Site ‘J’ it is! Comet lander readies for November touchdown

The primary landing site for the Rosetta lander Philae will be on the comet’s head in an area that’s both safe and shows interesting activity. Site J was chosen from the original “Top 5″. Credit: ESA/Rosetta/MPS for Osiris team/UPD/LAM/IAA/SSO/INTA/UPD/DASP/IDA

Site J it is! European Space Agency scientists have selected a safe but intriguing region on 67P/Churyumov-Gerasimenko for humanity’s first-ever soft landing on a comet. Site C, located on the larger ‘body’ of the comet, was selected as backup.

Close-up of Philae’s primary landing site J, which is located on the ‘head’ of Comet 67P/Churyumov–Gerasimenko. The image was taken by Rosetta’s OSIRIS narrow-angle camera on August 20, 2014 from a distance of about 42 miles. Click to enlarge. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Choosing a landing site hasn’t been easy. Safety was a prime concern. Site J appears to have relatively few boulders and receives sufficient daily sunlight to recharge Philae and continue science operations on the surface beyond the initial battery-powered phase. The majority of slopes there are sloped less than 30º relative to the local vertical, reducing the chances of Philae toppling over during touchdown. The J Site also allows the probe to be in regular communication with the orbiter as it passes overhead during each orbit of the comet.

Close-up of Site C,  Philae’s backup landing site.

“As we have seen from recent close-up images, the comet is a beautiful but dramatic world – it is scientifically exciting, but its shape makes it operationally challenging,” says Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center.

Since the descent to the comet is passive in the comet’s low gravity field – like dropping a piece of paper – it’s only possible to predict the landing point to within about 1,000 feet (~300 meters). Descent time to the surface is estimated at 7 hours. Landing has to happen by November before 67P/C-G becomes too active spewing dust and gas as it draws ever nearer the sun.

See it in 3D! You can roam Philae’s primary landing site with all its ridges and valleys if you have a pair of red-blue 3D glasses. Click to enlarge. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“There’s no time to lose, but now that we’re closer to the comet, continued science and mapping operations will help us improve the analysis of the primary and backup landing sites,” says ESA Rosetta flight director Andrea Accomazzo.

Site J offers scientists a place to analyze 67P/C-G’s ice and dust and study the ice vaporization processes that make comets such special critters.
During the descent, Philae will take photos and observe the comet’s dusty-gassy environment. Once it gently drops to the surface at the equivalent of a walking pace, it will fix itself to the comet’s crust using a harpoon-like device and ice screws in the landing legs. It will then shoot a 360-degree panorama of its surroundings.
Next comes an analysis of the plasma and magnetic environment, and the surface and subsurface temperature. The lander will employ a drill to collect samples from beneath the surface and analyze them inside its onboard lab. The interior structure of the comet will also be explored by sending radio waves through the surface towards Rosetta.
Source: Rosetta blog, ESA

Philae snaps photo of Rosetta orbiting comet / New mosaic may show ice particles

The Philae lander, stowed aboard Rosetta, took this photo of the side of the mothership and one of its 46-foot (14-m) long solar arrays with comet 67P/Churyumov-Gerasimenko in the background. Two images with different exposure times were combined to capture the bright, 2.5-mile-wide comet and dimmer hardware. Click to enlarge. Credit: ESA/Rosetta/Philae/CIVA

Seeing this makes me feel like I’m in orbit. What a nice perspective with the comet in the frame. It was taken by the Comet Infrared and Visible Analyzer (CIVA) on September 7th and shows part of the spacecraft and one of its solar arrays from a distance of about 31 miles (50 km) from the comet.

CIVA is several instruments in one. CIVA-P consists of seven cameras – five regular cameras and one dual-instrument stereo camera – that will take a 360º panorama of the landing site once it’s safely on the surface of the comet. A section of the panorama will be shot in stereo.  Landing is scheduled for November 11th.

Artist’s impression of Rosetta’s lander Philae (front view) on the surface of the comet. Philae will be deployed to 67P/C-G in November where it will make in situ observations of the comet surface, including drilling 9 inches (23 cm) into the surface to gather a sample for analysis in its on board laboratory. Copyright: ESA/ATG medialab

CIVA-M has a miniaturized visible light microscope and a coupled infrared spectrometer (an instrument used to measure light in the infrared part of the spectrum) that will be used to study a drilled sample from the comet’s crust. CIVA-M is designed to identify organic materials in the soil.

