The Solar Eclipse Caused a Bow Wave in Earth’s Atmosphere

This graphic shows atmospheric bow waves forming during the August 2017 eclipse over the continental United States. Image: Shunrong Zhang/Haystack Observatory
This graphic shows atmospheric bow waves forming during the August 2017 eclipse over the continental United States. Image: Shunrong Zhang/Haystack Observatory

It’s long been predicted that a solar eclipse would cause a bow wave in Earth’s ionosphere. The August 2017 eclipse—called the “Great American Eclipse” because it crossed the continental US— gave scientists a chance to test that prediction. Scientists at MIT’s Haystack Observatory used more than 2,000 GNSS (Global Navigation Satellite System) receivers across the continental US to observe this type of bow wave for the first time.

The Great American Eclipse took 90 minutes to cross the US, with totality lasting only a few minutes at any location. As the Moon’s shadow moved across the US at supersonic speeds, it created a rapid temperature drop. After moving on, the temperature rose again. This rapid heating and cooling is what caused the ionospheric bow wave.

The bow wave itself is made up of fluctuations in the electron content of the ionosphere. The GNSS receivers collect very accurate data on the TEC (Total Electron Content) of the ionosphere. This animation shows the bow wave of electron content moving across the US.

The details of this bow wave were published in a paper by Shun-Rong Zhang and colleagues at MIT’s Haystack Observatory, and colleagues at the University of Tromso in Norway. In their paper, they explain it like this: “The eclipse shadow has a supersonic motion which [generates] atmospheric bow waves, similar to a fast-moving river boat, with waves starting in the lower atmosphere and propagating into the ionosphere. Eclipse passage generated clear ionospheric bow waves in electron content disturbances emanating from totality primarily over central/eastern United States. Study of wave characteristics reveals complex interconnections between the sun, moon, and Earth’s neutral atmosphere and ionosphere.”

The ionosphere stretches from about 50 km to 1000 km in altitude during the day. It swells as radiation from the Sun reaches Earth, and subsides at night. Its size is always fluctuating during the day. It’s called the ionosphere because it’s the region where charged particles created by solar radiation reside. The ionosphere is also where auroras occur. But more importantly, it’s where radio waves propagate.

The ionosphere surrounds the Earth, extending from about 80 km to 650 km. Image Credit:  NASA's Goddard Space Flight Center/Duberstein
The ionosphere surrounds the Earth, extending from about 80 km to 650 km. Image Credit: NASA’s Goddard Space Flight Center/Duberstein

The ionosphere plays an important role in the modern world. It allows radio waves to travel over the horizon, and also affects satellite communications. This image shows some of the complex ways our communications systems interact with the ionosphere.

This graphic shows some of the effects that the ionosphere has on communications. Image: National Institute of Information and Communications Technology
This graphic shows some of the effects that the ionosphere has on communications. Image: National Institute of Information and Communications Technology

There’s a lot going on in the ionosphere. There are different types of waves and disturbances besides the bow wave. A better understanding of the ionosphere is important in our modern world, and the August eclipse gave scientists a chance not only to observe the bow wave, but also to study the ionosphere in greater detail.

The GNSS data used to observe the bow wave was key in another study as well. This one was also published in the journal Geophysical Research Letters, and was led by Anthea Coster of the Haystack Observatory. The data from the network of GNSS was used to detect the Total Electron Content (TEC) and the differential TEC. They then analyzed that data for a couple things during the passage of the eclipse: the latitudinal and longitudinal response of the TEC, and the presence of any Travelling Ionospheric Disturbances (TID) to the TEC.

Predictions showed a 35% reduction in TEC, but the team was surprised to find a reduction of up to 60%. They were also surprised to find structures of increased TEC over the Rocky Mountains, though that was never predicted. These structures are probably linked to atmospheric waves created in the lower atmosphere by the Rocky Mountains during the solar eclipse, but their exact nature needs to be investigated.

