Two planets far outshine all the stars this month. May through June, Venus, at magnitude -4 in the west-northwest at dusk, attains peak altitude of its current apparition and sets a maximum of 2.6 hours after sunset.
Jupiter—magnitude -2.5, its brightest of the year—passes opposition to the sun and all-night visibility on May 8. Watch it climb from very low in the east-southeast at dusk on May 1 to 30 degrees up in the southeast by month’s end.
Next in brilliance at dusk in May are blue-white Sirius (magnitude -1.4) until its departure in the west-southwest, and three zero-magnitude stars: golden-orange Arcturus, high in the east to east-southeast; blue-white Vega, rising into view in the northeast; and yellow Capella, sinking in northwest.
In the morning: Jupiter, in the west-southwest at dawn, is easily the brightest morning “star” until it drops out late in the month. It sets 4-5 minutes earlier each morning, so you can still catch it at month’s end simply by looking before dawn starts to brighten. Next is Mars, hovering in the south while doubling in brightness from magnitude -0.4 to -1.2. Ranking after Mars are Arcturus, sinking in the west to west-northwest; Vega, just northwest of overhead; and Saturn, 15 to 28 degrees west of Mars. Other stars visible include Altair and Deneb, completing the nearly overhead Summer Triangle with Vega; Antares, low in the southwest, below the line joining Jupiter and Saturn; and Fomalhaut, low in the southeast. Mercury, brightening slowly from magnitude +0.4 to -0.6 May 1-21, is very low in the east to east-northeast in dawn twilight. In this poor apparition, binoculars are recommended.
May through July 2018 is a special time for planets! On the morning of May 1, Spaceship Earth is heading toward a direction in space about 17 degrees east (left) of Mars. As our planet follows its nearly circular orbit around the sun, we will overtake all three bright outer planets within 79 days, the shortest interval since 1984, and shortest until 2078. Each body will take its turn at opposition and all-night visibility, starting with Jupiter on May 8.
For events at dawn, we suggest viewing at least one hour before sunrise. For events at dusk, look one hour after sunset.
Tuesday, May 1, at dawn: The moon is in the southwest, just past full, with Jupiter 9 degrees to its lower right. Mars-Saturn are 15 degrees apart in the south. Mercury is rising just north of east. Dusk: Venus and Jupiter are 160 degrees apart.
May 2, at dawn: Antares is 8 degrees to the lower left of the moon. Jupiter 21 is degrees to the moon’s lower right. Dusk: Aldebaran, eye of Taurus, is 6 degrees to the lower left of Venus. Binoculars show the Hyades star cluster in same field as Aldebaran. Together, they form the head of the Bull.
Before this month is over, we’ll lose our views of four of winter’s bright stars as they sink into the western twilight glow. In order of disappearance, they are Rigel, Aldebaran, Sirius and Betelgeuse.
May 3 at dawn: Saturn is 16 degrees, and Mars 32 degrees, to the east (left) of the moon. Through a telescope, the phase of Mars matches the moon’s this morning, 88 percent. The dark marking on Mars known as Syrtis Major appears closest to the center of the Martian disk at 3:43 a.m., and 39 minutes later each successive morning.
May 4 at dawn: Saturn is 4 degrees to the left of the moon. A telescope shows Saturn’s rings tipped nearly 26 degrees from edgewise.
May 5 at dawn: Mars is 9 degrees to the lower left of the waning gibbous moon. Four planets, from Jupiter low in southwest to west-southwest, to Mercury rising just north of east, span 150 degrees.
Sunday, May 6, at dawn: Mars is 3 degrees to the lower right of the moon. Can you detect the bright South Polar Cap, or its cloud cover? Later this month, spring will begin in the Martian southern hemisphere, and the frozen-carbon-dioxide polar cap is near its maximum extent.
May 7 at dawn: The moon, just over half full, is approaching last-quarter phase. Note Mars 14 degrees to the moon’s right.
May 8 at dawn: The moon, in the southeast, is less than 90 degrees west of the sun, so it has passed last-quarter phase and appears as a fat crescent. Find Mars 26 degrees west of the moon. All night: Jupiter is at opposition and visible from dusk until dawn as Earth passes between that planet and the sun.
May 9 at dusk: Venus and Jupiter are within 150 degrees and closing.