Four-image navigation camera mosaic of Comet 67P/Churyumov-Gerasimenko, using photos taken on September 10 when Rosetta was just 17.3 miles from the comet. Click for a giant version. Copyright: ESA/Rosetta/NAVCAM

Stephan Ulamec, Philae lander manager, will announce the primary and backup landing sites for the washing-machine-sized probe at a presentation tomorrow morning at ESA Headquarters in Paris. The challenges and scientific expectations of the sites will be presented and shared with the media. We’ll have photos and the latest information here when it arrives.

In this cropped version, a bright, reflective spot is visible. It doesn’t show in images taken before this one (although the shadow does, so it’s unrelated). It’s either a camera artifact or something reflective like a fleck of ice or dust that passed by in the foreground. Credit: ESA/Rosetta/NAVCAM

In the meantime, have fun exploring the huge mosaic of 67P/C-G snapped from just 17.3 miles (27.8 km) away. In the cropped closeup, there’s an interesting bright spot that may either be an artifact or a something reflective (ice?) in the foreground. Take a look at the image below, which I’ve lightened up to show the comet’s neck in reflected sunlight and a speckling of either “noise” from cosmic ray hits or a mix of noise and reflections from bright ice and dust particles released by the comet.

In this lightened up view of the comet, we can the neck area dimly illuminated by sunlight reflecting from the comet’s larger lobe. Small streaks and specks – cosmic ray hits, detector noise or dust and ice or some combination – are visible too. I’ve circled a few of the larger ones. Credit: ESA/Rosetta/NAVCAM

What happened to the aurora? / New forecast for tonight Sept. 13-14

Observers in Maine were treated to a very nice aurora early last night September 12th. Mike Taylor saw this “intense aurora” light up above the unused railroad tracks along Unity Pond at 8:38 p.m. Click to see more of Mike’s work. Credit: Mike Taylor

Feeling disappointed in the aurora last night? The storm happened as forecast only it petered out just about the time the sky was getting dark across much of North America. Observers in Maine caught a good show early, and the lights even put in an appearance here in northern Minnesota, albeit low in the north from behind clouds.

The Kp index, an indicator of magnetic disturbances in Earth’s magnetosphere, shot up to “7″ last night before dropping off to low activity, where it’s remained all day so far today. Credit: NOAA

NOAA space weather forecasters call for minor G1 storm tonight September 13 from about 10 pm to 4 a.m. Central Daylight Time tomorrow.

Minor usually means auroras in the bottom half of the northern sky for skywatchers living in the U.S.-Canada borderland region. You may choose to ignore the forecast and go to bed. I understand. You’re feeling a little burned. Those who feel like soldiering on, remain alert for possible auroras.

ACE orbits ahead of Earth toward the sun and can measure the clouds of plasma belted out by the sun about an hour before they arrive at Earth. Credit: NASA

It’s hard to blame NOAA. Predicting the magnetic inclination of a cloud of solar plasma at a distance is fraught with uncertainty. We get a little help from the Advanced Composition Explorer (ACE) which orbits at the L1 libration point, one of five places near Earth where the sun’s and Earth’s gravity are in balance, allowing a satellite placed there to remain relatively stationary. ACE pivots about some 932,000 miles (1.5 million km) from Earth and 92 million miles (148.5 million km) from the sun.

The probe detects the direction, strength and magnetic field particulars of incoming blasts of particles from the sun and provides advance warning of about one hour of dangerous storms. Storms that affect power grids, satellites and of course paint the sky in northern lights. It also measures the magnetic properties of the cloud and relays that data in real time for us to see in the ACE plots.

ACE plot of magnetic field direction or Bz from last night. You can see how the storm dissipated once the magnetic direction of the cloud changed from south (during the storm) to north (above the white horizontal line). Credit: NASA

Yesterday’s big puff of electrons and protons came packaged in a magnetic field that linked into Earth’s  - at first. But later in the evening, the cloud’s magnetic field changed from south to north and was effectively cut off from connecting with our planet’s magnetic bubble. Earth gave it the cold shoulder, and you and I lost some sleep.