This image of GNSS data shows the positive Travelling Ionospheric Disturbance (TID) structure in the center of the primary TEC depleted region. The triangles mark cities in or near the Rocky Mountains. Image: Coster et. al.
This image of GNSS data shows the positive Travelling Ionospheric Disturbance (TID) structure in the center of the primary TEC depleted region. The triangles mark cities in or near the Rocky Mountains. Image: Coster et. al.

“… a giant active celestial experiment provided by the sun and moon.” – Phil Erickson, assistant director at Haystack Observatory.

“Since the first days of radio communications more than 100 years ago, eclipses have been known to have large and sometimes unanticipated effects on the ionized part of Earth’s atmosphere and the signals that pass through it,” says Phil Erickson, assistant director at Haystack and lead for the atmospheric and geospace sciences group. “These new results from Haystack-led studies are an excellent example of how much still remains to be learned about our atmosphere and its complex interactions through observing one of nature’s most spectacular sights — a giant active celestial experiment provided by the sun and moon. The power of modern observing methods, including radio remote sensors distributed widely across the United States, was key to revealing these new and fascinating features.”

The Great American Eclipse has come and gone, but the detailed data gathered during that 90 minute “celestial experiment” will be examined by scientists for some time.

Tales From Totality: Standing in the Shadow of the Moon

A brilliant diamond ring punctuates totality. Image credit and copyright: Shahrin Ahmad.
A brilliant diamond ring punctuates totality. Image credit and copyright: Shahrin Ahmad.

They came, they saw, they battled clouds, traffic and strange charger adapters in a strange land. Yesterday, millions stood in awe as the shadow of the Moon rolled over the contiguous United States for the first time in a century. If you’re like us, your social media feed is now brimming with amazing images of yesterday’s total solar eclipse.

Already, we’ve seen some amazing reader images at Universe Today, with more to come. As a special look at a unique event, we’ve collected reader testimonies from every state along the path of totality of just what the eclipse was like.

Enjoy!

Oregon- Shahrin Ahmad (@Shahgazer)

We drove from Dalles at 3 AM. Nearing the observation spot, we got a flat tire! It was 5:30 AM, and no phone line! I sent a text to the land owner and somehow it reached him and we managed to be there by 6:30 AM. We observed from a secluded spot about 30 miles from Madras, with a 2 minutes and 2 seconds of totality. The sky was really clear during sunrise, but as totality approached we got some thin clouds hovering in the east. Luckily, it was thin enough to not spoil anything. The corona was incredibly beautiful with longer (streamers) jutting out at the 4 and 8 o’clock position. The first and second diamond ring were spectacular with the eye, probably with the help with the thin clouds. We calculated about 7 degree drop in temperature. The shadow was enormous, engulfing Mt Hood from the west and flew past above us towards and towards the Sun. Mesmerizing! 2 minutes simply was not enough, since this is probably my best view of a total solar eclipse so far!

The bright star Regulus, tangled up in the solar corona. Image credit and copyright: Shahrin Ahmad.

(Note: to our knowledge, no one witnessed the brief moments of totality as the umbra of the Moon brushed tiny corners of Montana and Iowa… if you’re reading this and did so, let us know!)

Idaho- Bruce McCurdy (@BruceMcCurdy)

