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28 Feb 2018

March Astronomy: The Month Brings Two Full Moons—and the Start of Spring!

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As was the case back in January, March 2018 has two full moons. The first one almost coincides with its rising at sunset on the March 1. The second full moon of the month, sometimes called a “Blue Moon,” occurs in morning twilight on the 31st, before its setting just after sunrise. The moon will not be unusually close this time, nor will there be an eclipse, so there won’t be much media hype about this year’s second “Blue Moon” on March 31.

In March 2018, the predawn sky continues to host the three bright outer planets, all fitting within a span of 44 to 46 degrees. Starting with the full moon on March 1, follow the waning moon each morning through the 15th, and watch it pass above bright Jupiter on March 7, then skip from right to left of Mars March 9 and 10, and from right to left of Saturn March 10 and 11.

In the early evening, Venus can be spotted as a bright point of light very low in the western twilight glow, its visibility improving as the time of its setting after the sun broadens from 57 minutes to 93 minutes. Mercury can be spotted in the same binocular field as Venus for first three weeks, but fades rapidly after mid-month. Catch the moon at dusk on March 18 as a thin crescent low in the west, to the lower left of Venus and Mercury. Then follow moon nightly until March 31, when it’s full, rising just south of east within a half-hour after sunset. After Venus, evening’s most prominent point of light is Sirius, the “Dog Star,” the in southern sky at dusk in March.

Our morning chart for March depicts daily positions of all three bright outer planets and the half-dozen stars of first magnitude or brighter visible all month about 40 minutes before sunrise, before they fade into twilight. Jupiter, of magnitude -2.2 to -2.4 in the south-southwest to southwest, is the brightest “star” at dawn. The brightest actual stars are golden Arcturus, high in the southwest to west, and blue-white Vega, very high in the east-northeast to northeast. Saturn, of magnitude +0.6 to +0.5 in the south-southeast, glows about 45 degrees east of Jupiter. Mars, brightening from magnitude +0.8 to +0.3 and starting to outshine Saturn, strangely seems to hover nearly stationary in the sky all month, just more than 30 degrees up in the south-southeast to south. But Mars is really shifting nearly 0.6 degrees per day east against the background of stars, which appear to be sliding westward behind the planet. Watch Mars narrow its distance west of Saturn from 15 degrees on March 4, to 10 degrees on March 14, 5 degrees on March 24, and 2 degrees on March 30. Against stars on March 29, Mars passes within 1.9 degrees north of (above) third-magnitude Lambda, or Kaus Borealis, the northern star of the bow of Sagittarius, the Archer.

Look about an hour before sunrise to catch these events:

March 1: Regulus is 4 degrees to the lower right of the full moon.

March 4 and 5: Spica is 10 degrees to the lower left, then 8 degrees to the lower right, of the waning gibbous moon.

March 7: Jupiter is 4 degrees to the lower right of the moon, now two-thirds full.

March 8: Antares, heart of Scorpius, is 9 degrees below the moon.

March 9: Mars is 6 degrees to the lower left of the last quarter (half full) moon. Also, Jupiter, two months before its opposition, is stationary against background stars 8 degrees east of third-magnitude Alpha in Libra, and begins to retrograde.

March 10: Mars is 7 degrees to the right of the fat crescent moon. Saturn is 6 degrees to the lower left of the moon.

March 11: Saturn is 6 degrees to the right of the moon.

March 15: This is the last chance for the thin old crescent moon, very low in the east-southeast in twilight.

March 31: Spica is 15 degrees to the upper left of the moon.

In the evening: Have you ever seen Mercury? Here’s your chance! Our solar system’s innermost (and smallest known) planet has its best evening appearance of this year in the first three weeks of March. It sets in a deeper stage of twilight than it will during either of its two other evening showings, in July and November. And in March, Mercury will be very easy to find, within the same binocular field as Venus. All you need is a clear sky very low in the west, and an unobstructed view, without nearby high mountains blocking your line of sight toward the planet duo. Choose your viewing site carefully!

