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Sky This Month: April 2021

Mars rules the night.
Moon and planets over Bursa, Turkey
A crescent Moon and three planets peek out from the early morning clouds above Bursa, Turkey. Most planetary observing this month is best done in the hours before sunrise.
Tunç Tezel

Planet viewing this month is limited to the evening and morning hours, with a broad stretch of nighttime devoid of major planets. Mars drifts through Taurus and into Gemini, while Mercury and Venus make a late April appearance. Jupiter and Saturn start the month in Capricornus, now appearing in the predawn sky. They climb higher as the month progresses.

Mercury passes through superior conjunction on the far side of the Sun April 18 and appears in the evening sky along with Venus, which passed through superior conjunction in late March. Both planets are slow to climb away from the Sun’s glow.

On the last day of April, both planets hang very low in the western sky 30 minutes after sunset. Venus is most challenging at only 2° in elevation and sinking fast, but at magnitude –3.9 you can catch it perhaps 10 minutes earlier. If you do spot Venus, it’s a useful guide to find fainter Mercury 4.5° above it. Mercury stands 6° high 30 minutes after sunset and glows at magnitude –1.2. It sets just after 9 P.M. local time. The visibility of both planets improves next month as they glide past the Pleiades star cluster (M45).

Star chart showing Mars on April 25, 2021
Mars cozies up to two star clusters
Grab your binoculars April 25, when Mars passes M35. Nearby is NGC 2158.
All Illustrations: Astronomy: Roen Kelly

Mars lies high in the western sky after sunset, joining the stars of Taurus the Bull. It shines at magnitude 1.3 on April 1 and fades to 1.6 by the end of the month. Compare the Red Planet’s signature orange-colored glow to Aldebaran, the Bull’s magnitude 0.9 star. Aldebaran, a glowing star emitting its own light, appears reddish due to its “cool” surface temperature around 6,700 degrees Fahrenheit. But Mars is a cold, rocky planet and has no light of its own. Instead, its ruddy color occurs because it reflects sunlight preferentially in the red part of the spectrum, due to the makeup of its rusty surface material.

Mars tracks slowly through Taurus. Between April 12 and 13, it passes between the two stars marking the horns of the Bull — Alheka (Zeta [ζ] Tauri) and Elnath (Beta [β] Tauri). The planet crosses into Gemini April 24, then early on the 27th passes delightfully 0.5° north of M35, a fine open cluster in Gemini with a few colored stars. Grab a pair of binoculars to soak in this stunning sight, as well as pick up an additional target: Located just 0.5° southwest of M35 is an even more distant cluster, NGC 2158.

On April 30, Mars forms a nice triangle with the well-known 3rd-magnitude pair Eta (η) and Mu (μ) Geminorum. By now, Mars sets shortly after local midnight, so it’s best to catch the planet soon after twilight for the best views. The disk spans only 5.3" on April 1 and slowly shrinks to 5" by the end of the month, so telescopic views are challenging at best.

Once Mars sets, it’s more than four hours before another major planet rises. You can use the middle of the night to search for the brightest of all asteroids, 4 Vesta, which reached opposition in Leo the Lion last month. Vesta is an easy binocular object at 6th magnitude, now moving across the central region of Leo. Comparing its position on consecutive nights will reveal its motion relative to the background stars. On April 1, it’s 2° due east of 51 Leonis. This star is most easily found by scanning midway between Algieba (Gamma [γ] Leonis) and Chertan (Theta [θ] Leonis). By mid-April, Vesta lies only 0.6° southeast of 51 Leonis; it then swings southward, ending the month just over 1° southeast of the star.

Star chart for the Moon and giant planets April 6, 2021
The Moon joins Jupiter and Saturn
Giant planets Jupiter and Saturn rise before the Sun this month. On April 6, a waning crescent Moon joins the pair in Capricornus.