How to describe such a magnificent spectacle in a “brief paragraph”? Our group from Edmonton observed totality under clear skies near Birch Creek, Idaho. After the Moon’s silhouette inexorably progressed & gradually swallowed up an impressive line of sunspots, the pace of dynamic events picked up dramatically in the minutes surrounding totality. The temperature dropped noticeably. Light faded & became “flat” while shadows became better defined & lost their fuzzy edges (penumbrae). The Moon’s onrushing shadow became visible on the mountains to our west, while rapidly-moving shadow bands squiggled on the ground around us. The sky took on an eerie indigo hue as the last vestiges of direct sunlight were obscured. A new & temporary centrepiece emerged in the sky: the black circle of the lunar night side highlighted by a spectacular corona, its far-flung pearly-white streamers contained within sharply defined edges. Around the black limb fiery coral pink prominences added intense colour highlights to the scene. Just beyond the corona gleamed Regulus, closer to the Sun than is possible for any other star of first magnitude or brighter, while off to one side Venus shone brilliantly, far higher in the sky than its customary window of dominance in normal twilight. All too soon the right edge of the lunar silhouette brightened, then blossomed in a brilliant diamond ring that continued to intensify for a couple of glorious seconds until filters again became a must. By now the mountains to our east were in darkness as the umbral shadow receded from our immediate location, leaving a number of our small party in tears from the intensity of the experience.

Wyoming- Kelly Kizer Whitt (@Astronomommy)

We woke up in the Tetons Monday morning to a sky streaked with clouds. But the hourly weather report showed clearing, so we headed to our spot before 7 AM. We were able to secure parking by our preferred observing location, the Mormon Barn with a view of the iconic Teton range in the background. Looking east, we saw the clouds slink away to the south until skies were blue and clear, despite lingering haze and smoke on the northern horizon from wildfires.

Crescent Suns along with the Tetons. image credit and copyright: Kelly Kizer Whitt.

Having been a science writer for two decades, I was well versed on total solar eclipses even though I’d never seen one first hand. But it didn’t unfold quite as I expected. The sky and air didn’t take on a twilight quality until the Sun was well over halfway obscured. Then when darkness fell, it came fast and the temperature dropped hard. We had on our eclipse glasses and were staring at the Sun, waiting to see bailey’s beads or the diamond ring. But first I glanced down and saw the slithering, wiggling lines of darkness and light known as the shadow bands. They have a truly creepy quality as they dance in the growing dark. Then we looked back up as the sliver of orange disappeared and the Sun winked out from our glasses. Pulling them off, my family let out cries of surprise when they saw the black hole where the Sun had been, surrounded by the long, wispy, intricate corona. The eclipsed Sun and corona took up a much larger space in the sky than I expected, but the photo I took (just like when photographing a full moon) does not give a true representation of what you can see with your eyes.

I only took three photos because I wanted to just enjoy the view. I almost forgot to look for the stars. We saw a plane, Venus, and Sirius. Our eyes never adjusted enough to spot Jupiter or the others and the rosy glow of a false twilight brightened all horizons in a 360-degree ring. So soon it was over. The bailey’s beads and diamond ring we missed as the total eclipse began, and appeared to us instead at the end. These phenomena were a bright and beautiful warning to get our eclipse glasses back on. The world returned to daylight fairly quickly, but the drop in temperature lingered a bit longer. Our memories will last a lifetime.

Nebraska- (@BigBadEd)

Having doubtful cloud forecasts for Scottsbluff & Carhenge,  we met on a foggy morning in Sidney, Nebraska with thoughts of changing plans to Wyoming for clear skies. As the forecast improved,  15 of us set off for Carhenge.  We arrived before 7 AM to plentiful parking & a few hundred people. Towards 9 AM the crowds started to swell, including aliens, welders and the governor of Nebraska. Joined by more people & dogs, I estimate around 3,000 people were at the site. Some clouds went by at mid-coverage, casting a spectacular crescent. Clouds cleared, and cheers rose as we went into totality,  such a beautiful sight some were moved to tears as the diamond ring emerged. A thoroughly wonderful experience shared with friends and spellbound crowd, definitely worth the trip from Florida.

Kansas- Michelle Tevis (@MichelleKTevis)

I saw it (the eclipse) from Weston, Missouri, just northwest of the Kansas-Missouri line. Clouds and rain obscured the sun for most of the eclipse, but the rain subsided during totality and allowed us to get outside for the quick move into darkness. Even though we couldn’t see the eclipse or corona, the atmosphere took on a different feel. There was a change in how things were colored — as if you were looking through darker and darker polarized glasses, and the silence took on a feeling, like a deep vibration.

