days and constantly changing relative positions. Enceladus — at magnitude 12 — lies near the edge of the rings and Saturn’s glare makes viewing it more of a challenge.

The positions of Jupiter's moons on May 7, 2021
Whose shadow?
On May 7, Ganymede’s shadow is already transiting when Jupiter rises. But Callisto is closer to the planet’s eastern limb, making it seem like it’s responsible for the dark spot. Callisto begins its own transit at 5:48 A.M. EDT.

Jupiter rises 45 minutes after Saturn and now resides in Aquarius, where it will remain for most of 2021. It starts out the month at magnitude –2.2 and brightens to –2.4 by late May. The gas giant is 30° high at the onset of morning twilight in the last week of May, offering a good chance for clear views of its cloud belts while avoiding some of the blurring effects of our own atmosphere.

Jupiter’s disk spans 41" by May 31. This is the start of its main 2021 observing season. With the bright outbreak in the Northern Temperate Belt and the sudden appearance of Clyde’s spot, among others, in methane-band images, 2020 offered lots of drama. Regular observers of Jupiter are sure to be watching closely. The gas giant’s Galilean moons are on show most nights: Io, Europa, Ganymede, and Callisto provide a constantly changing display and often produce shadow transits, occultations, and eclipses.

Neptune, in northeastern Aquarius, rises about an hour after Jupiter. It is best seen in late May, when it’s had time to rise higher by the onset of twilight. On May 31, it stands 5.6° east of Phi (ϕ) Aquarii and due south of the Circlet of stars in Pisces. A telescope will show it within 4' of a 7th-magnitude field star. Shining at magnitude 7.8, Neptune is within range of binoculars.

Uranus reappears low on the eastern horizon as dawn breaks, 11° below Hamal, the brightest star in Aries the Ram. At magnitude 5.9, Uranus is tricky to spot as the sky brightens. Its observability will improve late next month.

The position of the Moon before sunrise on May 26, 2021
Bright Antares sits about 7° southeast of the Moon on May 26 as a lunar eclipse gets underway. The complete event is not visible from the continental U.S.

A total lunar eclipse occurs May 26, coinciding with lunar perigee, when the Moon is closest to Earth (222,022 miles). The event is visible across western North and South America, the Pacific Ocean, Australasia, and eastern Asia.

The eclipse occurs near moonset early on May 26 across much of the U.S. In the Midwest, the setting Moon enters Earth’s shadow at 4:44 A.M. CDT, with the Moon about 10° above the horizon, amid the stars of Scorpius. The shadow progresses across the Moon as twilight grows. Totality begins at 6:11 A.M. CDT, just as the Moon is setting or shortly after it has disappeared across the Midwest.

Observers in the Mountain and Pacific time zones have better luck. In the Pacific time zone, the partial phase begins at 1:47 A.M., totality lasts from 4:11 A.M. to 4:25 A.M., and the partial phase ends at 5:52 A.M. — after the Moon has set for many locations.

Totality lasts only 14 minutes because the Moon cuts across the northern edge of Earth’s shadow. The Moon’s northern limb will appear brilliant orange, compared with a duskier southern hemisphere.

Rising Moon: Climb every mountain


The lunar Apennine Mountains

A half Moon is a wonderfully detail-packed world of dramatic contrasts. Look on the 18th to find a grand range of mountains straddling the middle of the disk north of the equator. The lunar Apennines are quite rugged compared to the smooth plains of frozen lava to the east. Their long shadows at sunrise tell us they thrust upward 3 miles. Find their black sawtooth shape reaching for the nightside, then come back every 10 minutes to watch the shadows grow shorter and shorter. We’re seeing the effects of sunrise on our sister, Luna. Gently curving to the north and east, the spine of the Apennines turns into the Caucasus. In the other direction, they continue into darkness but will be fully visible on the 19th.

Three decades after Galileo’s inaugural observation, lunar cartographer Johannes Hevelius published a map using names inspired by the layout of Europe’s great mountain ranges. He named this range for the earthly Apennines, which form the backbone of Italy. Of the nearly 300 features Hevelius labeled, only 10 mountains and ranges remain on maps today.