After tonight, calmer conditions are expected for a couple days. After that, it’s anyone’s guess. I’ll be watching tonight and report back.

Curiosity rover reaches the sublime Mt. Sharp

The next goal for NASA’s Mars Curiosity Rover – the beautiful layered rocks at the base of Mt. Sharp. Credit: NASA/JPL

You’ve seen it looming in the background of so many photo for more than two years. Finally, we’re there. NASA’s Curiosity rover rolled up to Mount Sharp in Gale Crater. With a peak 2.7 miles (4.4 km) high, Mt. Sharp stands more than a half mile higher than Mt. Ranier in Washington.

Orbital view of the 96-mile-wide Gale Crater, the peak Mt. Sharp and Curiosity’s landing site. The rover now begins its journey up the mountain’s slope. Credit: NASA

The mountain is built of layer upon layer of stratified rocks deposited by water and wind after the massive impact that excavated Gale Crater more than 3 billion years ago. From orbit, scientists have detected clays in some of the layers, an indication that water flowed here in the past.

Curiosity’s route up Mt. Sharp will first take it through the Pahrump Hills, which make up part of the Murray Formation of layered rocks. The white dashed line represents the border between rocks of Gale Crater’s plains, which Curiosity has investigated since landing, and those at the base of Mt. Sharp. Click to enlarge. Credit: NASA

As Curiosity begins its trek up the mountain’s slope, it will first trundle across the Pahrump Hills, a region of layered rocks that’s part of the Murray Formation. At some point within this broad expanse of soft rock, the rover will drill a sample and analyze it before continuing upslope. Several miles later, it will encounter a ridge of hematite-bearing rocks. Hematite is a gray version of iron oxide (rust) that precipitates in hot springs or in pools of standing water.

Orbital view of the 96-mile-wide Gale Crater showing Curiosity’s planned path up Mt. Sharp. The rover has traversed 5.5 miles to reach the mountain’s base. Credit: NASA image with illustration by T. Reyes

An intriguing layer of clay-bearing rocks that lies further upslope and offers the best opportunity of finding organic, carbon-containing minerals. A region containing sulfates, found earlier by Curiosity in the form of gypsum (calcium sulfate) extends beyond the clay layer higher yet. Gypsum is the same material used to make drywall back on Earth.

More detailed view of a potential path up Mt. Sharp from an earlier study this year showing the different terrains Curiosity will traverse. Credit: NASA/JPL

Scientists hope to study the transition between the two. Sulfates point to a time when the ancient, more watery Mars evolved from a wet, fresh-water climate to a drier one with acidic waters that favored the formation of sulfates instead of clays.

We all hope Curiosity’s wheels, poked and torn by sharp rocks, will be up for the long journey ahead.

“In late 2013, the rover team realized a region of Martian terrain littered with sharp, embedded rocks was poking holes in four of the rover’s six wheels. This damage accelerated the rate of wear and tear beyond that for which the rover team had planned. The team altered the rover’s route to a milder terrain, bringing the rover farther south, toward the base of Mount Sharp”, according to NASA.

A view of Mars taken on September 12, 2014 by the Curiosity rover’s hazard avoidance camera. Inset shows a big hole in one of the rover’s aluminum wheels from 2013. Looks rough out there! Credit: NASA/JPL

Curiosity has already fulfilled its initial goal of determining whether Mars ever offered an environment suitable for the formation and development of early life. Clay-bearing rocks in the Yellowknife Bay site revealed an ancient lakebed that once lapped with fresh water and contained the key elemental ingredients for life - sulfur, nitrogen, hydrogen, oxygen, phosphorus and carbon – as well as a sulfate energy source potential life could use to thrive.

Showtime! Aurora makes first appearance – updated

A diffuse double is seen low in the northern sky at 9:30 p.m. CDT this evening September 11, 2014. The Big Dipper is off to the left. Credit: Bob King

(Scroll down for the latest update)

Just came in from a check on the northern lights and they’re out. Just a quiet start, but I can see a classic green arc low in the northern sky. Once my eyes were dark adapted, faint rays streaked the sky above the arc. No doubt they would have stood out more boldly were it not for the rising gibbous moon off to the east. Stay tuned for more updates during the night.

