Astronomy Blue Moon

This page deals with the facts and news concerning the astronomy blue moon, a fascinating but highly confusing subject. Stay tuned with us, as we try to bring to you the latest about it!

May 2016 Astronomy: This Month Offers Epic Views and Alignments!

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 an astronomy blue moon and red Mars, at its brightest, team up on May 2

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 astronomy 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 an astronomy 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 astronomy 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 sky watching 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.

Fact Check 2016: Will a green moon be visible here?

Times-Union readers want to know:

I read on the Internet that on May 29, a “green moon” will be visible in the night sky. Is this true?

On March 25, a Facebook user published a meme claiming that on May 29 a rare “green moon” would appear for the first time since 1847 (other versions claimed the date would be April 20), and reported:

“For those who need information on how this works … All night long on Sunday, May 29th the seventh planet from the Sun, Uranus, will park itself near the moon.

The green giant is only 4 degrees away from the moon. The cosmic odd-couple will appear about four degrees apart in the sky — equal to 8 full moons side-by-side.

This week after darkness falls the near full moon acts as a convenient guidepost for finding Uranus. Credit: Starry Night Software/ A.Fazekas The green-colored ice giant has four times the width of Earth, but since it lies nearly 1.9 billion miles (3.1 billion kilometers) away from Earth, it’s barely visible to the naked eye — and only in very dark, pristine skies.

With the glare from the nearby moon, binoculars will be your best bet in spotting Uranus. Just look for a tiny greenish-blue disk in the field of view. By the way, the absorption of red light by methane in the atmosphere is what gives Uranus its cool coloring.”

This might sound scientific, but it isn’t. states that the photo of a green moon that accompanied the post appears to have been lifted from a Flickr account.

The user had Photoshopped a photo of the moon taken on Aug. 3, 2009, and, according to, posted a joking comment, “Who said the moon isn’t made of green cheese.”

The claim was similar to earlier rumors of an April “pink moon,” a phenomenon that referenced a real lunar event, although it didn’t involve a moon that was pink in color, found.

The credible science website Sea and Sky has a chronological listing of astronomical events in 2016. Six events are included for May 2016 (including a full, blue moon on May 21), but a “green moon” is nowhere to be found.

The original poster appeared to present the image as a joke, but it soon began spreading as true to Facebook users.

In addition, there were some cool astronomical events in 1847 — including a solar eclipse — but there’s no record of a green moon that year. And, considering that Neptune had just been discovered the year before, there was plenty of interest in astronomy, so it wouldn’t have gone unnoticed, the fact-checking websites noted.

While the “pink moon” meme had a marginal basis in fact, the “green moon” one was entirely humorous in nature,’s research found.

No astronomical events of note are expected for May 29, and the only remotely similar lunar phenomenon described by the Farmer’s Almanac is August’s “Full Sturgeon Moon” (also known as the “Green Corn Moon”).

That name, however, has nothing to do with the moon’s color, as described by the almanac and reported by

“The fishing tribes are given credit for the naming of this Moon, since sturgeon, a large fish of the Great Lakes and other major bodies of water, were most readily caught during this month.

A few tribes knew it as the Full Red Moon because, as the Moon rises, it appears reddish through any sultry haze. It was also called the Green Corn Moon or Grain Moon.”

Carole Fader: (904) 359-4635

Astronomy Blue Moon: What Does It Mean?

When the moon rises Tuesday night (Aug. 20), it brings us the August full moon and in addition, it will also technically be an Astronomy Blue Moon.

"But wait a minute," you may ask. "Isn't an astronomy blue moon defined as the second full moon that occurs during a calendar month? Tuesday’s full moon will be the only full moon of August 2013. So how can we call it a 'Blue' moon?"

Yet it still is an Astronomy Blue Moon, but only if we follow a now somewhat obscure rule of astronomy. In fact, the current "two full moon in one month" rule has superseded the rule that would allow us to call Tuesday’s full moon an astronomy blue moon.

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Confused yet? Well, as the late Paul Harvey used to say, here now, is the rest of the story:

An Astronomy Blue Moon: The Almanac Moon Rule

Back in the July 1943 issue of "Sky & Telescope magazine, in a question and answer column written by Lawrence J. Lafleur, there was a reference made to the term "Blue Moon." Lafleur cited the unusual term from a copy of the 1937 edition of the now-defunct Maine Farmers’ Almanac (NOT to be confused with The Farmers' Almanac which is still published in Lewiston, Maine).

On the Maine Farmers' almanac page for August 1937, the calendar definition of the Astronomy Blue Moon explained that occasionally "one of the four seasons would contain four full moons instead of the usual three."

"There are seven Astronomy Blue Moons in a Lunar Cycle of nineteen years," continued the Almanac, ending on the comment: "In olden times the almanac makers had much difficulty calculating the occurrence of the Astronomy Blue Moon and this uncertainty gave rise to the expression 'Once in a Blue Moon.'"

Unfortunate Astronomical Oversight

While LaFleur quoted the Almanac's account, he made one important omission: He never specified any date for the Blue Moon. And as it turned out, in 1937 it occurred on Aug. 21. That was the third full moon in the summer of 1937, a summer season that would see a total of four full moons.

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Names were assigned to each moon in a season: For example, the first moon of summer was called the early summer moon, the second was the midsummer moon, and the last was called the late summer moon. But when a particular season has four moons the third was apparently called a Blue Moon so that the fourth and final one can continue to be called the late moon.

So where did we get the "two full moons in a month is an Astronomy Blue Moon rule" that is so popular today?

Pruett's Mistake

Once again, we must turn to the pages of Sky & Telescope. This time to page 3 of the March 1946 issue.

In that issue, author James Hugh Pruett wrote the article "Once in a Blue Moon" in which he made a reference to the term "Blue Moon" and referenced LaFleur's S&T article from July 1943. But because Pruett had no specific dates to fall back on, his interpretation of the ruling given by the Maine Farmers' Almanac was highly subjective. Pruett ultimately came to this conclusion:

"Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called an Astronomy Blue Moon."

How unfortunate that Pruett did not have a copy of that 1937 almanac at hand, or else he would have almost certainly noticed that his "two full moons in a single month assumption" would have been wrong. For the Blue Moon date of Aug. 21 was most definitely not the second full moon that month!

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