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Articles by J. Alex Knoll

Just how rare are two full moons in one month?

The full moon lights up the night Friday, the second full moon of the month, a Blue Moon.
    The history of the phrase Blue Moon dates back several hundred years, but the meaning has evolved. As far back as the 16th century, it was an expression of absurdity. I’ll believe that when the moon is blue would have the same effect as saying I’ll believe that when hell freezes over.
    Atmospheric conditions can affect the color of the moon. Volcanic eruptions, dust storms and forest fires have filled the skies with enough airborne particulates to turn the moon blue. An 1883 eruption of the Indonesian volcano Krakatoa caused green sunsets and a blue moon for almost two years. So over time, the phrase became an expression of rarity, as in once in a blue moon.
    Perhaps it is this off-chance, ever-so-slight possibility that lent a dose of melancholy and longing, immortalized in Elvis’ ballad, Blue Moon. “You saw me standing alone, without a love of my own …” But just maybe that Blue Moon would deliver love and happiness.
    Then, in the 1980s, the meaning as a single month with two full moons went viral. In a 2012 column for Sky & Telescope Magazine, Philip Hiscock traces the Blue Moon phrase to a 1946 Sky & Telescope Magazine article that was then quoted in a 1980 radio broadcast of the NPR program Stardate. Read the story at
    By today’s definition, a Blue Moon isn’t all that rare. The moon travels through its phases from one full moon to the next over a period of 281⁄2 days. Whenever a full moon falls in the first few days of a month, it’s likely a second Blue Moon will follow at the end of the month. In fact, during leap year a blue moon can even fall in February. A Blue Moon happens on average once every two to three years. But every now and then you might have two Blue Moons within three months, when a typical 28-day February has no full moon; That would leave January with two full moons followed by March with two full moons itself. This will next occur in 2018.
    Blue Moons are in fact as predictable as clockwork. Every 19 years, in what’s called a Metonic cycle, the solar calendar and the lunar calendar are in synch. The ancient Greeks used this as the basis for their calendar, which stood until 46BC with the advent of the Julian calendar. Based on the Metonic cycle, 19 years from this Friday — or 235 lunar months — a Blue Moon will again fall on July 31.
    The next day, August 1, marks another calendar milestone — Midsummer, third of the four cross-quarter days midway between solstice and equinox. The actual midpoint of summer falls on August 7 this year. The day of midsummer, once a pagan holiday called Lughnasadh in honor of the waning, post-solstice sun god, was co-opted by the Church during the early spread of Christianity, becoming Lammas Day, the festival of the wheat harvest, celebrated August 1.
    Look to the west just after sunset for Venus and Jupiter. They’re still only six degrees apart and the two brightest star-like objects in the heavens. But they set within 45 minutes of the sun, and soon they will be lost in its glare.
    That leaves Saturn ruling the night sky. As bright as an average star, the ringed planet appears high in the south at sunset and doesn’t set until after midnight.

Can you spot the International Space Station?

As darkness deepens Thursday, you’ll find the first-quarter moon high in the south-southwest with the bright star Spica seven degrees to its lower right.
    Friday night the moon has pulled eastward and is equidistant from Spica to its right and Saturn to its left. Even closer to the left of the moon is the faint star Zubenelgenubi, which marks the fulcrum of the celestial scales Libra. Train binoculars on this star, however, and you’ll see that it is actually a double.
    Saturday the waxing gibbous moon is just a few degrees above and to the right of golden Saturn with the red star Antares, whose name translates to Rival of Mars, a little beyond the ringed planet. Sunday the moon, Saturn and Antares form a tight triangle, with Saturn a few degrees to the right of the moon and Antares about the same distance below the moon.
    Venus and Jupiter are the only other planets visible this week. They appear low in the west at twilight and are still within 10 degrees of one another this week. Regulus, the eye of Taurus the bull, is a little higher than the two planets, but they all creep a little lower each night.
    The near-full moon will limit the display from the Delta Aquarids meteor shower, which peaks in the wee hours between Tuesday and Wednesday, July 28 and 29. But with a little luck, you might still catch a few brighter meteors streaking across the sky. Best viewing is after midnight. The meteors can appear anywhere in the sky, but tracing their path backward they all appear to radiate from the constellation Aquarius.
    If you’re up before dawn Wednesday or Thursday, you might catch a fleeting glimpse of the International Space Station, which shines brighter than any star and moves as fast as a jet. Wednesday it is visible for six minutes, popping into view at 4:55am, 40 degrees above the northwest horizon and blinking out of sight just as quickly at 5:01am, 10 degrees above the east-southeast horizon. Thursday it appears 24 degrees above the north-northwest horizon at 4:02am and disappears five minutes later, 11 degrees above the east horizon. Learn more at