Missouri- Jeudy Blanco (@Jeudyx)

Totality from Missouri. Image credit and copyright: Jeudy Blanco.

It was amazing. We changed plans last night, instead of going to St Joseph we drove to Columbia. I was really worried the first few minutes of the eclipse because it was cloudy, my PST couldn’t resolve the image of the Sun! But quickly the clouds dispersed. We were on a property from the family of my friend, around 25 people of all ages. When it was around 70% (partial) you could feel in the environment that something was going on. Everything got a lot more quiet and the temperature dropped. Everybody was trying to get pictures of the Sun with their phones on the PST. Then totality started, it was indescribable for me. I was seeing the Sun’s corona with my bare eyes. I was really nervous and anxious, actually. We could see Venus near the Sun. Everybody was super excited, I almost cried. The experience was amazing, a total success, the long trip was worth it.

Illinois- The Universe Today expedition to the Prairie State led by Publisher Fraser Cain also managed to catch a brief glimpse of totality through a gap in the clouds:

Kentucky- Mike Weasner (@Mweasner)

Earthshine (!) on the Moon, seen during totality. Image credit and copyright: Mike Weasner.

About 400 eclipse enthusiasts from around the world including me were part of a Sky and Telescope tour group. We were at Hopkinsville Community College located in Hopkinsville, Kentucky, where totality lasted 2 minutes and 40 seconds, which was too short. We arrived at the viewing site about 4.5 hours before First Contact. Traffic was surprisingly light. There were a few thin clouds but nothing significant. Anticipation was high. Many of us set up cameras and were ready well before First Contact. First Contact occurred with a clear sky, and the sky stayed mostly clear until about 30 minutes before Second Contact. Then a large cloud covered the Sun. Fortunately the cloud moved on within a couple of minutes and the sky was mostly clear through Fourth Contact. Totality was beautiful. Most people saw Venus, some saw Jupiter too, but no one seems to have seen any stars although it did get dark at the site. Many people in the group left soon after totality ended, but I and several others stayed to view and photograph the eclipse through Fourth Contact. 

Tennessee- (Terry Horne @CapH_1)

My wife and I viewed the event from Sheep Barn Ridge, which is a few miles from Kingston, TN. We began the planning in late 2015 when we realized the shadow path was adjacent to our property near my folks in TN. Our location delivered 2 minutes and 29 seconds of totality, with clear skies, a valley pasture view among new friends, goats, llama, ducks, chickens and a few hounds.

An amazing expample of the “Diamond Ring” effect. Image credit and copyright: Phyllis Horne @sahgma

We experienced every awe & oddity we had studied during the ramp up to the event. My wife did an excellent job with her photo efforts. She balanced her personal viewing time and planned equipment duties well. This was a source of much worry and discussion during the months prior.

I’ll mention a few surprises. I was impressed by the amount of light cast on the landscape with barely a sliver of the Sun remaining. I suspect the ambient sunlight to the south east was the major source. The rapid transition to peak darkness was dramatic.

In contrast, I noticed a clear reduction of heat radiation on my skin with about 50% coverage. It was a hot day. I wished I’d had more time to observe the animals.

I found it somewhat humorous how many folks took all of the important PSA’s about retina damage to heart. Before totality they bowed their heads to the ground when they did not have their gasses on while walking, standing and sitting.

What I learned most was, to the inexperienced, East Tennessee Moonshine travels faster than the Moon’s shadow.

Be careful!

Georgia- Jeannette Iriye (@i_fridrich)

We found a lovely scenic overlook facing west in Sky Valley, just outside Dillard, Georgia. Skies were clear with only minimal cloud cover until about 13:30, when heavy cloud cover began to build in the south/southeast. The clouds obfuscated the remainder of our view of the eclipse directly. It did get much cooler, windy, and the crickets were singing just prior to and during totality.