Saturn rises shortly after 4 A.M. local time on April 1 and stands more than 10° high at the onset of twilight. This is still quite low for good viewing conditions, but Earth’s early-morning atmosphere is occasionally very steady before the heat of the day strikes it. Saturn shines at magnitude 0.6 all month. Viewing the ringed planet is possible toward the end of April because of its higher altitude (above 20°) as twilight begins. Since the planet has been out of view for a while due to conjunction with the Sun, check out its atmospheric features. Are there any new white spots?

Even at low altitude, moons of Saturn can be spotted in small telescopes. Titan shines the brightest at magnitude 8.8. On April 1, it lies 2.6' due east of the planet. It orbits every 16 days, making about two orbits per month. Titan passes due south of the planet April 5 and 21, and due north of the planet April 13 and 29.

Iapetus is a challenging moon to spot, swinging between 10th and 12th magnitude as it orbits the planet — a result of the varying albedo (reflectivity) of its surface. Saturn’s low altitude creates another challenge here. However, on April 16, Iapetus reaches its brightest western elongation, presenting a good opportunity to identify it with small telescopes. Iapetus lies 8' due east of Saturn, so use a low-power eyepiece with a field of view of at least 1/3° to search for the moon.

Chart showing Saturn and its bright moons April 16, 2021
Bright Iapetus
On April 16, Saturn’s moon Iapetus lies at its brightest due east of the planet, making it a good small scope target.

Jupiter rises 35 minutes after Saturn in early April, shortly before 5 A.M. local time. It lies in the northeastern part of Capricornus. On April 7, it stands 2° due north of Deneb Algedi (Delta [δ] Capricorni). Jupiter brightens by 0.1 magnitude to –2.2 during the month and treks eastward, crossing into Aquarius on April 25.

The best telescopic views are later in the month, when the giant planet has time to rise higher in the sky. It reaches 18° in elevation in the southeastern sky by 5 A.M. local time on April 30, just as dawn begins to break — a great time to check out atmospheric features on the 37"-wide disk. Its four Galilean moons wander around the planet in various intervals, and their changing positions are fascinating to track.

As Jupiter rises on April 9, Io is already transiting the disk. The moon is preceded by its dark shadow, farther west on the planet’s face. Io’s shadow slips off the disk at 6:28 A.M. EDT, visible in darkness across the U.S. But only those in western locations will be able to watch Io make its way off the western edge of the disk an hour later. There are many more transits and occultations throughout the month.

Between now and August, when Jupiter reaches opposition, the gas giant will climb higher in the eastern sky each month, gaining altitude and offering improved conditions for observing.

Neptune rises at the break of twilight at the end of April, having passed through superior conjunction in March. By 5:30 A.M. local time, the planet stands 10° high in the eastern sky, nearly 5° east of 4th-magnitude Phi (ϕ) Aquarii. Neptune is dim at magnitude 7.8; if you’re lucky, you can catch it before the sky brightens. Its visibility will improve through the summer.

Uranus is in conjunction with the Sun April 30 and only briefly visible low in the evening sky early in the month.

Rising Moon: A heartfelt sunrise

The Valentine Dome

The Sea of Serenity’s dark floor is strewn with pits, bumps and ridges when the Moon appears sliced in half. On the 17th, the snaking Serpentine Ridge captures our attention with its long play of light and dark across this huge impact basin just north of the lunar equator.

By the following night, the Sun has risen over the western half of Serenity’s ancient lava-filled bowl, bringing to light some comparatively young craters. Strung out in a line from north to south are Linné G, H, F, B and A, with F and B the biggest at 3 miles (1.6 kilometers) across.

The Valentine Dome is a gentle volcanic protrusion tucked against the western flank of Serenity, visible only at the lowest of Sun angles. Timed perfectly for the 18th, its somewhat heart-shaped swelling crests less than 400 feet (122 meters) above the floor. You won’t be able to miss the handful of older peaks sticking above the dome. One Earth night later (the 19th), the shade of the gentle slopes has disappeared under the higher Sun, leaving but a trace of the tiny tops.