Faint rays streak either side of this photo taken at 9:35 p.m. Moonlight lowered the contrast but they were dimly visible with the naked eye. Credit: Bob King

Here are some links for you to check out to help you plan through the night:

The approximate extent of the auroral oval forecast for 10:30 p.m. CDT from Ovation. Credit: NOAA

* Ovation oval – shows the approximate extent of the auroral oval that looks like a cap centered on Earth’s geomagnetic pole. During storms, the oval extends south into the northern U.S. and farther.

* Kp index – indicator of magnetic activity high overhead and updated every three hours. A Kp index of “5″ means the onset of a minor storm; a Kp of “6″, a moderate storm.

* NOAA space weather forecast

* Advanced Composition Explorer (ACE) satellite plots - The magnetic field direction of the arriving wind from the sun. The topmost graph, plotting Bz, is your friend. When it drops into the negative zone that’s good! A prolonged stay at -10 or lower increases the chance of seeing the aurora.

* UPDATE 8:15 a.m. Saturday Sept. 13: Well, well, well. Yes, the effects of the solar blast did arrive and we did experience a G3 storm, only the best part happened before nightfall had settled over the U.S. and southern Canada. The peak was also fairly brief. All those arriving protons and electrons connected for a time with Earth’s magnetic field but then disconnected, leaving us with a weak storm for much of the rest of the night. More activity is expected tonight but the forecast calls for a lesser G1 geomagnetic storm.

* UPDATE 10:30 p.m. : Although the aurora has died back, I just got the NOAA forecast update which still calls for a strong storm overnight. Crossing my fingers it happens.

Graph of Bz from the ACE spacecraft shows the past 24 hours of solar wind direction changes. Far right is 11 p.m. CDT Credit: NOAA

* UPDATE 9:30 p.m. : Definite aurora seen through breaks in the clouds low in the northern sky here in Duluth, Minn. After a big surge late this afternoon and during early evening, activity’s temporarily dropped off. The ACE plot has “gone north”. Will keep tabs and report back.

UPDATE Friday 7:30 p.m September 12: Wow! Kp=7 (G3 storm). Auroras should be visible now over the far eastern seaboard of Canada including New Brunswick and the Gaspe Peninsula. If I were a betting man, folks in Maine should see at least a low, glowing arc in the northern sky. Still dusk here in Duluth.

UPDATE Friday 3 p.m.: The Kp index is now at “5″ or minor storm. If you live in the Scandinavian countries or Iceland, you’re getting a very good show right now.

* UPDATE Friday 9 a.m. September 12: Auroras did appear as forecast overnight beginning at nightfall and continuing through about 1 a.m. this morning. Then the action stopped. The Kp index reached “5″ during that time leading to a G1 or minor geomagnetic storm. It wasn’t a particularly bright aurora, remained low in the northern sky and had to compete with moonlight, so many of you may not have seen it.

The stronger G3 geomagnetic storm from the second and more Earth-directed solar blast is still forecast for tonight. This should bring a much better display and should begin right at nightfall. Peak is expected between 10 p.m. and 1 a.m. Central Daylight Time.

My forecast is good, so I’ll be updating during the night. Good luck and clear skies!

Twin solar storms may stoke auroras tonight Sept. 11-12

A CME or coronal mass ejection from the sun on September 9 is expected to pass Earth later today and possibly spark auroras tonight. Credit: NASA/ESA

(Click HERE for updates))

Two bursts of solar particles propelled by flares on September 9th and 10th are expected to arrive starting tonight and possibly touch off a moderate G2 geomagnetic storm. Translation: auroras may bloom in the next few nights!

A moderate M4 flare occurred early on September 9th followed by a more powerful X1.6 yesterday afternoon. Provided the magnetic field the particles come packaged in points in the right direction – south – these bursts have good potential for creating auroras tonight and again over the weekend.

A second, Earth-directed CME leaves the sun in the wake of the X1.6 flare on September 10th. Credit: NASA/ESA

The timing is good because the moon is past full and won’t be too bright. During a moderate storm, auroras are often seen across the northern tier of states and Canada. According to the latest NOAA space weather forecast, activity should kick up but remain shy of storm level from 9 p.m.-midnight Central Daylight Time tonight September 11th.

