As twilight fades to night these May evenings, Mars stands out in the northern sky. It begins the month near the center of Cancer the Crab not far from the magnificent Beehive star cluster (M44). The planet passes less than 1° north of the cluster during May’s first week. Mars’ eastward motion carries it into Leo the Lion on May 26, and it ends the month slightly to the left of Leo’s famous Sickle asterism.
The Red Planet fades a bit during May, dipping from magnitude 0.9 to 1.2, but it remains prominent in part because of its striking color. Mars no longer appears great through a telescope, however, because its disk has slimmed to just 6″ in apparent diameter. Wait for moments of good seeing to view any details.
Jupiter shines at magnitude –1.9 in mid-May. It gleams 16 times brighter than Mars and appears conspicuous in the northwest after nightfall in early May. The giant planet sets during twilight as the month winds down, however. Jupiter lies among the background stars of eastern Taurus, passing 1° north of the Crab Nebula (M1) at midmonth.
A telescope reveals the gas giant’s 33″-diameter disk and two dark belts, one on either side of a brighter zone that coincides with the planet’s equator. Jupiter’s four bright moons — Io, Europa, Ganymede, and Callisto — also show up in the smallest of instruments.
Mars sets before midnight local time, leaving the sky free of naked-eye planets for several hours. The drought ends by 4 a.m. local time when Venus and Saturn clear the eastern horizon. The two stand 4° apart at the start of May.
Venus shines at magnitude –4.7 and makes an unmistakable beacon in the predawn sky. The planet lies among the background stars of southern Pisces the Fish during most of May, though it manages to clip the corner of Cetus the Whale on the 12th. Venus ends the month near its greatest elongation from the Sun, which will take place June 1.
A telescope shows the planet’s quickly changing size and phase. On May 1, Venus’ disk spans 36″ and appears 29 percent lit. At month’s end, the planet measures 24″ across and the Sun illuminates 49 percent of its Earth-facing hemisphere.
Saturn remains in Pisces all month, though it moves eastward more slowly than Venus and thus the two drift apart. By the 31st, nearly 25° separate the two planets. Saturn shines at magnitude 1.1, just 1 percent of Venus’s luminosity though still far brighter than any of Pisces’ stars.
With Saturn climbing higher in a dark sky every day, it puts on a great show through telescopes of all sizes. The rings remain backlit at the start of May, with Earth south of the ring plane and the Sun north of it. The Sun crosses to the south side on the 6th, returning the ringed planet to a more normal view that will last until we cross the ring plane again in October 2038. On May’s final morning, Saturn measures 17″ across while the rings span 38″ and tilt 3° to our line of sight. Also keep an eye out for 8th-magnitude Titan, Saturn’s brightest moon.
Our final planet graces the morning sky during the first three weeks of May. Mercury appears highest on the 1st, when it rises two hours before the Sun and lies about 15° below the Venus-Saturn pair. Mercury then shines at magnitude 0.1 and grows brighter as the days pass.
The best views through a telescope come early in the month, when the inner planet shows a slightly gibbous disk that spans 7″. Mercury disappears during May’s final week as it reaches superior conjunction on the 30th.
The starry sky
I’ve been presenting planetarium shows for decades now, and I always delight in projecting the current evening sky and pointing out the constellations. Sadly, most of the star patterns don’t strongly resemble the object or creature they were named for, and some seem to have no connection at all.
So, what constellations are the easiest for beginning stargazers to identify with their namesakes? Let’s start with those visible in our current evening sky.
Undoubtedly, Crux the Cross tops the list. The tiny constellation appears conspicuous now because it lies nearly overhead at midevening. Not far to Crux’s lower left is another easy one: Triangulum Australe. Its three brightest stars form a nearly perfect isosceles triangle. The angular distance from Alpha (α), at the bottom in our current evening sky, to Beta (β) is 7.7°, while from Alpha to Gamma (γ) is 8.1°.
Swing around to the eastern sky and you’ll find two more constellations you should have little trouble identifying. The dazzling stars of Scorpius the Scorpion make it a breeze to recognize. Corona Australis the Southern Crown proves a bit harder because its stars are not as bright. Look for it below the Scorpion’s tail as the two climb through the eastern sky.
Once you see the Scorpion’s shape, you might be surprised to learn that some observers see a different shape in its stars. Some Aboriginal people of northern Australia think it resembles a crocodile.
Dare I add another? Chamaeleon the Chameleon may be on the obscure side, but it definitely looks like an animal to me — though admittedly it’s upside down on May evenings. Look for it about 20° below Crux. You’ll need a dark sky to spot this constellation’s faint stars.
Star Dome
This map portrays the sky as seen near 30° south latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.
The all-sky map shows how the sky looks at:
9 P.M. May 1
8 P.M. May 15
7 P.M. May 31
Planets are shown at midmonth
