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30 Apr 2016

May Astronomy: This Month Offers a Blue Moon, Epic Pairings and a Transit of Mercury Across the Sun

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Make necessary preparations to safely observe the transit of Mercury across the sun on May 9.

Jupiter is brightest “star” in evening sky this spring until Mars offers serious competition in late May, as the red planet presents its brightest and closest approach since 2005. The moon and Jupiter will pair up on May 14, while a “blue moon” and red Mars, at its brightest, team up on May 21.

On our chart depicting the sky at evening mid-twilight in May, we find two bright stars—Rigel south of west, and Aldebaran in the west-northwest—departing early in the month. The brightest star, Sirius, the Dog Star, is next to go, in the west-southwest. All that then remains of the Winter Hexagon will be the “Spring Arch” of Procyon, Pollux (with Castor 4.5 degrees to its right) and Capella. Orion’s shoulder Betelgeuse, below the arch, drops out by late May, soon after Sirius.

On the chart, bright Jupiter follows Regulus across the sky’s vertical north-south-overhead line, crossing it high in the south. Golden Arcturus climbs high in the east, while blue-white Spica is in the southeast, climbing toward the south. In the southeast, Mars first appears in evening mid-twilight around mid-month—and competes with Jupiter in brilliance—while Saturn and Antares follow about a week later. But you can see Mars, Saturn and Antares earlier in May, simply by observing later in the evening, or before dawn.

Low in the northeast in May’s evening twilight, bright blue-white Vega appears, followed by fainter Deneb to its lower left.

For illustrations of the following sky events in May, you are encouraged to download and reprint the free May 2016 Sky Calendar and evening sky map available at

May 6: The new moon occurs at 12:30 p.m.

May 7: At dusk, look for the young crescent moon, age 31-32 hours, very low in the west-northwest. Aldebaran, eye of Taurus the Bull, is just to the moon’s upper left, beautiful in binoculars! On May 8, the moon will be higher, to the upper left of Aldebaran.

May 9: The transit of Mercury is visible from sunrise until 11:42 a.m.

If proper equipment is used, and precautions are taken to avoid eye damage, you will be able to observe Mercury in silhouette against the disk of the sun. During this transit, the tiny dot cannot be detected by simply looking through a solar filter without magnification. Instead, use a telescope suitably protected by a certified safe solar filter, securely installed at the front end of the telescope, before sunlight enters the optical system. Use a magnification of at least 50 power, or use your telescope to project an image of the sun on a screen or a piece of white cardboard. Whichever method is used, be sure to remove the finder scope so no one will be tempted to look through it at the sun.

From the Coachella Valley, the transit will already be under way at sunrise. The planet passes closest to the center of the solar disk at 7:58:31 a.m., with the sun 25 degrees up in the east. At 11:39:06 a.m., the leading edge of Mercury will meet the edge of the sun. Egress lasts 3.2 minutes, until 11:42:18 a.m., when Mercury moves completely off the solar disk.

May 10, 11: From one evening to the next, the moon leaps over the line joining Pollux and Procyon.

May 13: The moon, just past first quarter phase, is in the afternoon and evening sky. Note Regulus, heart of Leo, above the moon.

May 14: Using binoculars a few minutes before sunset, can you spot Jupiter not far to the upper left of the moon. Also: This is Astronomy Day! You’re welcome to attend our star party that evening; details below.

May 15-21: Mars, going west one-third of a degree daily against background stars, passes closely north of Delta, brightest and middle star of three in the head of Scorpius. Two hours after sunset, Mars is the brilliant reddish object low in the southeast.

May 17, 18: The bright star near the moon is Spica, in Virgo.

May 21: The full “blue moon” and red Mars hang out together from dusk until dawn. In spring 2016, we have four full moons: On March 23, April 21, May 21 and June 20. The third full moon of four within the same astronomical season is called a “blue moon.” Also tonight: Mars is at opposition—as Earth overtakes Mars, we observe the red planet all night long, from dusk to dawn, in the direction opposite to the sun.

Today’s “blue moon” rises in the east-southeast around sunset, with Mars quickly becoming visible 6 to 7 degrees to the moon’s right. Within two hours after sunset, below the moon and Mars, look for Saturn with the twinkling red first-magnitude star Antares, “Rival of Mars,” about 7.5 degrees to Saturn’s right. For the rest of the night, these four bright objects form a striking quadrilateral, in clockwise order: moon, Mars, Antares and Saturn.

Also that night: Syrtis Major, the most prominent of the dark markings on Mars, lies near the center of the Martian disk as the planet reaches its highest position in our southern sky, when telescopic viewing is best. This feature was discovered by Dutch astronomer Christiaan Huygens, who included it on a drawing of Mars in 1659. He used repeated observations of the feature to estimate the length of a day on Mars. Because Mars’ day is slightly longer than Earth’s, around opposition, we see the same face of Mars about 36 minutes later on each successive night.

If you’re inclined to observe the predawn sky at this time of year, despite the early sunrises, you’ll find the triangle of Mars, Saturn and Antares sinking into the southwest; Arcturus in the west to west-northwest; the Summer Triangle of Vega, Altair and Deneb overhead; and Fomalhaut low in the southeast.

Star Parties

On Saturday, May 14, from 8 to 10 p.m., the Astronomical Society of the Desert will be hosting the last star party of the season at the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument, on Highway 74, within four miles south of Highway 111 in Palm Desert. The society’s website at has directions and a map to our year-round high-altitude star parties at Sawmill Trailhead. Also check the separate link to our “Impromptu Star Parties,” which could be announced on short notice at any time.

Robert C. Victor was 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. Robert D. Miller, who provided the twilight charts and the planet orbit 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.

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