A partially eclipsed Sun versus clouds. Image credit and copyright: Jeannette Iriye.

South Carolina- Terri (@wizbee1)

We didn’t make it to South Carolina, and had to turn the plane back because of weather. Watched instead from Saint Mary’s Georgia. Did feel the temperature drop and experienced darkening but not in totality.

And us? We watched from the Pisgah Astronomical Research Institute in North Carolina as the shadow of the Moon draped over the landscape. The rolling afternoon clouds afforded only brief glimpses of the partially eclipsed Sun. Then, just prior to totality, we caught the final moments as the Sun withered to a brief diamond ring flash… and was gone. Magic! Unfortunately, the corona remained hidden behind high clouds for the 107 seconds of darkness, though we were treated to an unworldly 360 degree sunset below the cloud deck. Nocturnal mosquitoes, fooled by the false dusk, began their rounds, as a light “eclipse wind” kicked up.

Author and wife (@MyschaTheriault) standing in the shadow of the Moon, plus our view from the Pisgah Astronomical Research Institute (PARI) just before totality. Thanks to @Dayveesutton for snapping the pic!

Then, it was over. Got the eclipse bug? Well, another total solar eclipse crosses the U.S. in 2024… but you don’t have to wait that long, as we’ve got one coming right up crossing Argentina and Chile on July 2nd, 2019…

I’ll see you there!

Incredible Solar Eclipse Images From Our Readers

Totality of the August 21, 2017 solar eclipse, as seen from Waterloo, Illinois. Credit and copyright: Rob Sparks.

Holy moly, that was awesome! Incredible, fantastic, amazing…there just aren’t the words to describe what it is like to experience totality. While I’m trying to come down to Earth and figure out how to explain how wonderful this was, enjoy the beautiful images captured by our readers from across the US and those from across the world who traveled to capture one of nature’s most spectacular events: a total solar eclipse.

The images from those seeing partial eclipses are wonderful, as well, and we’ll keep adding them as they come in (update, we just got some from Europe too). Great job everyone!

Eclipse panorama. Got some cool Baily’s Beads and that prominence is nuts! Shot at 2000mm on an old Celestron 8in telescope! Credit and copyright: Kenneth Brandon.
2017 Solar Eclipse from Clayton GA, USA.
Celestron C8 Telescope on CGEM. Canon T3i (Modified IR enhanced), Solar Filter. Credit and copyright: Michael Bee.
The August 21, 2017 total solar eclipse over the Grand Tetons as seen from the Teton Valley in Idaho, near Driggs. ..This is from a 700-frame time-lapse and is of second contact just as the diamond ring is ending and the dark shadow of the Moon is approaching from the west at right, darkening the sky at right, and beginning to touch the Sun. The peaks of the Tetons are not yet in the umbral shadow and are still lit by the partially eclipsed Sun. ..With the Canon 6D and 14mm SP Rokinon lens at f/2.5 for 1/10 second at ISO 100. Credit and copyright: Alan Dyer.
Total Solar Eclipse, August 21, 2017 as seen from Tellico Plains, Tennessee. New City Expedition, photo by Igor Kuskovsky.
Total Solar Eclipse, Aug. 21, 2017, as seen from Charleston, South Carolina. Credit and copyright: Jason Major
Partial Eclipse montage from Charlottesville, Virginia. Credit and copyright: David Murr.
Partial Solar Eclipse August 21st 2017, as seen from Fullerton California USA. Sky: Partially Cloudy. Telescope: Nexstar 102 SLT Refractor, Camera: Fujifilm X-T1 @ Prime Focus. Credit and copyright: Jimmy CD.
From the total solar eclipse as seen in Columbia, Missouri, on Aug. 21, 2017. Credit and copyright: Wildhaven Creative.
Total Eclipse from Shaw Air Force Base (August 21, 2017). It was magical. Credit and copyright: Michael Seeley.