May 12 at dawn: The last-day crescent moon is above the horizon one hour before sunrise, just north of east, 5 degrees up, 11 percent full. Dusk: Venus passes 4 degrees south (to the lower left) of second-magnitude Elnath (Beta Tauri, Bull’s northern horn) tonight and tomorrow.
Sunday, May 13: Look about 40 minutes before sunrise to catch the 5-percent crescent old moon just risen 5 degrees north of east. Binoculars may show Mercury 4 degrees to the left of the moon and slightly higher.
May 13-21 at dusk: Look for the red-supergiant star Betelgeuse, shoulder of Orion, 18 degrees to the lower left of Venus. If you look daily at same stage of twilight, Betelgeuse will appear about 1 degrees lower each evening. On what date will you last spot it?
May 16 at dusk: The first crescent moon, 4 percent full, is low in the west-northwest, 11 degrees below Venus.
May 17 at dusk: The crescent moon, 10 percent full, is within 7 degrees to the left of Venus and slightly higher. Telescopes show Venus 84 percent full. Can you still see Betelgeuse? Look 13 degrees to the moon’s lower left.
May 18 at dusk: “Twin” stars Pollux and Castor are 4.5 degrees apart, 10-13 degrees to the upper right of the moon. Venus is 18 degrees to the moon’s lower right. Procyon is 16 degrees to the moon’s left and a little lower.
May 19 at dusk: The twins are 11-16 degrees right of the moon. Procyon is 16 degrees to the moon’s lower left. After dark: Binoculars show the Beehive star cluster within 6 degrees to the moon’s upper left.
Sunday, May 20 at dusk: Venus and Jupiter are 135 degrees apart and closing.
May 21 at dusk: The first-quarter moon is half full. Regulus, heart of Leo, is a degree to the moon’s lower right. As we look toward the moon and Regulus tonight, we are looking out the “rear window” of Spaceship Earth, opposite to the direction we are moving around the sun. Later this evening, Venus has its northernmost setting of this year.
May 23: It’s the first day Saturn rises before Venus sets, but only by 5 minutes. With mountains surrounding us, it is difficult to find a spot to see both planets simultaneously—but the “window” of opportunity opens 5 minutes wider each day.
May 25 at dusk: Spica in Virgo is within 7 degrees to the lower right of the waxing gibbous moon.
May 26 at dusk: Jupiter is within 9 degrees to the moon’s lower left.
Sunday, May 27, at dusk: The moon is in the southeast, with Jupiter 6 degrees to its lower right.
May 28: The nearly full moon rises some 43 minutes before sunset. Can you catch the sun and moon simultaneously? At dusk, note Jupiter 18 degrees to the moon’s upper right, and Antares 11 degrees to the moon’s lower right.
May 29: The moon, just past full, rises about 13 minutes after sunset. At dusk, Jupiter is 30 degrees to the upper right of the moon. Antares, heart of Scorpion, is 10 degrees to right of the moon.
May 30 at dusk: Venus is 10 or 11 degrees below the “Twins” tonight and tomorrow, forming a nearly isosceles triangle.
May 31 at dusk: Venus and Jupiter appear 120 degrees apart tonight. As we approach the dates of the oppositions of Saturn and Mars in late June and late July, respectively, those planets are beginning to appear in late evening: Saturn now rises within two hours after sunset, and Mars rises just two hours after Saturn.
The website of the Astronomical Society of the Desert at www.astrorx.org has a listing of our evening star parties at two locations. The primary, more accessible location is at the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument (on Highway 74, within 4 miles south of Highway 111 in Palm Desert). Our last sky viewing session for the season at that location is scheduled on Saturday, May 19, from 8 to 10 p.m.Sawmill Trailhead, our high-altitude site (elevation 4,000 feet), will have a star party starting at dusk on Saturday, May 12. (Star parties at that location will continue monthly.) Listings on the society’s website includes maps and directions for both locations. Star parties can be cancelled in poor observing weather. Also, check the link to “Impromptu Star Parties” on that website for star parties announced on short notice. There will be some in Palm Springs during May.
The Abrams Planetarium is making available online the May 2018 Sky Calendar with an evening sky map. It may be reprinted for free distribution. To download the calendar and map, visit www.abramsplanetarium.org/skycalendar.
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing sky-watching opportunities for a variety of groups in the Coachella Valley. Robert D. Miller 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.