Nine-year, three-billion-mile mission to study solar system’s outer limits

As the sky darkens, Venus and Jupiter appear low in the west. While the gap between the two planets is growing, they are both inching toward Regulus, with Venus two degrees below the star Monday and Tuesday.
    Dusk reveals Saturn above the southern horizon with the three stars marking the head of Scorpius beneath it and Antares, the heart of the scorpion, a dozen degrees below.
    Mercury makes an early morning appearance low in the east-northeast a half-hour before sunrise. Don’t confuse Mercury with the much-dimmer star Aldebaran, the red eye of Taurus the bull, high above to the planet’s right. Aldebaran is visited by the thin waning crescent moon before dawn Sunday.
    While you’d need a massive telescope to see it from your back yard, eyes will be on Pluto this week, as NASA’s New Horizons spacecraft, launched in 2006, comes its closest to the distant planetoid Tuesday after a three-billion-mile journey. Back then, Pluto was still the ninth planet. As I wrote at the time:
    Thursday, January 19, NASA launched the space-probe New Horizons on a nine-year mission to our solar system’s outermost planet, Pluto. Discovered by Clyde Tombaugh in 1930, Pluto is also the last planet not viewed by a passing earthen spacecraft.
    Two weather delays were not the only pressures weighing on the launch. The project sparked debate over the craft’s plutonium-powered engines and a possible lift-off explosion. But as New Horizons safely cleared earth orbit, it carried another payload, one closing the cosmic loop: a tiny canister bearing cremated remains of Clyde Tombaugh.
    Over the next five months, New Horizons will study Pluto and its moons, of which there are at least four detected by the Hubble Space Telescope. According to NASA, astronomers hope to determine “where Pluto and its moons fit in with the other objects in the solar system, such as the inner rocky planets (Earth, Mars, Venus and Mercury) and the outer gas giants (Jupiter, Saturn, Uranus and Neptune).
    On the 14th, New Horizons will be within 7,800 miles of Pluto’s surface. But at almost three billion miles from earth, data sent from the probe, travelling at the speed of light, will take more than four hours to reach us.
    Thanks to the near-endless power of its plutonium engine, New Horizons will leave Pluto and delve farther still, into the heart of the Kuiper Belt, a region at the solar system’s outer limits made up of countless icy mini-worlds akin to Pluto.

No need for fireworks here

While you’re waiting for fireworks in the gathering darkness, impress your friends and family with a quick orientation of the celestial lights popping into view.
    First to emerge in twilight’s glare is Venus, low in the west, so bright you might confuse its twinkling with a jet high overhead. With a little more darkness, Jupiter pops into view a little to the right of Venus. To the upper left of the two planets is Regulus, the heart of Leo the lion. The three provide a good contrast in brightness, with Venus blazing at –4.6 magnitude, Jupiter outshining any star at –1.8 magnitude, and Regulus still quite prominent at 1.6 magnitude.
    The month began with Venus and Jupiter less than one degree apart, and on July 4th they are still within two degrees of one another. But they are parting ways, with Venus moving closer to Regulus and Jupiter inching to the northwest and the wake of the setting sun.
    By 9pm, Arcturus, the brightest star of summer is directly overhead. Shining at magnitude –0.1, it is the lead star in the constellation Boötes the herdsman, who tends the bears Ursa Major and Ursa Minor, which contain the Big and Little Dipper respectively.
    To the east of Boötes is the Hercules, most notable for its trapezoid-shaped keystone. Look for the minor constellation Corona Borealis, the Northern Crown, between Boötes and Hercules.
    To the east of Hercules are the three constellations that host the Summer Triangle. The smallest constellation, Lyra, hosts the brightest star, zero-magnitude Vega. From there look for first-magnitude Deneb at the head of the Northern Cross, Cygnus the swan. The final point in the triangle is Altair, the eye of the eagle Aquilla, shining at magnitude 0.8.
    Low in the south-southeast at sunset is golden Saturn at the head of Scorpius. The heart of the scorpion, Antares, shines a dozen degrees to Saturn’s lower left of Saturn twinkles fiery orange Antares, not quite as bright.
    Early risers can spot Mercury above the east-northeast horizon about 40 minutes before sunrise. Binoculars will help pick it out of the growing glow of dawn. Don’t confuse it for Aldebaran, the eye of Taurus the bull, much higher overhead.

