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30 Apr 2020

May Astronomy: Say Farewell to Venus and Hello to Mercury at Dusk; at Dawn, the Giant Planets Hang Out Together

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May features the departure of Venus from the western evening sky just days after its close pairing with Mercury. In the southern predawn sky, Jupiter and Saturn remain in “quasi-conjunction,” within 5 degrees all month, while brightening Mars widens its distance to the lower left of our solar system’s giant planets.

Every spring, in nightly outings during the first hour after sunset, you can enjoy following the seasonal departure of bright stars into the western twilight glow. In order of date, they are Rigel, Aldebaran, the Dog Star Sirius and Betelgeuse, as well as—before the end of May this year—Venus. A close conjunction of departing Venus with emerging Mercury will take place on May 21. By the start of June, of winter’s luminaries, only the spring arch of four bright stars remains: from left to right, Procyon, the “Twins” Pollux and Castor, and Capella.

Using a telescope or binoculars held steady during bright twilight, follow the changing crescent phases of Venus, just after sunset in May, and just before sunrise in June and July.

You can also follow the moon nightly at dusk, as it waxes from a thin crescent in the west to full in the east. Then you can switch to predawn viewing to follow it waning from full in the west to a thin crescent in the east.

Early risers this spring can follow the three bright outer planets in the southeast to southern sky: Bright Jupiter pauses within 5 degrees west of Saturn through early June, while red, brightening Mars pulls away to their east.

Nearly all of the lunar and planetary events mentioned here are illustrated on the Abrams Planetarium Sky Calendar. The issue for May 2020, including evening sky maps, are available free at www.abramsplanetarium.org/skycalendar.

Evenings: Even in ordinary years, the May evening sky undergoes rapid change, with the departure of several bright winter stars. But evenings in May 2020 feature the dramatic departure of Venus—watch it get noticeably lower each night until it disappears into bright twilight in month’s final days—and the fast emergence of Mercury from the far side of the sun, around midmonth.

Venus in the foreground, and Mercury in the background, moving in opposite directions, speed past each other on May 21. That evening, they’re just 1 degree apart, allowing the large, thin crescent Venus and tiny, gibbous Mercury to be viewed in the same low-power telescope field! Be sure to catch the compact gathering of the young crescent moon, Venus and Mercury low in the very early evening twilight on May 23. To spot this moon, which begins the month of Shawwal and ends Ramadan on the Islamic calendar, it’ll be essential to find a place where a mountain or a nearby obstruction won’t block your view toward the west-northwest.

At month’s end, the only bright objects remaining in the low western sky in deepening twilight form the “Arch of Spring” in the west to west-northwest, consisting of, from left to right, Procyon, Pollux, Castor and Capella, with Mercury below. Leo’s brightest star, Regulus, at the end of May is still well up in the west-southwest, to the upper left of the arch. The Big Dipper’s curved handle points the way to Arcturus high in the east-southeast, and Virgo’s brightest star, Spica, well up in the south-southeast: Follow the arc to Arcturus, and drive a spike to Spica. The second member of the Summer Triangle to rise, Deneb, appears very low in the northeast, to the lower left of Vega, its brightest member. Finally, locate the red supergiant star Antares, heart of the Scorpion, low in the southeast at dusk in late May. As Earth passes between Antares and the sun on night of May 30-31, Antares stands at opposition and is visible almost all night: Low in the southeast at dusk, at its highest in the south in middle of night, and low in the southwest at dawn.

The moon at dusk, on its way toward full on the night of May 6, passes near Regulus, heart of Leo the Lion, on May 1; and near Spica on May 5. On its next pass through the early evening sky May 23-June 5, catch the moon as a thin crescent below Mercury and Venus on May 23; as a thickening crescent near Pollux on May 26 and near Regulus on May 28; and at first quarter phase, half full, on May 29.

Venus on May 1 is still near peak brilliance, at magnitude -4.7. Slight optical aid reveals it as a crescent, 24 percent illuminated and 40 arcseconds across, large enough to resolve even through 7x binoculars if observed in daylight or soon after sunset, before there’s too much contrast of the brilliant planet against a darkened sky. At sunset on May 1, Venus is 38 degrees to the upper left of the sun and sets more than three hours later. Beginning retrograde, Venus hovers 1.5 degrees from Beta Tauri May 9-12 in a quasi-conjunction, without moving on past the star. Dramatic changes: By May 16, Venus is 25 degrees to the upper left of the setting sun; sets two hours after the sun; and shows a thinner crescent—10 percent lit and 51 arcseconds across. On May 22, Venus is 18 degrees from the sun; sets 90 minutes after sunset; and shows a crescent just 5 percent lit, 55 arcseconds across. Can you still see Venus on May 28? It’s moved to within 9 degrees of the sun; sets 45 minutes after sunset, and shows a very thin crescent of just over 1 percent, 58 arcseconds across.

Mercury passes superior conjunction behind the sun on May 4, and perihelion only five days later, so it emerges fast and bright into evening sky. Using binoculars, can you spot Mercury on May 9, when it shines at magnitude -1.7 and sets just 30 minutes after sunset? If not, it’ll be easy even for unaided eye a few days later. Capture gibbous Mercury and crescent Venus in same telescope field on May 21.

Morning: The morning twilight chart shows bright Jupiter in the southern sky, and Saturn in quasi-conjunction, within 5 degrees to Jupiter’s east (left) all month. Brightening Mars is in the southeast, 20 to 41 degrees to the lower left of Saturn. Watch the moon pass the three bright outer planets May 12-15. Bright stars are the Summer Triangle of Vega, Altair and Deneb overhead, with Arcturus sinking in the west to west-northwest, Antares sinking in the southwest, and Fomalhaut in the southeast, below Mars.

The moon waxes from full in the west-southwest to a thin crescent in the east-southeast from May 7-20. It appears near Antares on May 8 and 9; near Jupiter and Saturn on May 12; at last quarter phase, half full, on May 14; and near Mars on May 15.

Brightest Jupiter (magnitude -2.3 to -2.6 in eastern Sagittarius) is in the south-southeast to south. Saturn (magnitude +0.6 to +0.4 in western Capricornus) is within 5 degrees to Jupiter’s east all month. They’re in quasi-conjunction, with a minimum separation of 4.7 degrees on May 18. They move little this month against stars, as Saturn commences retrograde on May 11, and Jupiter on May 14. A telescope shows Jupiter’s cloud belts and four Galilean moons, and Saturn’s rings. Mars (+0.4 to 0.0) is in the south-southeast, to the lower left of Saturn, by 20 degrees on May 1, by 30 degrees on May 16, and 40 degrees on May 30. Watch Mars pass 0.9 degrees north of two stars in tail of Capricornus May 1-4, and 2 degrees south of a star in Aquarius on May 30. Through a telescope in May, Mars shows a tiny gibbous disk 7.6 to 9.2 arcseconds across, but it is early spring in the red planet’s southern hemisphere, and the large, bright carbon dioxide south polar cap should be readily seen.

For more on Sagittarius, the Territory of Dogs, and resources for telescopic observation of Mars in 2020, visit the Abrams Planetarium Extra content page, at www.abramsplanetarium.org/msta.

Star parties hosted by the Astronomical Society of the Desert have been canceled through at least the end of May. Watch www.astrorx.org for updates.

Wishing you clear skies!

Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. Now retired, he often collaborates with John S. French on the Sky Calendar, and enjoys providing sky-watching opportunities for a variety of groups, mostly in the California desert and in Michigan.

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