The fast-moving inner planets are gathered in the western sky at dusk in May. Venus and Mercury are low in twilight, so find a spot where mountains aren’t blocking your view—and bring binoculars. Mars, though faint, has many groupings with the stars of Gemini.
In the morning, stars and the gas giants Jupiter and Saturn are about where you will find them at dusk in the late summer and early fall.
However, the top event this month is a predawn total lunar eclipse on Wednesday, May 26. Two days later, you’ll have a chance to view an unusual eclipse of one of Jupiter’s moons.
Evenings: Venus, nearly of magnitude -4, is bright enough to be seen in twilight quite soon after sunset. It follows the sun over the west-northwest horizon by only three-quarters of an hour on May 1, increasing to 1.3 hours by month’s end. Mercury is nearby, opening May at magnitude -1.1, fading to 0.0 by May 13, to magnitude +1.0 by May 21, and to +1.5 by May 24. Mars, though dim at magnitude +1.6 to +1.7, is higher and easily seen in a dark sky. Using binoculars, track Mars’ motion of 0.6 degrees per day as it passes by third-magnitude stars in Gemini through May 2, and on May 9 and 23. It forms noteworthy configurations with brighter Pollux (+1.2) and Castor (+1.6) on May 15 (an isosceles triangle) and on May 31 (when it passes closest to Pollux).
The bright winter stars, like snowbirds, have begun their annual exodus. On May’s evening chart, four bright stars visible at the start of May—Rigel, very low in the west-southwest; Aldebaran, in the west-northwest; Sirius, the Dog Star, in the southwest; and Betelgeuse, in the west—all depart, in that order, below the horizon over the course of the month. At month’s end, an arch of winter stars—from left to right, Procyon; the “Twin stars” Pollux and Castor; and Capella—still remains in the west to northwest at dusk.
Regulus, the heart of Leo, is usually considered a star of late winter, though it remains visible at dusk into July. Look for it high in the south in early May, and still well up in the west-southwest at month’s end. Golden Arcturus, climbing high in the east to east-southeast, and blue-white Spica, 33 degrees to its lower right, are bona fide spring stars, since they’re up all night early in the season. Watch for a brighter blue-white star, Vega, harbinger of summer, rising in far the northeast by early May.
Watch for Antares, the red supergiant star marking the heart of the Scorpion, rising in the southeast in evening mid-twilight in the last days of May. It’s at oppositionto the sun on the night of May 30.
Mornings: Jupiter, shining at magnitude -2.2 to -2.4, rises ahead of the sun by nearly three hours on May 1, and by more than four hours on May 31. An hour before sunup, it’s the prominent “morning star” in the southeast, while Saturn, magnitudes fainter at +0.7 to +0.6, is 15-18 degrees to Jupiter’s upper right. Jupiter passes 1.3 degrees north of 4.3-magnitude Iota in Aquarius on May 6, and, after beginning retrograde on June 20, will come back to pass it twice more. This is an especially good month for viewing Saturn through a telescope, since the planet’s shadow on the rings gives the system a beautiful 3-D appearance.
The brightest morning stars in May are golden Arcturus, getting lower in the west to west-northwest, and blue-white Vega, nearly overhead. Other bright stars visible each clear morning in May are Altair and Deneb, completing the Summer Triangle with Vega, and Antares, sinking low in the southwest.
Moon doings: Catch a waxing crescent moon near each evening planet: Venus on May 12 (look early!), Mercury on May 13, and Mars on May 15. Catch a waning moon near giant planets at dawn on May 3-5, and May 31-June 1.
The main event: On the morning of Wednesday, May 26, there will be a brief total lunar eclipse centered at 4:19 a.m., low in the southwest, within 7 degrees to the right of Antares. Southern California has the best view of this eclipse in the contiguous 48 states. Hawaii has an even better view, with the moon high in a dark sky, along with the star clouds and central bulge of our Milky Way.