Earth’s 23.5-degree axis gives us summer, winter and everything between

As evening twilight settles Thursday, look to the western horizon for the nascent crescent moon. Above it are Venus and Jupiter. The bright star Regulus is up there, too, forming a line with Venus and Jupiter, each roughly a dozen degrees from the next. Keep an eye on the two planets as they inch closer together over the next two weeks before a spectacular end-of-month conjunction when they are within one-third degree of one another.
    Friday and Saturday the moon, Venus and Jupiter form a loose triangle. By Sunday the moon is far to the left of the planets but is just a few degrees below Regulus and the Sickle of Leo. The Sickle, which looks like an inverted question mark, outlines the head of the lion.
    While Venus and Jupiter glimmer above the western horizon, Saturn shines off to the east. By midnight it is at its highest almost due south. Don’t confuse its steady golden glow for the brighter red star Antares, which is a dozen degrees below. With greater magnification there’s no confusion: Saturn’s rings appear as a distinct bulge with even binoculars, and seen through even a modest telescope the rings themselves come into view.
    You’ll need binoculars to spot Mercury, which is emerging from the sun’s glare before dawn low in the east-northeast. Early Wednesday morning provides the best view of Mercury as it reaches its greatest eastern elongation, its point farthest from the sun in our sky, when it will peak 22 degrees above the horizon and remain visible 45 minutes before obscured by daybreak. Mercury is bright, but a couple degrees below it the star Aldebaran shines brighter still.
    Sunday at 12:38pm marks the summer solstice, when the sun reaches its northernmost position in the sky. ­Because the earth spins at an odd, 231⁄2-degree angle as it orbits the sun, the Northern Hemisphere is currently positioned to receive far more sunlight than the Southern Hemisphere. On the summer solstice, the sun appears to stand still above the Tropic of Cancer, which straddles the earth 231⁄2 degrees north of the equator. After the solstice, the sun will slowly shift southward from day to day until six months from now it is at its southernmost extreme over the Tropic of Capricorn, which is 231⁄2 degrees south of the equator. The sun shifting from one extreme to the other is what causes our seasons here on earth. Were the planet to spin upright like a top instead of tilted, the seasons would never change, with perpetual summer along the equatorial band and growing ever darker and colder the farther north or south you went.

The sun follows its own clock

As darkness falls, first Venus then Jupiter pop into view in the wake of the setting sun. Venus blazes at magnitude –4.4, exponentially brighter than Jupiter at magnitude –2, which still outshines any star. The two planets are inching closer on their way to an end-of-month rendezvous. This week the gap between the two shrinks to 10 degrees — close enough to obscure both with your fist held at arm’s length.
    Keep an eye on Venus this weekend. It is near the center of the dim Y-shaped constellation Cancer, which in itself is not that interesting, boasting no star brighter than 3.5 magnitude. In fact, were it not placed on the ecliptic, the path of the sun, moon and planets as they circle overhead, Cancer would be a minor constellation, certainly not a denizen of the zodiac or even an astrological sign. Cancer’s claim to fame is no star but what appears as a faint, fuzzy patch of light to the unaided eye, M44, the Beehive Cluster.
    Viewed with binoculars, however, the Beehive comes to life with dozens of stars, and with a telescope it swarms with hundreds of stars. Friday and Saturday Venus is less than one degree from the Beehive in the western sky at nightfall and visible for at least an hour after sunset.
    Friday marks our earliest sunrise of the year, almost 10 days before the solstice. The discrepancy is the result of several factors, but in short, our clocks keep different time than the sun. While the clock measures a day with the passing of each 24 hours, a solar day is measured from the time of one high noon to the next. Hereabouts on the 12th, that is 24 hours and 15 seconds. Prior to the solstice this temporal slack is made up for at dawn, while after solstice the difference is at sunset. So while the summer solstice is June 21, our earliest sunrise is always some 10 days before, and the latest sunset another 10 days after solstice. In winter, the same phenomenon is at play, only reversed, and more pronounced, with nearly two weeks between earliest sunset and solstice and solstice and latest sunrise.