The low angle returns at lunar sunset, when the Sun is shining from the Moon’s west. This aptly named “reversed light” occurs about two weeks earlier on April 3, and again on May 2.

Meteor Watch: A springtime show

The springtime Lyrid meteor shower is like a welcome friend returning with the warmer weather. This annual shower is active from April 14 to 30 and peaks the morning of April 22. The radiant, located in the compact constellation Lyra the Harp, rises in the late evening and stands about 20° high at local midnight. A 10-day-old Moon will affect observations until it sets around 4 A.M. local time, offering an hour of dark skies before twilight begins. This early morning hour is the best time to view the shower, which can generate an average of up to 18 meteors per hour when the radiant is overhead. Most meteors are best seen from dark sites, well away from streetlights, and with dark-adapted eyes, which allow fainter members of the shower to be observed.

Lyrid meteor shower radiant April 2021
Lyrid meteor shower
The Lyrids’ radiant will reach 80° altitude around 4:30 A.M., slightly attenuating the shower’s maximum rate.

Comet Search: Constellation crosser

Like a sidekick to the bright galaxies in the Coma-Virgo Cluster, Comet C/2020 R4 (ATLAS) makes a wonderful addition to the spring sky. It launches from a late-night perch in Aquila and flies into evening skies by month’s end, its apparent speed due to the comet and Earth orbiting in opposite directions.

The late-night observing window opens on the 6th after 1 A.M. local time, with the best views coming before moonrise and the dawn, when ATLAS is higher in the sky and well above the horizon haze.

Lunar interference begins on the 19th, but by evening’s astronomical darkness on the 29th, ATLAS is hoofing it out of Boötes and into galaxy country at 4° each night. The comet’s brightness of 11th magnitude will depend on its dust and gas activity. Some detail is visible through an 8-inch scope for this typical appearance. Look for a northward-pointing dust tail with a sharp western flank; imagers should collect some blue or green ion emission.

ATLAS’ brightness will soon plunge, worsened by the increasing Earth-comet separation after its closest pass to our planet (0.46 astronomical unit [AU], where 1 AU is the average Earth-Sun distance) on the 23rd. If that pass had occurred three months later in ATLAS’ nearly 1,000-year period, the geometry would have made it as bright as last year’s C/2020 F3 (NEOWISE).

Path of Comet C/2020 R4 (ATLAS) in April 2021
Comet C/2020 R4 (ATLAS)
This month’s Messier marathons feature a Kuiper Belt arrival in front of the Milky Way’s Great Rift in Aquila. Comet ATLAS covers a large swath of sky during April, passing a plethora of deep-sky objects on the way

Locating Asteroids: A walk in the park

Simple, relaxing asteroid tracking like this happens only every three years or so. This month, asteroid 4 Vesta shines at magnitude 6.6, easy pickings with binoculars from the city. Its background constellation of Leo is recognizable from light polluted areas, with the blue-white luminary Regulus leading it high in the southeast. Combine this with mild spring evenings and it rarely gets better for asteroid watchers.

After two or three sightings, you won’t even need a chart to find it. For the first search, though, use our chart to guide you to the spot near Leo’s hindquarters, notable for its perfect right-angled triangle. In a logbook or on a blank sheet or paper, place the brightest six dots with a pencil. Create a large version so you have enough room to add Vesta’s slow shift every three or four nights.

A typical amateur won’t be able to detect Vesta moving during one observing session this month because it appears to slow down and turn in a tight curve. Earth passed it by on our faster inside track, causing this part of a retrograde loop. The bright light of the waxing gibbous Moon on the 21st through the 23rd interferes, so give these nights a pass.

Spanning 310 miles (499 km), Vesta is a large object in the main asteroid belt, second only to dwarf planet 1 Ceres.

Path of asteroid Vesta in April 2021
Making the turn
Asteroid Vesta is easy to spot this month as it makes a quick turnaround in Leo the Lion.


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