The brunt of the storm is expected from 1-4 a.m. tomorrow morning the 12th with effects lasting until 7 a.m.

This may only be the start of an even stronger storm anticipated Friday night and continuing into the weekend beginning from yesterday’s flare. That one blasted material directly toward Earth. Always a good omen for auroras.

Earth’s magnetic bubble, generated by motions within its iron-nickel core and shaped by the solar wind, is called the magnetosphere. It extends some 40,000 miles forward of the planet and more than 3.9 million miles in the tailward direction. Credit: NASA

As always with northern lights, keep in mind they’re fickle. Most of the time, Earth’s magnetic defense – a humongous, teardrop-shaped bubble of magnetism called the magnetosphere –  acts as a bulwark against strong solar winds, letting them slide by harmlessly. We’ll see what happens on this round, but I’m optimistic.

The Earth weather forecast for my locale is mostly clear tonight, so I’ll be monitoring the sky. Stop back later for an update.

* UPDATE 9 p.m. CDT: Quiet so far. Auroras still holed up in Hudson Bay and Quebec. The magnetic field direction of the arriving wind from the sun shows a lot of variation (see ACE satellite plot, topmost graph showing Bz) rising and falling from positive to negative. Negative is good! A prolonged stay at -10 or lower increase the chance of seeing the aurora.

Struggled to find Uranus? Let the moon take you there tonight

Once you’re done chuckling, we’ll move on. Ahem!

The waning gibbous moon will near the planet Uranus tonight September 10, 2014. From northeastern U.S. it will be covered by the moon. These views show moon and planet from Syracuse (eastern U.S.) and the Midwest at the times shown. Source: Stellarium

If you’ve ever had trouble finding the remote planet Uranus, Luna can help you tonight. The waning gibbous moon will occult or cover up the planet for observers in northeastern North America, Greenland, Iceland and northern Scandinavia around 8 p.m. Eastern Daylight Time this evening.

If you have a small telescope, you’ll be able to watch the bright eastern (left) edge of the moon slowly approach and then hide the planet. Unlike a point-like star, which winks out in a split second when covered by the moon’s edge, Uranus shows a small disk and will fade more gradually over several seconds.

Observers in the wedge-shaped zone that spans the Northeastern U.S., Canada and other northern countries will see the moon cover Uranus. Those living in the U.S. and Canada will spy the planet very close to the moon’s west rim. Credit: USNO

But let’s say like me you live outside the occultation zone. What will we see? From the Midwest, Uranus will be just less than 1° to the west (right) of the moon as it comes up in the eastern sky in late twilight. Over the hours, it will appear to move gradually drift to the west away from the moon as the moon moves eastward in its orbit.

The farther west you live, the farther Uranus will be from the moon’s western edge. But not too far. Even from the California Coast, Uranus strays only about 2° (four moon diameters) to the right of the moon.

The planet may even be easier to see in binoculars from points west because it will be further from the lunar glare. No matter what, it’ll be easy to find the planet, which shines around 6th magnitude.

The view from the U.S. West Coast around 10 o’clock local time tonight. Source: Stellarium

Remember, you’ll need 50 mm binoculars, or better, a small telescope, to view the planet near the moon. Telescope users are encouraged to crank up the magnification and see Uranus’ diminutive disk next the moon, which appears gigantic in comparison. In reality, the 7th planet is nearly 15 times as large.

Uranus only a degree east of the totally eclipsed moon seen from the Midwest on October 8, 2014. Stellarium

Get ready for an even better shot at seeing Uranus. On the morning of October 8th, the full moon will be in total eclipse and the planet will lie very close due east. With no glary moonlight and everyone focused on the eclipse, more people will probably see Uranus at one time than perhaps any time in history.

Harvest Moon glow show / Rosetta captures pieces of a comet

The Full Harvest Moon rises from Lake Superior in Duluth, Minn. as the HR Maria heads down the lake last night September 8, 2014. Credit: Bob King

We luckily avoided the clouds last night to see a big pink moon rise from Lake Superior. The moon languished near the horizon for a long time as if reluctant to meet the deck of clouds waiting for it higher up.