Seach the sky for Berenice’s hair and Ariadne’s crown

The moon wanes to last-quarter Tuesday, rising more than a half-hour later each night, providing an increasingly darker backdrop for sky-watching.
    As the evening sky begins to darken, the first lights to appear are the planets Venus and Jupiter high in the west. Then you might notice golden Saturn aglow in the southeast. The next brightest object to appear is Arcturus, almost directly overhead.
    Arcturus is the third brightest star in the heavens and is the lead star in the constellation Boötes. It is a red giant 36 light years away burning more than 100 times brighter than our sun. Its name is derived from the Greek word Arktouros, meaning guardian of the bear. Boötes follows Ursa Major along the ecliptic, while behind it is the constellation Hercules. Closer to either side of Arcturus, however, are two lesser-known constellations.
    To the east of Boötes shines a semi-circle of severn stars, Corona Borealis, the northern crown. By about 11pm, this constellation is almost directly overhead. In Greek mythology, this is the crown Dionysus gave to his bride Ariadne. Celebrating after their wedding, Dionysus threw the crown into the sky, where the jewels turned into stars and the crown became a constellation. The lead star in Corona Borealis is Gemma, almost as bright as the North Star.
    To the west of Arcturus is Coma Berenices, or Berenice’s Hair, most notable by three not-so-bright stars making a 90-degree angle. The legend of this constellation dates back to Queen Berenice II of Egypt, whose husband Ptolemy III Euergetes was away in battle. Praying to the goddess Aphrodite, Berenice swore to cut off her long, blonde hair if Ptolemy survived. Upon his return, the queen kept her word and placed her locks on an altar in Aphrodite’s temple. The next morning the hair was gone: the goddess of love was was so pleased with Berenice’s beautiful hair that she placed it forever in the heavens.
    Venus is at its best this week, ­Saturday reaching greatest eastern elongation — or in layman’s terms, its farthest from the sun, 45 degrees as seen from our earthbound vantage, and thus at its highest point in our sky. As the sun sets, look for the Evening Star high in the west. Hereafter, Venus ever so slowly inches toward the setting sun. By mid-August, Venus disappears behind the sun, reappearing in the pre-dawn sky a couple weeks later.
    Jupiter shines a dozen degrees to the upper left of Venus. The two planets are closing in on each other on the way to a close conjunction at the end of the month.
    Sunset reveals Saturn in the southeast, and by midnight it is high in the south. Even a modest telescope will reveal the planet’s famous rings, which are right now tilted at their best angle for viewing. Roughly 10 degrees below Saturn is orange Antares, the lead star in the constellation Scorpius.

Our atmosphere tints summer moons

The moon waxes through the weekend, reaching full phase Tuesday, June 2. This time of year the moon follows a low, lazy arc above the southern horizon. At such a low angle to the horizon, before reaching our eyes the moon’s light must cut through much more of earth’s atmosphere than in winter, when the moon shines high overhead. Gases and trapped moisture within the atmosphere combine to tint the image we see a red, orange or yellow, which explains the names of June’s full moon: the Strawberry Moon, the Rose Moon and the Honey Moon.
    Friday night, the moon is joined by Spica, the blue-white first-magnitude star of Virgo, which is just a few degrees below and to the right of the moon.
    Saturday the moon has pulled westward and is midway between Spica to its right and Saturn to its left. Sunday the moon is less than 10 degrees to the left of Saturn, and Monday it is just two degrees to the the left of Saturn with red-orange Antares a few degrees below.
    Having just reached opposition, its closest point to earth and dead-opposite the sun, Saturn rises as the sun sets, is high overhead at midnight and sets with daybreak. Shining at zero magnitude, Saturn is brighter than it’s been in eight years. Better yet, its rings are positioned to allow the best possible viewing and appear all the brighter so close to opposition.
    Saturn isn’t the only planet visible after dark. Sunset reveals Venus high in the west and Jupiter 20 degrees higher still. Night to night, Venus is gaining ground on Jupiter leading to a grand conjunction in late June. Monday Venus forms a near-stright line with the Gemini twins Pollux and Castor.
    By late-evening the stars of the Summer Triangle are perched above the east horizon. Farthest west is the brightest and the fifth-brightest star, Vega, of the constellation Lyra. Off to the southeast of Vega and almost as bright is Altair of Aquila the eagle. Closing the triangle is first-magnitude Deneb, the head of Cygnus the swan.