Partial solar eclipse, seen from the west coast of France, August 21, 2017. Credit and copyright: Frank Tyrlik.

Great American Eclipse, 21-08-2017. Silver Falls Oregon 10:17-10:19 local time. Raw straight out of the camera. 65mm Refractor / Canon 700D. Credit and copyright: Alexandra Hart.

NASA Live-Broadcasting 2017 Solar Eclipse!

Today, the NASA TV Public Channel is live-streaming their coverage of the totality of the 2017 Solar Eclipse as it covers a path reaching across the continental United States – from Oregon to South Carolina. The event, titled “Solar Eclipse: Through the Eyes of NASA“, begins at 1 p.m. EDT (11 am PDT). Be sure to check it out by following the link below:

https://www.nasa.gov/eclipselive/#NASA+TV+Public+Channel

Also, NASA has promised a plethora of information on this eclipse, which will include “images captured before, during and after the eclipse by 11 spacecraft, at least three NASA aircraft, more than 50 high-altitude balloons, and the astronauts aboard the International Space Station – each offering a unique vantage point for the celestial event.”

If you’re just reading this now, there’s still time! Head on over and see it all unfold!

Further Reading: NASA, NASA -Eclipse 2017

“Eclipse Across America:” Could This Event Bring Us All Together?

If you are looking forward to the August 21 solar eclipse as much as we are, you’ll want to check out a new series on CuriosityStream called “Eclipse Across America.”

The four-part series takes an inside preview at this highly anticipated event. NASA experts, an international group of astronomers and seasoned eclipse chasers reveal secret viewing spots and tips on how to safely view what could be for millions of people a once-in-a-lifetime opportunity to see one of the most spectacular sights in nature: a total solar eclipse.

Our friend and Astronomy Cast co-host Dr. Pamela Gay, who is also the Director of Technology at the Astronomical Society of the Pacific, is part of the all-star “astronomy cast” (pun intended) of experts, and she talked with us about her part in the show.

“I get to talk about the really cool parts of an eclipse, where you can experience totality and actually feel the temperature change,” she said. Using state-of-the-art special effects, Pamela gets to show off what the eclipse will likely look like at spots around her location in the St. Louis, MO area. “There’s the fickleness of it, where some buildings will see totality and another won’t,” she said. “Of course, the closer you get to the centerline the longer totality will be, and there are some beautiful locations where you can see it.”

A special effects image from CuriosityStream showing how the solar eclipse could look in St. Louis, Missouri. Credit: CuriosityStream.

The really fun part of this particular eclipse is that it takes a path across the entire continental U.S. I asked Pamela if, given the rather divisive political and social climate these days in this country, could this total solar eclipse become a possible unifying moment?

“I think that during the event there will be a lot of overwhelming experiences that people sometimes have of being in a crowd – such as will there be enough bathrooms,” she said. “But afterwards, I think it will be this shared ‘do you remember when’ moment that we’ll have for the rest of our lives. We all have certain moments that we remember and currently here in the US a lot of those memories are ‘where were you when 911 happened? Or where were you when we started bombing Iraq.’ This will be ‘where you when the sun was eclipsed,’ and that is so much better than ‘where were you when the towers fell down.’”

If you already have a CuriosityStream account, you can watch the series starting with Episode One here. If you don’t, you can take advantage of a 7-day free trial in order to watch this series, and all the other great science offerings available, such as Stephen Hawking’s Universe, Brian Cox’s Wonders of Life, and other topics from astronomy observing tips to info about various missions to theoretical physics. Check it out. If you’re interested in continuing after your free trial, the ad-free streaming service costs $2.99, $5.99 and $11.99 per month for standard definition, high definition, and ultra high definition 4K respectively.