The opening partial phase of the lunar eclipse begins at 2:45 a.m., as the east (left) edge of the moon begins to enter the umbra, or dark, central core of Earth’s shadow. Within a few minutes, the circular shape of the shadow edge will become evident. At 3:18 a.m., half the moon’s width will be immersed in deep shadow. By then, the typical rusty color of deep lunar eclipses should be noted, from sunlight which has passed through Earth’s atmosphere and been bent, or refracted, deep into the shadow. It will take another 53 minutes until the entire disk of the moon is immersed in shadow, and total eclipse begins, at 4:11:30 a.m.
In this lunar eclipse, the moon skims through the outer part of the Earth’s dark shadow, so the total phase of the eclipse is predicted to last only 14 1/2 minutes. Even at deepest eclipse, the moon’s northern (upper) edge will be barely within the outer edge of the Earth’s shadow, and so is likely to appear lighter and yellowish or bluish in color, in contrast to the deep rusty or reddish brown tones projected on the southern part of the moon, nearer to the center of Earth’s shadow. At mid-eclipse, the moon will be only 14 degrees up in the southwest, within 7 degrees to the right of Antares. In the Coachella Valley, the sun will be less than 15 degrees below the northeast horizon. and twilight will be under way. The sky will brighten as the latter half of the eclipse unfolds.
Totality ends at 4:26 a.m., and 53 minutes later, at 5:19 a.m., half the moon’s diameter will be out of the deep shadow. But by then, as seen from the Coachella Valley, the moon will be only 4 degrees up, some 30 degrees south of west. For this eclipse, especially the later stages, choose your vantage point carefully to avoid mountains blocking your view. Locally, the moon sets several minutes before the moon finally leaves the umbra, at 5:52 a.m.
The next lunar eclipse, on the morning of Nov. 19, will be nearly total (97 percent, at 1:03 a.m.). It should be a spectacular event, with the moon high in the sky, not far from the Pleiades star cluster—with no mountain-dodging required!
Two days after this month’s lunar eclipse, viewers with telescopes will get a chance to witness an eclipse of one of Jupiter’s satellites, volcanic Io, as it passes through the shadow of Ganymede, Jupiter’s largest moon, on May 28, from 2:50 a.m. to 3:02 a.m. PDT. Set up your scope several minutes early to identify the four satellites visible before the eclipse gets underway. On the east side of Jupiter, you’ll notice two moons very close together. (You may need to use a medium or higher magnification to split them.) The brighter member of the close pair is Ganymede, and the other is Europa. Closer in toward Jupiter on the same side is the innermost of the Galilean moons, Io. Both Io and Europa are on the far sides of their orbits and so are moving outbound, farther away from Jupiter. Ganymede is on the near side of its orbit, and so is moving inbound, toward Jupiter. Callisto, the faintest and outermost of the four Galilean satellites, happens to be near greatest elongation on the morning of May 28, farthest west of Jupiter, far out on the opposite side of the planet from the other three moons.
The eclipse of Io by Ganymede’s shadow, and many similar events, are occurring because these four moons revolve very nearly in Jupiter’s equatorial plane. Early in May 2021, Jupiter presents that plane edgewise to the sun, marking the beginning of spring for Jupiter’s northern hemisphere. The satellite orbits are also now presented nearly edge-on to the sun, so the moons are frequently casting shadows on one another.
You can also watch Ganymede and Europa move past each other. Before and during the eclipse, brighter Ganymede is slightly farther from Jupiter, and the more southerly of the close pair. As the eclipse of Io begins, Ganymede-Europa are 8.3 arcseconds apart. As Io fully emerges from shadow, Ganymede-Europa have closed to 5.8 arcseconds apart. Their closest encounter occurs after the eclipse, at 3:15 a.m., when Ganymede, moving inbound, passes 4.5 arcseconds south of outbound Europa.
Wishing you clear skies!
Robert Victor was a staff astronomer at Abrams Planetarium at Michigan State University. Although now retired, he is often involved in the production of their Sky Calendar. To subscribe or to view a sample, visit www.abramsplanetarium.org/skycalendar. Robert D. Miller, who provided the twilight charts, did graduate work in planetarium science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.