After almost 4,000 days in space, this probe died for science

This week Mercury shows its best face in homage to the Messenger spacecraft, which crashed into the planet early morning April 30.
    The craft was launched in August 2004 and reached Mercury in March 2011, the first to orbit the innermost planet. Since then it has circled Mercury more than 4,100 times, compiling more data in the process than everything combined before that.
    Thursday, April 30, scientists at Johns Hopkins Applied Physics Lab cut life support on their prodigy, and shortly thereafter the craft fell from orbit, plummeting to the surface at more than 8,750 miles an hour. Messenger was already running on borrowed time, having exhausted its traditional fuel supply, but programmers were able to supplement it with helium onboard for other reasons. Now that, too, is exhausted.
    “I guess the end is coming,” the Messenger team posted earlier on Twitter “After 10 years, spacecraft will end life as just another crater on Mercury's surface.” A big crater, more than 50 feet wide.
    The elusive planet is not only difficult to spot, it had been difficult to study before Messenger. Less than 45 percent of Mercury’s surface had even been photographed, and that decades ago. In its four years in orbit, Messenger sent more than a quarter-million photos back to earth. it found volcanoes, discovered polar caps of frozen water and studied Mercury’s chemical makeup.
    Mercury “is crucial to developing a better understanding of how the planets in our solar system formed and evolved,” NASA explains on its Messenger website. “Mercury is an extreme: the smallest, the densest, it has the oldest surface, the largest daily variations in surface temperature — and it’s the least explored.”
    Mercury reaches its greatest elongation from the sun on May 7, climbing 20 degrees above the horizon. Look for it low in the west just after sunset this week. You may need binoculars close to sunset when it’s at its highest, but by 9pm it should easily be visible as it prepares to set. Much-brighter Venus is 20 degrees higher, and Aldebaran, the red eye of Taurus the bull is 10 degrees to Mercury’s left.
    These are the last nights to see Mars, which is even deeper in the glare of sunset than Mercury. Jupiter shines high in the south at nightfall, while Saturn rises around the same time. Look for the just-past-full moon near Saturn Monday and Tuesday night.

The moon visits the Beehive Cluster and more

The waxing moon reaches first-quarter phase Saturday, shining below and to the left of Jupiter. The moon is near the center of the constellation Cancer.
    Were the crab not on the ecliptic, it’s doubtful it would hold its place in the zodiac, as none of its stars are brighter than 3rd magnitude. (The ecliptic is the path of the sun, moon and planets as they circle through our skies.) But what it lacks in bright stars it makes up for in sheer quantity, as it hosts the Beehive Cluster. The Beehive is near the center of Cancer, about halfway between Regulus in Leo and Pollux in Gemini. Seen with the unaided eye, it appears as a dull smudge of light. However, binoculars or a small telescope reveal dozens of individual stars. But there’s even more to the Beehive than that, as it is a stellar incubator with thousands of infant stars.
    With binoculars at hand late Saturday and early Sunday, see if you can spot the ninth-magnitude asteroid Juno just above the moon. Your best chance is around 2am when the moon is about to set and the two appear above the west-northwest horizon.
    Sunday the moon is to the lower left of Jupiter, while above and to the left of the moon is the first-magnitude star Regulus, the three forming a near-perfect triangle. By Monday the moon lies just four degrees below Regulus.
    The five naked-eye planets are visible this week. Venus blazes in the west in evening twilight and sets in the west-northwest nearly two hours after dark. Scour the horizon below Venus for Mercury, appearing 30 minutes after sunset. The innermost planet is surprisingly bright, unlike Mars, which is just a few degrees to Mercury’s upper left. Jupiter shines high in the south as darkness falls and sets a little after 2am. Saturn rises with Scorpius around 10pm and is high in the south come dawn.