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Articles by J. Alex Knoll

Its far side is always dark to us

The dark hours at week’s end are still brightened by the glow of the waning Hunter’s Moon, which rises mid-evening and dominates the night sky until daybreak. On clear days this week, you may even see the moon in the west after sunrise.
    Over the weekend, the moon travels with the constellation Taurus. Friday it is 10 degrees to the right of the Pleiades star cluster while the bull’s red eye Aldebaran is a little farther below the moon. The brightests stars of the Pleiades form a small but distinct dipper, which makes up the bull’s back. Saturday the moon is much closer to Aldebaran and the Hyades star cluster, which makes up the bull’s face. If the moonlight is too much to discern these stars, wait a day or two until the waning moon has shifted to the west.
    Just like here on earth, half the moon is always illuminated by the sun and the other half facing away from the sun. But as the angle between the sun, earth and moon changes, so does the portion of the moon’s illuminated face that we can see. With the moon waning, the angle is closing, obscuring more of the lunar surface behind earth’s shadow. This darkened section of the moon still faces earth and should not be confused with the so-called dark side of the moon. Better to think of that as the far side of the moon, which faces away from earth. The far side is still bathed in sunlight — we are just never in a position to witness it.
    The moon rotates on its own axis, with one side facing the sun for about two weeks and then facing away from the sun the next two weeks. Over billions of years, earth’s stronger gravitational pull has slowed the moon’s rotation to the point that it spins in synch with its pace around the earth. As a result, one side of the moon faces earth only during new phase, when it is between us and the sun, obscured by the light of day. So we never see the far side of the moon.
    Mars and Saturn pop into view in the wake of the setting sun. Saturn is sinking fast and is visible for less than an hour. Mars is well to the east of Saturn but not quite as bright. Don’t confuse it for the similarly hued star Antares, the heart of the Scorpion wriggling below.
    Jupiter rises around 2am and is high overhead in the east as morning approaches. Over the next month this gaseous giant climbs higher and grows brighter in our pre-dawn sky.

Earth’s shadow blots out this week’s full moon

With sunrise now after 7am, perhaps you’ve seen an exceptionally bright light in the south before dawn? Looking up, did you see the constellation Orion? That’s Sirius, the brightest star in the heavens, known as the Dog Star for its place amid Canis Major. Sirius rises around 2am in the east-southeast. Close to the horizon, it pulses a dazzling rainbow of colors, its light refracted by earth’s atmosphere like that from a prism. Closer to daybreak, when Sirius is high in the south, its light cuts through less of our atmosphere and appears a brilliant, cold white. As if Sirius didn’t stand out from its brightness alone, the three belt stars of Orion point straight down to the Dog Star.
    Perhaps, though, you’re looking due east in the hour before sunrise. In that case you’re seeing Jupiter, which outshines even Sirius. The giant planet rises around 3am, and by 6am it is high in the east. Below Jupiter is the constellation Leo with its upper body looking like an inversed question mark with Regulus at the bottom.
    You’ll want to get up before dawn Wednesday to catch the total lunar eclipse of the full, Hunter’s Moon. An eclipse of the moon only occurs during full moon, when it is aligned just so with the earth and sun, which casts earth’s shadow over the lunar surface. The process begins after 4am as the moon grows darker under the outer, penumbral, shadow. A little after 5am, the full, umbral, shadow begins to take a growing bite from the moon’s left edge until it is completely over-shadowed by 6:30. Mid-eclipse is at 6:54, after which point the rising sun breaks the spell.
    At the other side of darkness, Mars and Saturn appear at sunset. Saturn is fast on the heels of the sun, low in the southwest at dusk. Night by night the ringed planet is sinking lower and inching closer to the sun until it disappears around month’s end. Mars trails Saturn by roughly 20 degrees. Normally its red hue makes it easy to spot, but you may think you’re seeing double, as Mars is less than five degrees above another red light, Antares, the heart of Scorpius, whose name means rival of Mars. Over the coming weeks Mars creeps higher while the scorpion sets from view for winter.

The equinox ushers in fall

Week’s end finds the waning crescent moon in the company of Jupiter before dawn. Around 6am Friday morning, look for the moon high in the east with Jupiter to its lower left. The same time Saturday the moon shines just six degrees from Old Jove. Then Sunday, the now razor-thin crescent is well below Jupiter, while the first-magnitude star  Regulus, is just six degrees away.
    While you should have no trouble spotting the waning crescent moon and Jupiter in the east before dawn, Venus is a trickier target. This Morning Star rises less than an hour before the sun, and that window of visibility shrinks by about a minute each day.  At best Venus is only 10 degrees above the horizon before sunrise, so you may need to scour the eastern skyline with binoculars to pinpoint Venus’ otherwise dazzling glow.
    This time of year before dawn offers the best chance to spot the eerie zodiacal light also called false dawn. You need dark skies away from any urban glare to see the zodiacal light, which glows like milky pyramid of light rising from the horizon an hour or two before actual dawn.
    Unlike true dawn, the zodiacal light is a pale, ghostly glow devoid of the rosy tint from the coming sun, which is caused by light entering earth’s atmosphere. The zodiacal light is actually sunlight reflecting off countless bits of dust and detritus within our solar system that orbit the sun along the same path as the planets. This time of year the ecliptic — the path of the sun, moon and planets — stands nearly straight up with respect to the eastern horizon before dawn.
    At the other end of darkness, Mars and Saturn shine low in the southwest in the early evening. Of the two, Saturn is slightly brighter and is farther west, setting around 9:30pm. Mars isn’t far behind, setting shortly after 10. But while the ringed planet is weeks away from disappearing amid the glare of the sun, Mars remains a fixture in our early evening skies for weeks to come.
    A clear view to the west-southwest immediately following sunset may reveal Mercury burried in the horizon. Binoculars will help, but don’t confuse it with nearby Spica, which is only a couple degrees away through the weekend. They are so close that both will appear in the same field of view using binoculars or a telescope.
    Monday at 10:29pm EDT, the sun is poised directly above the equator somewhere in the vicinity of New Guinea. On that day or the following, the sun rises due east and sets due west, dividing the day between near-equal amounts of daylight and darkness for everyone around the globe. For the 90 percent of us living in the Northern Hemisphere, this equinox marks the beginning of autumn.
    Because of the earth’s 231⁄2-degree tilted axis, the Northern and Southern Hemispheres each receive more direct sunlight and warmth than the other for half the year. Twice a year, earth’s tilted axis and its orbit around the sun come together just so that the amount of light and dark are equal around the globe. Hereafter our time in the sun will grow shorter each day as the sun creeps ever southward of due east until reaching winter solstice in late December, its farthest point south in our skies.

A pair of planetary pairings

The waning moon rises in the late evening at week’s end and is high in the southwest with the approach of the rising sun. Each night it rises a half-hour later, so that by last-quarter on the 15th it crests the northeast horizon at midnight.
    The moon Saturday night and Sunday before dawn shines a few degrees below the smallest of the three celestial dippers: the Pleiades star cluster. This grouping of stars marks the shoulder of Taurus the bull and is a few degrees to the northwest of Aldebaran, the bull’s red eye. Aldebaran marks one end of another star cluster within Taurus, the Hyades, which is the V-shape of the bull’s face.
    Monday before dawn the moon trails just a few degrees behind Aldebaran. Early morning Tuesday the moon shines less than 10 degrees above Betelgeuse, the shoulder of Orion the hunter.
    Far to the east of Orion is Jupiter, which rises around 3:30am and is high overhead as daybreak approaches. By that time you might be able to spot Venus low against the horizon. The Morning Star is exponentially brighter than Jupiter, but so close to the horizon and the approaching sun that you may need binoculars to pick her out of the haze. Aligned halfway from Venus to Jupiter is Regulus, the blue-white heart of Leo the lion.    
    The other two planets visible to the unaided eye, Mars and Saturn, form a line of their own with the star Antares to the southwest in the evening sky. The two planets shine at nearly the same magnitude, but their colors make it easy to tell them apart. Not so with Antares, whose name literally translates to Rival of Mars. The red heart of Scorpius is farther to the east and is not as bright as the red planet. In the coming days Saturn sinks ever closer to the horizon, while Mars gains ground moving to the east. By month’s end Mars will be just a few degrees from Antares.
    Summer may be on the wane, but the season’s stars still command the evening sky. As the sun sets, look directly overhead for the zero-magnitude star Vega in the constellation Lyra. By 10pm first-magnitude Deneb, the head of Cygnus the swan, has taken the perch atop the celestial zenith. South of Vega and Deneb is Altair, the eye of the eagle Aquila, and the third point in the Summer Triangle.

Close as Mercury, far as Neptune

The moon waxes through our evening skies from a thin crescent at week’s end to first-quarter Tuesday September 2. Friday Luna shines just two degrees above the first-magnitude star Spica low in the southwest.
    Sunday the moon appears farther east at sunset, forming a tight triangle with Saturn to the west and Mars to the south. The two planets appear equally bright, shining at magnitude 0.6, but Saturn’s golden glow and Mars’ red hue make them easy to tell apart.
    Monday evening the moon shines at the head of Scorpius, which is marked by three slightly mis-aligned stars. The moon is just one degree above the northernmost of the three, Graffias, shining at magnitude 2.5. Ten degrees southeast of the moon is the red giant Antares, the heart of the scorpion.
    As the sun dips beneath the horizon around 7:40, look in its wake for Mercury low in the west. Binoculars may help you pinpoint this elusive planet. When most people spot Mercury, they are surprised by its brightness, shining around zero magnitude, brighter than most stars. But the innermost planet orbits so close to the sun that it never appears more than a dozen degrees above the horizon during dark hours.
    Mercury has been a fixture of our night sky since the dawn of civilization. The first telescopic observations of the planet were made by Galileo in 1610, but unlike his viewing of Saturn, there was no eureka moment. It was not until more than 350 years later with the fly-by of the Mariner 10 space probe that astronomers learned much more about this elusive planet.
    You may be able to see the farthest planet from the sun, Neptune, before dawn this week. On Friday the outermost planet reaches opposition, when earth is directly between it and the sun. Even with binoculars or a small telescope it will appear as little more than a small blue dot low in the west-southwest amid the dim stars of Aquarius.
    Venus and Jupiter will greet you in the east before dawn amid the glow of the coming sun.

Escape urban lights to see this sight

     Each August, as the kids head back to school, the galaxy is tilted  in such a way that the Milky Way stretches overhead in full glory. With Monday’s new moon, this may be the best week of summer to gaze on this river of stars. To fully appreciate it you’ll need dark skies away from any urban glow well after sunset. Give your eyes a few minutes to get accustomed to the darkness, tilt your head back and get lost in the glow.
    From Perseus and Cassiopeia in the north-northeast, the Milky Way flows down through Cygnus the swan onto Aquila the eagle. From the eagle’s tail, the glow of stars splits, one section continuing to Sagittarius, the other to Scorpius in the southwest; the dark space in between is called the Great Rift. But this patch of the heavens is not bereft of stars. Our view of them is blocked by masses of interstellar gas and dust. Of course it isn’t just the flowing river of stars that make up the Milky Way but almost every star we see with the unaided eye, including our own sun. All are part of the same spiral galaxy. Our solar system is at the end of one of the spiral’s arms. When we look at the river of stars, we are looking toward the center of the galaxy, through layers of light that combined form the glowing band that we see on a beautiful dark night.
    As evening twilight gives way to darkness, Mars and Saturn appear in the southwest. Mars has been inching toward the ringed planet night by night and will pass below it over the weekend, coming within three degrees. The two planets appear equally bright, but you should have no trouble telling Mars’ reddish hue from Saturn’s golden glow. While you’re comparing colors, look a few degrees to the north of the two planets for the star Zubeneschimali in the constellation Libra. This is the only star with a greenish glow visible to the unaided eye — at least to some. What about you?
    Venus and Jupiter rise in the east-northeast before dawn. Jupiter is first to crest the horizon, but once Venus appears a few minutes later you’ll have no trouble telling the two apart, as the morning star is six times brighter than old Jove. The two planets are joined by the ever-so-thin waning crescent moon early Saturday morning.
    The last of the naked-eye planets returns to view late this week. Look for Mercury Wednesday the 27th immediately in the wake of the setting sun and just a couple degrees from newly emerged waxing crescent moon.

The sun stands still for just a day before again heading south

In the early morning Saturday, at 6:51am EDT, the sun reaches its northernmost point in the sky for the year, with its center hovering directly above the Tropic of Cancer somewhere in Africa. On solstice, the sun appears to pause in place, holding steady for several days directly overhead at high noon — solstice in fact means sun standing still. You can see proof of the sun standing still in this week’s times of sunrise and sunset, listed below, which barely change.
    This solstice marks the Northern Hemisphere’s longest day of the year, with 14 hours 54 minutes of sunlight here along Chesapeake Bay. And while there is no universal body that dictates the start of the seasons, this celestial phenomenon is universally seen as the start of summer for the Northern Hemisphere and the start of winter for those below the equator.
    The earth spins at a 231⁄2-degree tilt, causing the north side of the planet to more directly face the sun for half the year and the south side to more directly face the sun the other half of the year. At the time of the June solstice, the North Pole points almost directly at the sun, while December’s solstice has the South Pole pointing sunward. Right now, we’re enjoying that sunward tilt, and all those extra hours of daylight add up to the season’s much warmer temperatures.
    While it is only the start of summer and the days will continue to grow warmer for some time to come, it is also the beginning of summer’s end. The very next day after solstice, the sun begins its southward march, albeit ever so slightly at first, and the length of daylight wanes.
    For millenia, cultures have tracked the sun’s path across the sky, measuring the length of daylight and the location of the sunrise and sunset throughout the year. The ancient Celts built Stonehenge, built at least 5,000 years ago in alignment with the solstices and sunrise. Around the same time, the Egyptians were building their own monuments to the sun and the passing seasons. From a vantage atop the Great Sphinx on the day of June’s solstice, the sun set directly between the oldest of the Great Pyramids.
    The sun may be the star this week, but the waning crescent moon makes good showings with Venus low in the east before dawn Monday and Tuesday, when only two degrees separate the two. Early Wednesday the moon is just above of the bright star Aldebaran in Taurus.

Mars still lights up the night
Thursday’s first-quarter moon appears high in the southwest at sunset and sets in the west around 1am. Each following night, darkness finds the waxing gibbous moon a dozen degrees farther east, providing almost an hour of additional moonlight. 
 
Friday, the moon is 15 degrees to the right of Mars, but come Saturday the two are practically on top of one another, separated by only two degrees. The red planet is just to the upper right of the moon as darkness falls, and they stay quite tight until setting around 2am. At -0.8 magnitude, Mars outshines any star — only Sirius is brighter, and the Dog Star is gone from view for the season. 
 
Two months ago Mars was even brighter, as the planet was at opposition from the sun with earth directly between the two. Imagine opposition as if you were seated at the movie theater, the light from the projector streaming from behind you to the screen. The screen itself isn’t illuminated, but instead it reflects the projected light back to your eyes. As you turn your gaze from straight ahead, or if you shifted the projector, the reflected image grows dimmer. That’s what’s happening now, as earth’s faster orbit hustles it away from Mars, diminishing the angle of reflected light.
 
To see a simulation of the intricate dance between earth and Mars as they travel around the sun, go to http://tinyurl.com/9dtvspa.

Sunday the moon has another partner, the first-magnitude star Spica. The brightest star in the constellation Virgo, Spica is just a couple degrees to the moon’s right. A dozen degrees to the west of the pair is Mars, while a dozen degrees to their east is Saturn.
 
Monday the moon shines 10 degrees to the right of Saturn, while Tuesday it is five degrees to the left of the ringed planet.
 
As the sun sets Wednesday, the moon appears low in the southeast. Just a few degrees below the nearly full moon is the first-magnitude red-giant Antares, the heart of Scorpius the scorpion. Antares means literally the opposite, or rival, of Mars, because of its own reddish hue. Compare the two for yourself.

Let it guide you through the night

Friday evening, look in the wake of the setting sun low in the west-northwest for the nascent crescent moon and Mercury. Mercury is just a few degrees to the upper right, but both are so close to the horizon that you may need binoculars and you won’t have long. Within 90 minutes of sunset Mercury is gone. And that window is shrinking each day. Mercury is surprisingly bright — equal to any star. But don’t confuse its white glow with the much brighter and golden hue of Jupiter, 20 degrees higher.
    By sunset Saturday, the moon has climbed well above the horizon, leaving Mercury in the dusk. Now the thin crescent is just seven degrees below Jupiter, easily the brightest object other than the moon. The moon, Jupiter and Pollux higher still form a near-straight line.
    Sunset Sunday finds the waxing crescent moon well positioned in the west. Jupiter shines 10 degrees to its right, while below and to the left, making a wide triangle, is the first-magnitude star Procyon. The eighth-brightest star in the heavens, Procyon is one of two bright stars in the constellation Canis Minor, the Little Dog.
    Monday the moon is amid the dim stars of Cancer. Look a few degrees to the right of the moon for a dim patch of light at the constellation’s center. Unlike the sharp, clear light of a star, the hazy glow you’re seeing is the combined light of hundreds of newborn stars within the Beehive Cluster 570 light years away. While our own sun is 4.5 billion years old, the stars of the Beehive Cluster are only 600 million years old, mere infants in the life of a star. Binoculars are enough to distinguish dozens of these lights; a modest telescope reveals many more.
    Tuesday and Wednesday the moon is several degrees to either side of Regulus, the blue-white heart of Leo the lion. Regulus marks the dot at the base of what looks like an inverted question mark, called the Sickle of Leo.
    As twilight turns to darkness, Mars glows like an ember in the south. Far to the lower left is Spica. The red planet sets around 3am.
    Saturn shines in the southeast at sunset, is high in the south around midnight and sets in the west around 4:30am. The ringed planet is flanked by the two brightest stars of Libra — both second-magnitude — Zubenelgenubi and Zubeneschamali.
    In the hour before sunrise, Venus appears low in the east. At –4 magnitude, there’s no confusing the Morning Star for anything but an airplane or satellite — except that it holds steady in place until daybreak.

Hundreds of shooting stars possible this weekend

If you’re not a night owl, you’ll want to set your alarm clock for early Saturday. Before dawn that morning, earth will plow through the trail of a newfound comet, providing what many astronomers are predicting to be the best meteor storm in years.
    Comet 209P/LINEAR orbits the sun over a five-year period, yet it was only discovered in 2004 by the Lincoln Near-Earth Asteroid Research project. It belongs to a family called the Jupiter Comets, which are steered and propelled by the gas giant’s own gravitational pull. Every so often, these comets come a little too close to Jupiter, and like a leaf buffeted by the wind, their course is set anew.
    That’s what happened in 2012 to Comet 209P/LINEAR. Not only did it alter the comet’s path, but it also warped the centuries-old trails of debris left with every five-year passing. Now the comet’s path comes within 280,000 miles of our own orbit — little more than the moon’s distance from earth.
    Not that the comet will actually pass that close to earth. At its innermost point of orbit, 209P/LINEAR is 90 million miles from the sun — roughly the same distance from the sun as earth, give or take a few million miles. May 29 the comet will be just 5.15 million miles from earth, the ninth closest approach by a comet ever recorded.
    Even at its closest, it’s doubtful you’ll be able to spot this small, dim chunk of ice and rock without a decent-sized telescope.
    In this case the sizzle is far better than the steak, thanks to the countless bits of dust trailing in the comet’s wake after hundreds of years circling the sun. Between sunset Friday and dawn Saturday, earth plows full-steam through these bands of accumulated inter-stellar flotsam.
    Astronomers are predicting a brief but intense period of activity, peaking around 3am with anywhere from dozens to hundreds of meteors an hour. Some models even predict a meteor storm with as many as 400 meteors an hour! And unlike most prolific meteor showers, which streak past in a matter of seconds, those from P/209 LINEAR will drift through the sky like falling snowflakes.
    Cloudy skies? Check out the meteor shower via a live feed starting at 1:30am Saturday at http://tinyurl.com/olywq6k.
    The waning crescent moon won’t interfere with the meteors, but it will make a nice appearance with Venus low in the east before dawn Sunday morning.