The full moon prepares to enter the cloud deck shortly after moonrise last night. Credit: Bob King

Watch for it to rise tonight about a half hour later. Will you be able to tell it’s no longer a perfect circle? A shadow has crept up along its western edge transforming it from a full to a waning gibbous moon.

Because the moon orbits Earth, we see it from a slightly different angle in relation to the sun each night, causing different parts of the globe to be in sunlight and shadow.

COSIMA catches bits of the comet. Left: an image of the target plate (measuring about 1/2-inch on the side) on which the grains were collected; right: a section of the plate showing it on August 17 when no dust grains were visible and August 24 (bottom) when some large dust grains were detected. The plate is illuminated from the right by LEDs and the length of the shadows is proportional to the height of the dust grains. Credit:  ESA/Rosetta/MPS for COSIMA Team et. all

While you and I may prefer our dust swept away, planetary astronomers feel otherwise. Scientists at the European Planetary Science Conference (EPSC) in Lisbon, Portugal, recently got a look at photos of the first dust grains collected by Rosetta’s COSIMA instrument. The specks were gathered between August 11th and 24th  from a distance of around 62 miles (100 km) from the nucleus of comet 67P/Churyumov-Gerasimenko

Many tiny grains showed up on the plate; the two largest, each about the width of a human hair, stand out in the photo above. Some of the samples will be selected for further analysis on board the probe. Here’s how it works.

Jagged cliffs and prominent boulders are visible in this image taken by the OSIRIS camera on September 5, 2014 from a distance of 39 miles (62 km) from comet. The left part of the image shows a side view of the comet’s ‘body’, while the right is the back of its ‘head’. One pixel corresponds to 3.6 feet (1.1-m). Click for a huge picture! Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The target plate will be moved to place each selected grain under an ion gun, a device that blasts the particle with a beam of ions. Ions are atoms which have gained or lost an electron and become either positively or negatively charged. The ion beam will ablate or vaporize the grain layer by layer. The material is then analyzed in a secondary ion mass spectrometer to determine its composition.

Several different regions are indicated in this preliminary map, which is oriented with the comet’s ‘body’ in the foreground and the ‘head’ in the background. The brown smooth terrain contrasts with the rugged blue region.

Astronomers are quite excited about getting these early dust grains studied as they’re the first to be retrieved from the solar system’s “snow line”, the distance from the sun at which stable ice grains can form.

A new color-coded terrain map of 67P/C-G has also been created based depicting regional variations in the comet’s landscape. Some terrains are smooth, others dominated by cliffs and still others by craters and depressions. The maps will play a key role this weekend (Sept. 13-14) as Rosetta’s Lander Team and the Rosetta orbiter scientists determine primary and backup landing sites for the Philae lander.

Asteroid 2014 RC makes astronomers’ heads spin / Crater appears in Nicaragua

Multiple images of asteroid 2014 RC made with the Lowell Observatory 42-inch Hall telescope showing its motion on September 7, 2014.

Not only did it make a very close approach to Earth yesterday, but astronomers got a better handle on 2014 RC’s size and how fast it spins during the flyby.

The space rock was originally thought to be 60-feet (20-m) across but we now know it’s closer to 40-feet (12-m) or about the size of a school bus and spinning very rapidly. Try one rotation every 15.8 seconds!

That makes 2014 RC the fastest rotating asteroid (by 50%) observed to date. Astronomers using NASA’s Infrared Telescope Facility (IRTF) on September 6 observed the space rock in infrared light to determine its composition. It reflects about much light (25%) as green grass and belongs to the “Sq class” of stony asteroids. Rocky asteroids are the most common kind. Based on its reflectivity, 2014 RC is much brighter than a typical comet and several other classes of asteroids.

The suspect crater near the Managua airport. Credit: AP

In a story begging to be related but isn’t, a purported meteor crater showed up near Managua, Nicaragua’s international airport this weekend. Whatever caused  it, the blast occurred during the early morning of September 6th, 13 hours before 2014 RC’s close flyby, which passed beneath Earth’s orbit anyway. A loud bang was heard and the ground shook, but there are no eyewitness reports or video of a meteor or meteorite fall. The crater is about 40 feet (12-m) across.