On March 1, Mercury will shine at magnitude -1.3, only 1.7 degrees to the lower right of 10-times-brighter Venus, at magnitude -3.9. On March 2, they’re 1.3 degrees apart, and on March 3, Mercury passes within 1.1 degrees to the right of Venus, with the pair setting just within an hour after sunset. Night by night, Mercury creeps a little farther to the upper right of Venus, to 2 degrees by March 6, on to a maximum of 4.1 degrees of Venus on March 14.

On March 15, Mercury, still bright at magnitude -0.3, reaches greatest elongation, 18 degrees from the sun, and at the end of its apparent orbit as we observe it from Earth. Mercury is then rapidly approaching Earth and begins to curve around to the near side of its orbit, where telescopically, it presents an ever-thinner crescent, fading as the planet’s surface features, large and small, cast shadows. Faded to magnitude +0.3, Mercury reaches a secondary minimum distance of 3.8 degrees to the upper right of Venus on March 18, the same evening when the 37-hour-old crescent moon passes 4 degrees to the south (lower left) of brilliant Venus.

On March 20, spring begins at 9:15 a.m., as the sun passes directly over the equator. That evening, the two inner planets set together, Mercury faded to magnitude +0.9, appearing 4.1 degrees to the right of Venus. Thereafter, Mercury quickly drops into bright twilight and fades. On March 21, it has faded to magnitude +1.2 and appears 4.5 degrees to the lower right of Venus. On the 22nd, Mercury is 5.2 degrees to the lower right of Venus, and is a challenging magnitude +1.5.

Orion’s three-star belt, not shown because its stars are of just second magnitude, is prominent in the south to southwest at dusk. It is flanked by red Betelgeuse, his shoulder, and blue-white Rigel, his foot. Extend the belt eastward to Sirius, the brightest star. Extend the belt in the opposite direction, and bend north a bit to Aldebaran, eye of Taurus. Some 14 degrees farther, locate a compact cluster of stars, the Pleiades, or Seven Sisters, a beautiful sight for binoculars! Procyon, the lesser Dog Star, completes the Winter Triangle with Betelgeuse and Sirius. North of the Winter Triangle, Orion and Taurus lie Pollux and Castor, the twin stars of Gemini, just 4.5 degrees apart, and Capella, the Mother Goat Star. Regulus, heart of Leo, follows the whole shebang across the sky, as if chasing his dinner menu items.

The moon is near bright objects in evening sky:

March 1: Regulus is 13 degrees to the upper right of the full moon.

March 18: Venus and Mercury are 4.1 and 7.6 degrees, respectively, to the upper right of a young crescent moon low in the west.

March 22: Aldebaran is 2 degrees below the moon.

March 25: Pollux is 9 degrees north of the moon; Procyon is 14 degrees south of the moon.

March 27 and 28: Regulus is 6 degrees to the lower left, then 8 degrees to the upper right, of the moon.

March 31: Before the end of twilight, watch for the rising of Spica, within 8 degrees to the lower right of the moon, just past full.

On Friday, March 2, Dennis Mammana, author of six books on astronomy and the “Stargazer” nationally syndicated newspaper column, will present a lecture at the Portola Community Center, 45480 Portola Ave., in Palm Desert. Doors open at 6:30 p.m. for refreshments, and the talk begins at 7 p.m.

The website of the Astronomical Society of the Desert has a listing of our lectures, and of our evening star parties at two locations. The primary 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). A sky viewing session is scheduled there on Saturday, March 10, from 6 to 9 p.m. Sawmill Trailhead, our high-altitude site (elevation 4,000 feet), will have a star party starting at dusk on Saturday, March 17. Star parties can be cancelled in poor observing weather. See also the link to “Impromptu Star Parties” on the website.

Also, check out the Sky’s the Limit Observatory and Nature Center in Twentynine Palms, at

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.

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