A mere 100 years ago, it would have boggled the minds of observers to hear that the mountain chain is but a small section of a vast bowl 750 miles across, formed when a small protoplanet slammed into the young Moon. The Imbrium basin filled with lava from the Moon’s molten interior millennia later. Look closely along the shoreline to detect partially filled craters and blocks of rock collapsed away from the wall. Out on the smoother plains, wrinkle ridges formed wherever material was squeezed horizontally. These gently sloping buckles are visible only at low Sun angles.

Meteor Watch: Early morning springtime shower


The springtime Eta Aquariid meteor shower is active from April 19 through May 28, peaking late on May 5. This coincides with a 24-day-old Moon, so conditions are favorable during the early hours of May 6. The radiant in Aquarius rises three hours before dawn and two hours before the crescent Moon. The 10° to 20° altitude of the radiant in the hour or two before dawn reduces the observable number of meteors below the predicted zenithal hourly rate of 50, but the fast-moving meteors — many with persistent trains — make it worth spending a few hours to see perhaps a dozen good ones.

The Eta Aquariid shower is one of two during associated with debris from Comet 1P/Halley. Earth crosses Halley’s orbit in May and again in October, when the Orionids occur.

Chart showing the radiant of the Eta Aquariid meteor shower
Eta Aquariid meteor shower
Although the Eta Aquariids peak late May 5, the radiant won’t rise until early in the morning on May 6.

Comet Search: Into the distance


This month is our last chance to see C/2020 R4 (ATLAS). Next month, we’ll hit 7P/Pons-Winnecke at its best, while C/2017 K2 (PanSTARRS) will remain in our sights for the next three years. And with a little luck, a newer, brighter visitor will outdo them all.

Discovered September 12, ATLAS appears similar to a cyclist passing us while we’re on foot: We first rapidly turn our gaze to track their motion, then barely adjust to watch them diminish into the distance. ATLAS exited Boötes at 3.5° per night in late April and slows to a 20'-per-night crawl on Leo’s back by May’s end.

During the first week of May, warm up your observing skills on the compact galaxy M94 in Canes Venatici before dropping 10° south to the fainter and smaller comet. You’ll likely need a 10-inch scope at 150x or more to pick up its soft glow.

Moonlight puts an end to visual sightings midmonth, leaving the next good look at the comet to our descendants 1,000 years from now.

Path of Comet C/2020 R4 (ATLAS) in May 2021
C/2020 R4 (ATLAS)
On May 5, NGC 4631 shines at 9th magnitude less than 3° west of Comet ATLAS. The edge-on galaxy NGC 4656 glows at 10th magnitude between the two. You’ll find numerous other galaxies of scattered along the comet’s path this month.

Locating Asteroids: Track asteroids across the Lion


Join one of the easiest asteroid hunts, already underway high in the south as darkness descends. From the suburbs, you can pop outside with binoculars and catch main-belt asteroid 4 Vesta masquerading as a magnitude 7.5 field star just to the right of Leo’s hindquarters.

The only trick is that you’ll need more than one evening to positively identify the 300-mile-wide space rock. There are a handful of stars just a little fainter and brighter here, but they won’t shift night to night. Place them as dots in a logbook or on paper, and see which one changes compared to the others. By the third evening, you’ll catch it in just a few minutes.

Skip the nights from the 18th to the 21st, when the waxing gibbous Moon passes by, causing glare and washing out the background sky.

If you want to see an asteroid move during one observing session, try and catch 29 Amphitrite on May 13 as it gives Leo’s luminary a heart attack, almost blocking out Cor Leonis (Regulus). Use a nearby 8th-magnitude field star as a reference; Amphitrite will be three times closer to Regulus. Visit www.asteroidoccultations.com for a list of actual “hits,” or occultations, to watch for.

Paths of asteroids Vesta and Amphitrite in May 2021
Heart and tail
Asteroid 4 Vesta offers easy viewing in Leo’s hindquarters, while 29 Amphitrite makes a close pass by the Lion’s heart.
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