In May, four of the 16 stars of first magnitude or brighter visible from Southern California begin their annual leaves of absence, sinking into the western twilight glow.
In order of departure, they are Rigel, Aldebaran, Sirius and Betelgeuse. In June, Procyon, Capella and Pollux will follow—all the stars of the huge Winter Hexagon, with Betelgeuse inside.
It’s enjoyable and relaxing to look for these stars within an hour after sunset on clear spring evenings. Those who watch regularly are certain to notice the stars appearing lower each evening at the same stage of twilight, and eventually dropping out of view. This change is a direct consequence of Earth’s annual revolution around the sun.
As seen from Earth from late April through mid-July, the sun appears to move from Aries through Taurus into Gemini, causing these zodiac constellations and their neighbors to sink into the evening twilight glow, and, after several weeks, to reappear in the eastern sky at dawn. (Exception: Far northern Capella emerges at dawn before it leaves the evening sky.)
The moon and naked-eye planets provide additional spice for skywatchers. In early May 2015, as many as four planets can be viewed simultaneously by observers with unobstructed views toward the west-northwest and east-southeast. During May’s first week, Mercury shines near magnitude 0, and on May 6, reaches its greatest angular distance from the sun this time around, 21 degrees. Around that date, Mercury also attains its highest position for this year at dusk. The best time to find it may be nearly an hour after sunset; look about 22 degrees to the lower right of brilliant Venus. Also on May 6, find bright Jupiter 45 degrees to Venus’ upper left. On May 6, wait until almost an hour and a half after sunset, when Mercury is just 3 degrees up in the west-northwest. Then turn around to find Saturn at about the same height above the opposite horizon, in the east-southeast.
Look a couple of minutes earlier each evening until May 11, and you’ll find Mercury and Saturn 5 degrees above opposite horizons, 1.2 hours after sunset. But by then, Mercury has faded to magnitude +1.0, and it will fade further in the following days.
Much easier than catching four planets simultaneously in May 2015 is viewing three, and you can do so in all of May and well into July! The span of Venus-Jupiter-Saturn starts out 159 degrees long on May 1, shrinking through 150 degrees on May 9, 135 degrees on May 22, and to 125 degrees on May 31. Venus and Saturn, the endpoints of the lineup, are both 10 degrees up 2.5 hours after sunset on May 1, improving to 23 degrees up at a very convenient viewing time of 1.3 hours after sunset by end of May.
As a wonderful bonus, these three planets are the most impressive for telescopic observation:Venusstarting in gibbous phase, two-thirds full at the start of May, passing through half full in early June, through ever larger and thinner crescent phases as the planet draws closer to Earth; Jupiter, with its cloud belts, and its four bright satellites discovered by Galileo (as were the phases of Venus); and Saturn,with its amazing rings, now 24 degrees from edge-on!
Excellent views of these showpiece planets make this a superb time to schedule evening sky watching sessions—star parties! The Astronomical Society of the Desert will host a star party on Saturday, May 30, at the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument on Highway 74, about four miles south of Highway 111 in Palm Desert. For more info, including dates of and maps to locations of star parties planned for this summer, visit www.astrorx.org.
If you haven’t been following the 5-months-long approach of Venus to Jupiter in the evening sky, start now! Venus and Jupiter are 50 degrees apart on May 1, closing to 35 degrees apart on May 16, and 20 degrees on June 1. The gap between the two brightest planets continues to narrow, to 10 degrees on June 14, to 5 degrees on June 21—and only 1/3 of a degree apart on June 30.
Many moon and planetary events in the evening sky in May and early Juneare illustrated on the Abrams Planetarium Sky Calendar. A downloadable PDF of the May issue, with an evening sky map, may be reprinted and distributed free of charge. Go to www.abramsplanetarium.org/skycalendar.
On May 1 and 2 in evening twilight, watch the waxing gibbous moon leapfrog past Spica. The full moon occurs on the evening of May 3. On May 4-6, two hours after sunset, watch the waning gibbous moon go past Saturn and Antares.
The moon returns to the early evening sky on May 19, as a thin crescent low in the west-northwest, 21 degrees to 22 degrees to the lower right of Venus. Two days later, on May 21, the moon passes 9 degrees to the lower left of Venus, which now forms an isosceles triangle with Gemini’s “Twin” stars Pollux and Castor. Jupiter is now 30 degrees to Venus’ upper left. On May 23, the fat crescent moon appears 6 degrees to the lower left of Jupiter. Catch the half-full first quarter moon near Regulus on May 24, and a gibbous moon near Spica on May 29.
On June 1, the nearly full moon appears in the southeast near Saturn and Antares, while Venus aligns with Pollux and Castor in the west-northwest. That same evening, Jupiter appears 20 degrees to Venus’s upper left, with just 29 days to go until their spectacular conjunction on June 30!
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs.