Tag Archives: orbit

In darkness the moon is reborn

Posted on by
2

If we could see the moon today, it would be a very thin crescent only a few degrees from the sun. Tomorrow it’s in new moon phase. Maps created with Stellarium

What’s old today but instantly becomes young again tomorrow? If you guessed the moon, you’re right! Today the moon winds up its current cycle of phases as an exceedingly thin crescent so close to the sun it’s invisible in the solar glare.

The moon’s cycle has always been a metaphor for life. Every month it’s born again as a thin crescent in the western evening sky, grows to a half-pie seven days later and reaches its full power and radiance when full at 14 days. After full, the moon’s radiance declines as its phase wanes to last quarter (21 days) and then to a whiskery crescent at dawn. Before it finally disappears in the sun’s glare, the moon, now 28 days old, reaches the end of its “life” cycle. But only briefly. The very next day, moments after new moon phase, it’s reborn again as an evening crescent.

When we run into troubles in our lives, we might look to the ever-renewing moon for inspiration.

The sky looking west-southwest a half hour after sunset Monday evening Feb. 11, 2013. You might be able to spot dimmer Mars in binoculars.

Tomorrow morning at 2:20 a.m. (CST) the moon will be exactly lined up with the sun and pass through new moon phase. Skywatchers in the western hemisphere won’t see the moon either day because it’s in the same direction as the sun and swamped by glare.

By Monday Feb. 11 however, the moon’s orbital motion will remove it far enough from the sun to be visible during evening twilight.  And there’s a bonus. The crescent will float a few degrees above the planet Mercury.

I’ve removed the atmosphere in this illustration so you can see where the moon is today at noon (CST), tomorrow morning when it reaches new moon phase and tomorrow Feb. 10 at noon. It passes north of the sun, which is in the constellation Capricornus. Notice all the planets in the neighborhood.

If we were to follow the moon today through new moon and into tomorrow, we’d notice it passes well north of the sun. Most of the time, the new moon is either north or south of the sun because its orbit is tipped about 5 degrees relative to Earth’s orbit.

The moon’s tilted orbit causes it to swing north or south (pictured here) of the sun from Earth’s perspective. A couple times a year however it crosses directly in front and a total solar eclipse is visible from somewhere on Earth. Illustration: Bob King

Over the course of its monthly cycle, it bobs up and then down along its tilted orbit. But 2 or 3 times a year, when the moon intersects the plane of Earth’s orbit at the same time as new moon phase, it crosses directly in front of the sun and we see a total solar eclipse. In fact, this is the only time we can see a new moon with the naked eye. It looks exactly like what you’d expect – a blank, black disk scrubbed free of its past life, waiting to begin the next as a tender crescent.

The new moon – black disk – is plainly visible silhouetting the sun during a total solar eclipse. Credit: Luc Viatour

Tonight’s Full Wolf Moon makes for happy howlers

Posted on by
13

Last night’s moon paints the bare trees with light. Tonight the moon will be full. Photo: Bob King

Tonight Jan. 26 the Full Wolf Moon will crest the horizon around sunset and appear high in the southern sky in the constellation Cancer around midnight. If you want to know exactly when the moon will rise for your location, click HERE and select your city.

January’s full moon is named for the wolves that are active this time of year hunting and looking for mates. Wolves howl for many reasons including to let the pack know their location, as a rallying cry to gather the pack together and to warn other wolf packs to keep off their territory.

A wolf points its face nearly straight up when howling so its voice can carry over a long distance.

We’ve come to associate wolf howling with the full moon but studies have shown there’s no direct connection between the phase of the moon and howling. Maybe it’s the way they point their muzzles to the moon and stars that makes us think they’re directing their calls to the sky. Wolves are just taking advantage of good acoustics. If you point your face upward and howl, your voice will carry much farther. Wolf howls can travel up to 6 miles in the forest and 10 miles across open terrain.

I said there was no direct link between full moon and howling frequency, but a bright moon makes hunting at night easier by providing a brighter light than say, a crescent moon. So yes, when wolves are actively hunting and howling, we might hear them more often during near-full or full moons.


I don’t know if it’s hidden in our DNA, but I’ve known lots of people over the years who’ve attempted to communicate with wolves – or get their dogs stoked up – by imitating a wolf howl. If you’ve ever wanted to howl but needed a few tips, I recommend this humorous 2-minute video.

The full moon lies directly opposite the sun in the sky. When the sun sets at our backs, the moon rises in our face in the eastern sky. Sunlight strikes the moon square-on at full phase, illuminating the entire disk. Photo: Bob King

The full moon will be spectacularly bright as always. Outside of the sun, it’s the brightest celestial object in the sky. Funny though. Astronomers have measured the light reflected by the moon and found it’s a near perfect match to the a freshly-paved asphalt parking lot.

It appears bright compared to the darkness of the sky. Our eyes also become more sensitive to dim light at night, contributing to the impression of the moon’s brilliance.  Like other animals including wolves, we’re equipped with night-vision ”goggles” in the form of some 120 million “rod” cells in our eyes. They’re far more numerous and much more sensitive than the “cones” we use for daytime vision.

The moment of full moon – when the moon is directly opposite the sun – happens at 10:38 p.m. (CST) tonight. That’s when the entire half of the moon facing Earth will be flooded with light. The shadows of mountains and craters that characterize the moon during its other phases will be absent … or nearly so. Ironically, a completely shadowless moon is only possible when the moon is exactly opposite the sun, at which time it moves into Earth’s shadow during a total lunar eclipse.

If the moon’s orbit wasn’t tilted with respect to Earth’s, we have a total lunar eclipse every month. Because it is, the moon “misses” our planet’s shadow most months. The imperfect lineup during tonight’s full moon is visible as a faint shading along the moon’s northern limb or edge. Look for crater and mountain shadows there through a telescope. Diagram not to scale. Illustration: Bob King

Because the moon’s orbit is tilted about 5 degrees with respect to Earth, it usually passes north or south of this ideal sun-Earth-moon line. Tonight it will be a full 5 degrees south of  that line at the moment of full moon. If you have a small telescope, look at the moon around that time. You’ll notice shadows of mountain walls and craters within its northern limb or edge. Oh, and don’t forget to let out a howl of delight when you do.

If you’re looking to enhance your full moon experience, check out my 10 Ways to Enjoy a full moon.

C/2012 K5 – an evening comet worth chasing

Posted on by
21

Comet C/2012 K5 photographed this morning Dec. 16, 2012 from Austria. Two tails are visible – the obvious one and a faint dust fan to the upper left of the comet’s head. Credit: Michael Jaeger

Sure I love the moon. Last night’s walk with the dog wouldn’t have been nearly as romantic without it. But tonight the moon won’t rise for an hour after twilight ends. That means the return of dark skies and the Milky Way. It’s also a perfect time to follow what has now become 2012′s brightest comet – C/2012 K5 LINEAR. Just in time it would seem!

This picture of Comet C/2012 K5 on Christmas Eve morning was made with an 8″ telescope and nicely shows the comet’s two tails. Credit: Gerald Rhemann

On Christmas morning I saw it in plain old 8×40 binoculars as a fuzzy glow near Big Dipper’s Bowl. Through a 15-inch telescope the comet was sheer beauty with a compact bright head and tail nearly as long as the full moon is wide (1/2 degree).

Currently shining around magnitude 8.5 and moving swiftly as it makes its closest approach to Earth tomorrow, C/2012 K5 is now out during convenient evening viewing hours.

The comet moves swiftly through Auriga and n. Taurus in the coming nights. Watch especially on Jan. 3 when it passes next to the bright star cluster M36. Stars shown to mag. 7.3 and map dates are for 7 p.m. CST.  Right-click, save and print a copy for use at the telescope. Created with Chris Marriott’s SkyMap software

You’ll find it still around 8-8.5 magnitude during the coming week as it skims through the bright constellation Auriga not far from Jupiter.  I have to be honest – while visible in binoculars from a reasonably dark sky, it’s no great shakes, just a patchy glow. Through a small telescope however, you’ll see the little head and at least a hint of the tail stretching off to the west.

Consider the comet a warm-up for the brighter fare coming this March when C/2011 L4 PANSTARRS makes its appearance in the evening sky. You can read more about that one  and another bright comet in my best sky events of 2013 blog.

C/2012 K5 orbit is steeply inclined to the plane of the solar system, which is why it’s been visible in the far northern sky of late. Now the comet’s rapidly moving southward as it plunges through the plane. Credit: NASA/JPL

C/2012 K5 LINEAR was discovered earlier this year by the automated Lincoln Near-Earth Asteroid Research (LINEAR) project. The joint effort by the Air Force, NASA and MIT’s Lincoln Laboratory uses a 1-meter (39-inch) telescope to discover and track Earth-approaching asteroids. In addition to thousands of new asteroid finds, the survey has picked up a few comets along the way. K5 was discovered on images taken May 25, 2012.

Facing east around 7 p.m. local time Dec. 30. Use this wide view map to locate Auriga and then the more detailed view above to find the comet. Created with Stellarium

The comet comes closest to Earth on Dec. 31 at a distance of 27.3 million miles. Now at its brightest, the comet will soon fade after about the middle of January. Stop by for a look the next clear night.

Long-lost comet Pons-Gambart finally returns home

Posted on by
17

Comet Pons-Gambart, also known as C/2012 V4,  photographed on Nov. 30 by Andres Chapman from his Observatorio Cruz del Sur (Southern Cross Observatory) in Argentina. The comet’s bright, condensed center is clothed in a fuzzy coma of vaporizing ice and dust. Click for more photos. Credit: Andres Chapman

A comet long considered lost has been found again!

Comet Pons-Gambart was discovered on June 21, 1827 by Jean Louis Pons, observing from Florence, Italy and Adolphe Gambart in Marseilles, France. It brightened to the naked-eye limit (between 5th and 6th magnitude) and then quickly faded. Pons last observed the comet on July 21, 1827 calling it “very faint”.

With only a month of observations, no one bothered to determine an orbit for the comet until a year later. Based on those calculations, it appeared that Pons-Gambart was like many comets, one-hit wonders destined to never return.

Fast forward to 1917 when a second look at the data by astronomer Dr. S. Ogura led him to calculate an elliptical orbit. A comet in an elliptical orbit regularly cycles around the sun making repeat visits. The length of time between visits is called its period. Ogura predicted a period of just under 64 years for Pons-Gambart with an uncertainty of +/- 10 years. Further investigation of its orbit in 1978 gave a period of 58 years plus or minus 10.

We knew it was a repeat or periodic comet, but with no observations since 1827, it appeared hopelessly lost. Hence its official name as D/1827 Pons-Gambart, the “D” meaning lost or deceased.

Color image of C/2012 V4 – the likely return of D/1827 Pons-Gambart taken on Dec. 2, 2012. Credit: Rob Kaufman

Then came a happy turn of events. In November, space scientist and amateur astronomer Rob Matson of Newport Coast, Calif. spotted and tracked a comet in pictures taken by the SWAN camera aboard the Solar Heliospheric Observatory (SOHO). Matson put out a call via e-mail for confirmation photos from the ground. Battling trees and moonlight, Australian amateur Terry Lovejoy photographed and confirmed the comet just two days later on Nov. 29.

Preliminary calculation of its orbit shows an excellent fit to Pons-Gambart’s.  Despite being missed on two previous flybys, this welcome stranger has returned home.

Reincarnated now as comet C/2012 V4, it reaches perihelion, the point in its orbit closest to the sun, on Dec. 18 at a distance of about 75 million miles. Astronomer Rob McNaught described its appearance as a “spring onion” with bright fuzzy core and short spike of a tail to the northeast.

Based on the present return date, we can figure Pons-Gambart’s period at about 62 years. After discovery in 1827, its appearances in 1889 and 1951 were missed, but thanks to space-based cameras and Matson’s keen eye, this time the comet didn’t get away.

Amateur astronomers will find the comet near the well-know Dipper-like asterism in Sagittarius called the “Milk Dipper” shown here an hour after sunset from the northern U.S.  Comet positions are shown for 5:30 p.m. (CST) every five nights. Stars plotted to mag. 8.5. Click for detailed timetable and orbital elements. Created with Chris Marriott’s SkyMap software

Next question – can you see it? No problem if you live in the far southern U.S. and points further south. Skywatchers in South America and Australia have the best view. It’s currently about magnitude 9.5 and visible in 4.5-inch and larger telescopes in the constellation Sagittarius. From mid-northern latitudes, it might be visible with a bigger scope (8 inches and up) very, very low in the southwestern sky at dusk in the coming week.

You can use the map above to help you find it – just right-click, save and print out a copy. One big help in locating the comet is its proximity to the planet Mars (not for real, just line of sight). On the 7th, the two will be in conjunction less than 1 degree apart!

Over the next few weeks, C/2012 V4  will stay fairly close to the sun; not until mid-January will northerners have a better shot at seeing it, when the comet tracks through the constellations Scutum and Aquila in the pre-dawn sky. Pons-Gambart shines brightest now through mid-December and then slowly fades. I hope at least some of you will be able to put out the welcome mat for this lost soul. There’s a thrill in finding a comet that no one’s seen hide nor hair of in 185 years.

Thank God it’s finally autumn this weekend

Posted on by
4

As the days grow shorter, green chlorophyll breaks down in these sugar maple leaves unmasking yellow and orange pigments that have been there all along. Reds are produced in late summer and early fall from excess sugar in the leaves. Photo: Bob King

Summer’s slipping away. Back in July, when every day was sunny and hot, many of us couldn’t wait for fall to get here. Our fondest hopes will materialize this Saturday September 22 at 9:49 a.m. (CDT) when autumn finally comes a-knockin’.

Astronomers call the first moment of the new season the autumnal equinox. It’s one of two times a year when the sun’s path intersects with the celestial equator, a projection of Earth’s equator onto the sky. The spring or vernal equinox is the other.

As seen from the equator, where the celestial equator is directly overhead, the sun will be overhead at local noon. People there who look down at their feet will discover they’re standing directly on the shadow of their head! From mid-northern latitudes, the celestial equator arcs approximately midway between the overhead point and the horizon at noon. Up at the north pole the celestial equator it’s a hula-hoop encircling the entire horizon. If you were standing there this Saturday, you’d see the sun circle the horizon for 24 hours straight, never rising higher.

The orientation of Earth’s axis to the sun changes during our yearly orbit – the reason for the changing seasons. Notice that the tilt of Earth’s axis remains fixed in space and does not flip-flop back and forth. Credit: National Weather Service

Seasons are caused by the 23.5 degree tilt of the Earth’s axis. As we orbit the sun during the year, the north-south position of the sun changes because of the changing orientation of our axis. When the north polar axis is pointed toward the sun, our star reaches its most northerly point in the sky and we experience long days and summer heat.

During northern hemisphere winter, our axis points away from the sun and our star is southernmost and lowest in the sky. Shorter days and a low sun make for cold weather.

The sun’s been sliding south in the sky each day since the beginning of summer. This Saturday it’s exactly halfway between its highest point (June 20) and lowest (December 21). Photo: Bob King

The first day of fall is special because Earth’s axis points neither toward nor away from the sun. Instead, we’re broadside to the sun, and day length is approximately equal to night nearly everywhere across the planet. If you’re into equality of light for all, the equinoxes are your symbols of emancipation.

The word equinox comes comes from the Latin words for equal and night because both day and night are approximately 12 hours long. Prior to September 22, days are longer; after the 22nd they get shorter. Shorter days are caused by the sun dropping farther south in the sky (lower altitude). The lower the sun, the less time it spends crossing the sky and the shorter the hours of daylight.

Interestingly, day and night are not exactly equal at the equinoxes. Yes, it’s true that the center of the sun sets exactly 12 hours after it rises on the first day of fall. Problem is, we determine sunrise at the first sighting of the sun, when its upper edge (not center) breaches the horizon. Similarly, sunset occurs when the last bit of sun disappears below the horizon. That adds about two minutes to daylight’s tally.

The sun in this beautiful sunrise photo is an illusion caused by the thick atmosphere bending the real sun (below the horizon) into view. Credit: Lyle Anderson; illustration: NOAA

We get another few minutes thanks to atmospheric refraction. That’s our atmosphere’s freaky ability to act like a prism and bend the sun’s rays upward into view when it’s still below the horizon. If you’ve ever seen the sun directly on the horizon at sunset or sunrise, you’ve witnessed one of nature’s grandest illusions. The sun’s not really there. The air is thick enough across your sightline to “lift” the sun into view about two minutes before it rises for real.

As astronomer George Greenstein, who worked for years at the Old Farmer’s Almanac, once said: “If the Sun were to shrink to a starlike point and we lived in a world without air, the spring and fall equinoxes would truly have ‘equal nights.’” To whittle away those excess minutes of daylight gained by these parlor tricks, we have to wait until September 25 for day and night to momentarily be equals.

Any planet with a decent amount of axial tilt will experience seasons. How many do? All but Venus, Mercury and Jupiter. Venus’ axis is tipped nearly 180 degrees and rotates backwards compared to the other planets, Mercury’s is 0 degees and Jupiter just 3. Mars’ axis is tilted closest to Earth’s at 25.2 degrees, but since that planet is about 2/3 farther from the sun than ours, its seasons are that much longer.

PANSTARRS – the next bright comet?

Posted on by
47

Comet C/2011 L4 PANSTARRS photographed on May 18. Credit: Ernesto Guido, Giovanni Sostero and Nick Howes

Last June astronomers at the University of Hawaii announced they’d discovered a comet with the 1.8 meter (70.7 inch) telescope atop Mount Haleakala as part of the Panoramic Survey Telescope & Rapid Response System or Pan-STARRS. The survey’s goal is to photograph the entire sky several times a month in search of Earth-approaching comets and asteroids that could pose a danger to our planet.

At the time, Comet C/2011 L4 PANSTARRS (or PANSTARRS for short) was extremely faint and nearly as far away as the planet Saturn.

After more observations pinned down the comet’s orbit, predictions showed it would pass perihelion – its closest point to the sun – at a distance of 28 million miles on the evening of March 9, 2013. That’s close enough to vaporize a lot of cometary ice, releasing the dust needed to form a bright coma and tail.

Just how bright, no one can be certain. We all know how unpredictable comets can be; the break up and fading of Comet Elenin is just one recent example. But estimates based on the PANSTARR’s distance from the sun and Earth at the time of perihelion put it at magnitude 0 or as brilliant as Vega or Arcturus.

Comet C/2011 L4 PANSTARRS will appear in the evening sky just days after perihelion. The date shown here is March 12, 2013 about 40 minutes after sunset. The ultra-thin crescent moon will lie just five degrees to the north of the comet. Maps created with Chris Marriott's SkyMap software

Circumstances for viewing a bright comet couldn’t be better. PANSTARRS pops into the evening sky only a few days after closest approach to the sun. Moving rapidly northward, it soon becomes visible all night long from mid-northern latitudes in April.

PANSTARR's orbit is steeply inclined (84 degrees). Right now it's below the plane of the solar system (dark blue) but after perihelion next March its orbit takes it quickly above the plane (light blue). Credit: JPL/NASA

You might be wondering why I’d bother writing a blog about something happening 10 months down the road. Let’s just say I want as many amateur astronomers as possible to have the opportunity to see the comet early.

Die-hard comet observers have been photographing and observing the comet since late this winter, more than a year before perihelion. I’ll take that as a good sign that PANSTARRS is on schedule.

I sought the comet a week ago using a 15-inch reflecting telescope and was surprised at how easy it was to see. Located near the bright star Antares in Scorpius the Scorpion, I estimated the comet’s brightness at magnitude 12.5 (at discovery it was 19 — faint!). PANSTARRS was a very small but dense knot of light about 20 arc seconds in diameter with a faint star-like center. Its compact appearance is a good indicator of lots of dust activity in the comet’s nucleus – another positive sign for the coming apparition. A second look this past Saturday morning showed it smidge brighter yet.

Use this map to help you find the comet in your telescope. Antares is at left and the head of Scorpius is outlined. The comet is low in the southern sky from mid-northern latitudes. Stars shown to about 10.5 magnitude. PANSTARRS look like a very small "cottonball" with a brighter center at medium and high powers.

If you start observing now, you’ll have the pleasure of watching Comet PANSTARRS brighten and develop on its journey to perihelion and beyond. Following a comet night by night can be very rewarding, comparable to studying a species of bird to better understand and appreciate its behavior. For the moment, you’ll need a 10-inch or larger scope and dark skies but as the weeks and months advance,  it will gradually brighten.

Skywatchers in mid-northern latitudes will be able to follow PANSTARRS through early August before it’s too low to view and lost in the glow of evening twilight. Our next opportunity won’t be until next March post-perihelion. Southern hemisphere observers will fare much better with the comet high in the sky and well-placed for viewing for months to come.

To assist you in your quest, either download comet orbital elements for your favorite star charting program at the IAU Minor Planet Center site  or use the map above which shows the comet’s position around 11:30 p.m. CDT every five nights. PANSTARRS is still 325 million miles from Earth or more than halfway to Jupiter.

Comet Hartley 2 photographed by NASA's EPOXI mission. The bowling pin shaped nucleus is about 1.2 miles long. Jets of material, from ice vaporized by the sun's heat, are being ejected from the nucleus. Credit: NASA/JPL-CalTech/UMD

Returning to the question of the comet’s brightness, that may depend on whether it’s making its first or hundredth trip around the sun. On a first swingby,  exotic ices of nitrogen and carbon dioxide, long preserved in the deep freeze of the outer solar system, vaporize at great distances from the sun, making the comet appear unusually bright. If we’re not careful, we might extrapolate that behavior to the time of closest approach and predict a very bright passage. Unfortunately, once those ices are gone, the comet may have only a modest amount of water ice remaining for the sun to vaporize and not brighten as expected when closer to the sun.

Three types of orbits are possible for bodies in the solar system. Most orbit in ellipses around the sun. Credit: Oracle ThinkQuest Education Foundation

Comets that return time and again all have elliptical orbits around the sun like the planets but more stretched out or elongated. Comet PANSTARRS’ orbit appears for the moment to be nearly parabolic. A parabola is a sort of open-ended ellipse with one end near the sun and the other a return trip to infinity. Most comets on parabolic orbits come from the far edge of the solar system and have their orbits reworked by giant planets Jupiter and Saturn into very long but closed ellipses with orbital periods of hundreds of thousands to millions of years. PANSTARRS might be one of those “fresh” comets and putting on a good show now despite its distance. We’ll have to just wait and see.

Minute by minute daylight takes back the night

Posted on by
8

Detail from a sundial in Krakow, Poland

One little minute. The drip of time toward more daylight hours began December 14 when the sun set one minute later than the previous week’s run of earliest sunsets of the year. Yet the amount of daylight remained static at around 9 hours 6 minutes until late in the month, because the sun continued to rise later and later each morning, mostly offsetting the gains made by later sunsets.

Tomorrow morning January 9 that all changes when the sun rises one minute earlier, the first time it’s done so since early June of last year. With earlier sunrises and later sunsets working in tandem, time’s drip will soon become a trickle and then a rushing stream as the additional minutes cascade into noticeably longer days. Daylight weighs in at 9 hours 17 minutes tomorrow. A week from now, we’ll pack on an additional 10 minutes and in two weeks another 13 minutes.

It's the tip of Earth's axis that makes the sun appear to travel north and south in the sky as seen from different sides in our orbit. Credit: NOAA

During a day, the sun rises in the east and sets in the west. Easy enough. But each day, unnoticed by almost all of us, it also moves a small distance to the east. Its eastward movement is a reflection of Earth’s orbital motion. Our planet travels an average of 1.6 million miles a day. When we wake up each new morning, the Earth’s ceaseless motion has quietly nudged the sun eastward against the distant background stars compared to the morning before.

This illustration shows the sun's travels across the sky in the course of a year. The sun moves north (up) and east from its lowest point in the sky starting on December 21 causing the day length to gradually increase. The green arrows show the direction of the sun's travel. Illustration adapted from John Lucey's (Durham University) animation. Click photo to see the full animation and learn more.

That’s not all. At the same time it’s moving east, the sun travels northward in the sky between Dec. 21 and June 20, then reverses direction and drops southward starting on June 20 (or so)  through Dec. 20. If you guessed that the sun’s north and south swings were due to the tip of Earth’s axis, give yourself a big pat on the back.The sun’s greatest height in the sky is on the summer solstice when it’s 23.5 degrees north of the celestial equator, an imaginary projection of the Earth’s equator into the sky. It sinks lowest on the winter solstice at 23.5 degrees south. Two pats on the back if you remembered that Earth’s axis is tipped 23.5 degrees.

Let’s get back to the here and now. Daylight increases at a faster and faster rate as we approach spring, because the sun’s path in the sky moves upward to the north at an ever steeper angle. It may cover the same distance in the sky each day, but as January gives way to February, most of that distance is to the north and less to the east, which means a higher sun and rapidly accelerating day length.

You may not care a whit for the mechanics of all this. That’s perfectly fine. Like me, you’ll probably just smile one morning when you open the shade and see the sun in your face when a week ago it hadn’t cleared the trees.

Follow the moon to twilight’s end

Posted on by
2

The moon travels upward from the western horizon the next few nights to line up with Saturn and then Spica in Virgo. Created with Stellarium

After a good pounding from thunderstorms last night, we’re looking forward to clear skies in the region this evening. Good thing. A fingernail crescent moon will be there to usher in the night. Look for it very low in the western sky around sunset. If you follow the moon over the next few evenings, it will direct your gaze first toward Saturn and then Spica, Virgo’s brightest star. Both these luminaries will soon will lost in twilight.

View of Earth's orbit around the sun seen from above the northern hemisphere. As our planet moves to the left, the background constellations appear to drift to the right or westward. Illustration: Bob King

Every night, the stars rise four minutes earlier than the night before. Over the days and weeks, the minutes accumulate into hours. When stars rise earlier, that means they also set earlier, causing them to drift westward over time. This seasonal drift of constellation and planets is caused by Earth’s yearly revolution around the sun.

As Earth zips along its orbit, our view of the constellations constantly changes the same way an athlete on a track passes different sections of the cheering crowd during a running race. Every new lap, the view of the crowd ‘repeats’. Earth’s ‘running track’ is 584 million miles in circumference and we dash along at 18.5 miles per second, completing one lap in a year’s time. The constellations ahead of Earth rise in the east and those behind set in the west at the rate of 4 minutes earlier each day. The view repeats – we come back to where we started – one year later.

If you see Jupiter or the constellation Pegasus in the eastern sky, they’ll be gradually rising earlier as the days and weeks go by, while Saturn and friends in the west will set sooner.

Twilight length depends on the angle the sun makes to the horizon. A steep angle means that the sun gets farther below the horizon in an hour's time compared to a shallow angle. The farther below the horizon the sun is, the darker the sky. Illustration: Bob King

Early August is the time most of us notice that night comes earlier than it did a month ago. Part of the reason is because the sun’s been setting earlier each evening since the June solstice. Another has to do with the length of twilight. For the northern U.S. and southern Canada, twilight length varies from a minimum of about 1 hour 45 minutes to 2 1/2 hours. Short twilights occur from early September to early April, but from late April through midsummer, twilight time reaches its maximum.

Twilight is the 'tween' time when sunlight reflecting off Earth's atmosphere lights the sky and provides gentle glow across the landscape. Credit: Bob King

Since the first day of summer, twilight has shrunk by nearly a half-hour at Duluth’s latitude of 47 degrees north. That means real night begins a half hour sooner. Twilight length depends on the angle the sun makes to the horizon when it rises and sets. That angle varies during the year. Starting in August and continuing through fall, that angle is steep, causing the sun to ‘hurry’ away below the horizon and twilight to shorten. In spring and summer, the angle is shallow or oblique. The sun takes its sweet time to get far enough below the horizon for night to finally begin.

However and whenever you take your night tonight, I wish you clear skies with a minimum of mosquitos.

Comet Elenin will not destroy the Earth

Posted on by
541

Comet Elenin photographed on April 2 by German astrophotographer Bernhard Häusler. Because the comet moves in relation to the stars, the stars appear trailed during the exposure. Credit: Bernhard Bernhard Häusler

I did a search for Comet Elenin the other day and was surprised at how much gloom-and-doom blather has been written about this modest comet. The misinformation covers the full range of nonsense – everything from the name ‘Elenin’ being a coded message for ‘Extinction Level Event Notable Impact Nemesis’ to the comet being a secret brown dwarf star called Nibiru (which doesn’t exist). Some are even spreading the rumor that Leonid Elenin the person doesn’t actually exist.

Come on people!

Russian amateur astronomer Leonid Elenin

Let’s shed a little light. Comet C/2010 X1 (Elenin) was discovered by Russian amateur Leonid Elenin on December 10, 2010. Although he lives in Lyubertsy, Russia, like many amateur astronomers, Elenin takes astrophotos ‘remotely’ using telescopes that can be controlled by a home computer. The night of his discovery he was taking routine photos of the sky at the independent Russian remote observatory ISON-NM (International Scientific Optical Network) near Mayhill, New Mexico.

By the way, Elenin is real guy who lives near Moscow, loves astronomy and studies asteroids and variable stars. He
a researcher at the Keldysh Institute of Applied Mathematics and volunteers at the International Astronomical Search Collaboration. Elenin also hosts a great website, where he frequently posts updates about his comet.

Elenin’s photos showed an extremely faint, tiny, teardrop-shaped fuzzball. After additional observations, astronomers determined that the comet’s orbit would bring it near the Earth later this summer, when it might become bright enough to see with the naked eye.

An animation of Comet Elenin in March compiled from multiple still photographs. Credit: Bernhard Häusler

When I heard the news, I cheered. While naked eye comets aren’t rare, they’re not common either, with one making an appearance every couple years. Each is a joy to follow. Every comet observer hopes a bright one will show interesting activity in its nucleus and develop a pretty tail. People love a nice comet – remember Hale-Bopp back in 1997? A thing of beauty.

Further study of Comet Elenin’s orbit appeared to indicate that it was making its first-ever visit to the inner solar system. This happens routinely with comets, since, like your in-laws, so many come from great distances. Other comets, like Halley’s for instance, travel on closed oval orbits called an ellipses and revisit the Earth’s vicinity periodically. These are called the periodic comets.

Leonid Elenin has studied the orbit of his comet closely and predicts that gravitational tugs from the giant planets Jupiter and Saturn will likely reshape its orbit into a very long, cigar-shaped ellipse. The closed track would return the comet for another visit in about 10,000 years. Further observations will no doubt refine Elenin’s orbit and narrow the possibilities.

Comet Elenin orbits nearly in the same plane as the planets do. It’s currently in the constellation Leo the Lion and still very faint at 16th magnitude, well below the visual limit of most amateur telescopes. As it slowly moves closer to the sun, the comet should become visible in large scopes by early summer. Right now, it’s cruising out beyond Mars in the asteroid belt some 180 million miles from Earth and looks like a small, fuzzy spot in photographs.

On September 5, the comet will be near the orbit of Mercury and closest to the sun. Unfortunately, from our vantage point on Earth, Elenin will appear in the same direction in the sky as the sun and be lost in the solar glare. Things get better later that month, when the comet moves away from the sun’s direction and makes an appearance in the morning sky at dawn in Leo the Lion.

This simplified diagram shows Comet Elenin as it swings through the inner solar system, passing by Earth on the dates shown. The two bodies are closest on October 17. Positions shown are approximate. Illustration: Bob King

Closest approach to Earth happens on October 17 when the comet will be 21 million miles away from Earth and 2.4 million miles above our orbit. While we might appear close to each other in the diagram above, we’re still millions of miles apart. After that, we each go our separate ways as Elenin plunges back into deep space.

Here are some more facts:

* Comet Elenin doesn’t appear to be any different from numerous other comets. It’s orbiting the sun in a predictable manner.

* Much baloney has been sliced about the comet or its tail striking the Earth and wreaking havoc. The truth is Comet Elenin is so tiny, its gravitational effect on Earth is practically zero. Even at closest approach it’s still 21 million miles away. Every few years we get almost that close to Venus (23.7 million miles), a rocky planet nearly the same size as our own, and don’t suffer any ill effects. Like the comet, its pull on Earth is tiny, tiny, tiny.

What about a meteor shower from the comet’s tail? After all, most meteors we see originate from comet dust lost through their tails. Elenin appears to cross right through Earth’s orbit in the diagram above. Unfortunately my little picture doesn’t show the third dimension, but this one does.

The comet actually passes above our orbit, missing Earth in the vertical direction by some 2.4 million miles. The chance for an impact is zero. Even stray meteors are extremely unlikely. We’re close but not that close. Keep this in mind too – comet tails point away from the sun. When Elenin and Earth are closest, its tail will be directed back and away from us as depicted in the diagram above.

Even if Earth were to pass through the tail, at worst we’d see a meteor shower. That would be awesome!

I’ll settle on getting up before dawn with binoculars in hand, enjoying the subtle beauty of our fuzzy visitor while trying to comprehend the amazing fact that it arrived in Duluth’s sky after a journey of billions of miles.

Daylight saving time – love it or hate it

Posted on by
1

Because of daylight saving time, you'll have to stay up an hour later to see Saturn in the east. Created with Stellarium

Daylight saving time. I don’t whether to love it or hate it. We lose an hour of early morning daylight but gain an hour in the evening. After nearly five months of winter, more light at the dinner hour is most welcome.

On the other hand, a later sunset means a later start to the night. Yesterday, twilight ended and true night began a little before 8 p.m. Tonight that becomes 9.  Saturn rose at 8 p.m. last night and will do so at 9 p.m. tonight. Since we’ll have to wait an additional hour for stars to come up in the east, daylight saving time has the effect of retarding every star’s rising by an hour.

Wait a minute. I was just getting used to seeing Saturn and the stars of early spring up in the east long before the 10 o’clock news. Does that mean we’re stuck with winter stars for a while longer? Yes. That’s the part of daylight time I don’t like. That and the hour of sleep I have to wait 7 months to get back.

As always, patience is necessary in astronomical pursuits. Soon enough, Earth’s journey around the sun will compensate for the later rising time. In only two weeks, Saturn and friends will be up where they used to be at 9 o’clock.

It works like this. Every night, the stars rise four minutes earlier than the night before. Over the days and weeks, the minutes accumulate into hours. When stars rise earlier, that means they also set earlier, causing them to drift westward over time.

View of Earth's orbit around the sun seen from above the northern hemisphere. As our planet moves to the left, the background constellations appear to drift to the right or westward. Credit: Bob King

The spring constellations are now low in the eastern sky, but in two months they’ll all be high in the south. As Earth travels in its orbit around the sun, we peer out into different sectors of the sky at night as the weeks and months pass. Think of sitting on one of those merry-go-round horses and looking out into the carnival crowd. As the merry-go-round turns, we look out at a different part of the fairground during our little spin. Now substitute the Earth for the horse and our orbit for the merry-go-round.

This time exposure shows stars trailing across the sky from east to west (left to right). This nightly motion due to Earth's rotation is different from the slow drift caused by Earth's revolution around the sun. Photo: Bob King

During Earth’s “little spin” around the sun, we see the stars and constellations drift from east to west across the sky as we pass them by.

This isn’t the same as the nightly rising and setting of stars – that’s due to Earth’s rotation. Every star you see makes a complete circle around the sky in 24 hours. The much more leisurely seasonal drift is superimposed on that pattern. It reveals itself to sky watchers who spend time regularly under the stars.