The surest way to confirm if a meteorite fell would be to look for fragments in and around the crater. No news yet on that. I’ll update when more information becomes available.

What Makes the Harvest Moon Special

“The Harvest Moon”, a circa 1833 oil painting by Samuel Palmer. Closely spaced moonrises meant extra light to bring in the crops in the days before electric lighting.

Today, September 8, the Harvest Moon will rise the color of a fall leaf and spill its light across deserts, forest, oceans and cities. The next night it rises only a half hour later. And the next, too. The short gap of time between successive moonrises gave farmers in the days before electricity extra light to harvest their crops, hence the name.

The Harvest Moon is the full moon that falls closest to the autumnal equinox, the beginning of northern autumn. As the moon orbits the Earth, it moves eastward about one fist held at arm’s length each night and rises about 50 minutes later. You can see its orbital travels for yourself by comparing the moon’s nightly position to a bright star or constellation.

Around the time of Harvest Moon, the full moon’s path is tilted at a shallow angle to the eastern horizon making with successive moonrises only about a half hour apart instead of the usual 50 minutes. Source: Stellarium

50 minutes is the usual gap between moonrises. But it can vary from 25 minutes to more than an hour depending upon the angle the moon’s path makes to the eastern horizon at rise time. In September that path runs above the horizon at a shallow angle. As the moon scoots eastward, it’s also moving northward this time of year.

This northward motion isn’t as obvious unless you watch the moon over the coming week. Then you’ll see it climb to the very top of its monthly path when it’s high overhead at dawn. The northward motion partly cancels out the eastward motion, keeping the September full moon’s path roughly parallel to the horizon with successive rise times only ~30 minutes apart.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King

Exactly the opposite happened 6 months earlier this spring, when the moon’s path met the horizon at a steep angle. While it traveled the identical distance each night then as now, its tilted path dunked it much farther below the horizon night to night. The spring full moon moves east and south towards its lowest point in the sky. Seen from the northern hemisphere, that southward travel adds in extra time for the moon to reach the horizon and rise each successive night.

If all this is a bit mind-bending, don’t sweat it. Click HERE to find when the moon rises for your town and find a spot with a great view of the eastern horizon. You’ll notice the moon is orange or red at moonrise because the many miles of thicker atmosphere you look through when you gaze along the horizon scatters the shorter bluer colors from moonlight, tinting it red just as it does the sun.

A series of photos of the full moon setting over Earth’s limb taken by from space by NASA astronaut Don Pettit on April 16, 2003. Refraction causes a celestial object’s light to be bent upwards, so it appears higher than it actually is. The bottom half of the moon, closer to the horizon, is refracted strongest and “pushed” upward into the top half, making it look squished. Credit: NASA

The moon will also appear squished due to atmospheric refraction. Air is densest right at the horizon and refracts or bends light more strongly than the air immediately above it. Air “lifts” the bottom of the moon – which is closer to the horizon – more than the top, squishing the two halves together into an egg or oval shape.

How we perceive the moon’s size may have much to do with what’s around it. In this illustration, most of us seen the bottom moon as smaller, but they’re both exactly the same size. Crazy, isn’t it? Credit: NASA

You may also be entranced Monday night by the Moon Illusion, where the full moon appears unnaturally large when near the horizon compared to when viewed higher up. No one has come up with a complete explanation for this intriguing aspect of our perception, but the link above offers some interesting hypotheses.

Can you see craters with your naked eye? Yes! Try tonight through Wednesday night. Plato is the trickiest. Credit: Bob King

Finally, full moon is an ideal time to see several lunar craters with the naked eye. They’re not the biggest, but all, except Plato, are surrounded by bright rays of secondary impact craters that expand their size and provide good contrast against the darker lunar “seas”. Try with your eyes alone first, and if you have difficulty, use binoculars to get familiar with the landscape and then try again with your unaided eyes.

In contrast to the other craters, Plato is dark against a bright landscape. This one’s a real challenge – I’ve tried for years but still haven’t convinced myself of seeing it. The others are easier than you’d think. Good luck and clear skies!