Gorgeous Images of the August 2017 Partial Lunar Eclipse

The partial lunar eclipse on August 7, 2017 as seen from Bucharest, Romania. Credit and copyright: Loredana Jucan.

Just to get you in the mood for the upcoming total solar eclipse — now less than two weeks away — our Solar System put on a little eclipse display of the lunar kind on August 7. The full Moon passed through part of the Earth’s umbral shadow, and the timing made this partial lunar eclipse visible in parts of Europe and Africa.

Thanks to our friends around the world who posted in Universe Today’s Flickr page, we’ve got images to share! Enjoy the views! Click on all the images to see larger versions of them on Flickr. The lead image link is here.

And for those of you in the path of the August 21 solar eclipse, please feel free to share your images on our Flickr page, and we may feature them in an upcoming article.

A composite of images take during the August 2017 lunar eclipse, as see from Kuala Lumpur. Credit and copyright: Shahrin Ahmad.
Partial lunar eclipse seen from Lausanne’s lakeshore in Switzerland … The Moon had just moved up from behind the Tour d’Aï Peaks. Credit and copyright: Hicham Dennaoui.
Partial Eclipse of Moon over the Church of Our Lady of the Bell in Casarano, Sicily, Italy. Single photo taken with a Konus 80/400 telescope and Canon 700d camera. Credit and copyright: Gianluca Belgrado.

Eclipsed full moon over the eastern horizon as seen from Treppendorf, Brandenburg, Germany. Credit and copyright: Andreas Schnabel.
Partial lunar eclipse of August 7th 2017, as seen from Bavaria, Germany at around 19:17 UTC. Shot with an EOS 550D mounted to a Meade ETX 70 Telescope. Exposure was 1/125 seconds with ISO 100. Credit and copyright:
Stephan Haverland.
The partial lunar eclipse as see from Czolpino, Pomerania, Poland. Credit and copyright:
Pawel Warchal.
A view of the partial lunar eclipse on August 7, 2017 as seen from Malta in the Mediterranean Sea. Credit and copyright: Leonard Ellul-Mercer.

Here is a video of additional images from Leonard Mercer:

You can watch a reply of a live webcast from the Virtual Telescope Project of the partial lunar eclipse seen from Rome:

A Partial Lunar Eclipse Ushers in Eclipse Season

partial lunar eclipse
The partial lunar eclipse of June 4th, 2012. Credit: Dave Dickinson
partial lunar eclipse
The partial lunar eclipse of June 4th, 2012. Credit: Dave Dickinson

Live on the wrong continent to witness the August 21st total solar eclipse? Well… celestial mechanics has a little consolation prize for Old World observers, with a partial lunar eclipse on the night of Monday into Tuesday, August 7/8th.

A partial lunar eclipse occurs when the Moon just nicks the inner dark core of the Earth’s shadow, known as the umbra. This eclipse is centered on the Indian Ocean region, with the event occurring at moonrise for the United Kingdom, Europe and western Africa and moonset/sunrise for New Zealand and Japan. Western Australia, southern Asia and eastern Africa will see the entire eclipse.

The path of the Moon through the Earth’s shadow Monday night. Credit: adapted from NASA/GSFC/Fred Espenak

The penumbral phase of the eclipse begins on August 7th at 15:50 Universal Time (UT), though you probably won’t notice a slight tea colored shading on the face of the Moon until about half an hour in. The partial phases begin at 17:23 UT, when the ragged edge of the umbra becomes apparent on the southeastern limb of the Moon. The deepest partial eclipse occurs at 18:22 UT with 25% of the Moon submerged in the umbra. Partial phase lasts 116 minutes in duration, and the entire eclipse is about five hours long.

The viewing prospects for the partial lunar eclipse. Credit: NASA/GSFC/Fred Espenak.

This also marks the start of the second and final eclipse season for 2017. Four eclipses occur this year: a penumbral lunar eclipse and annular solar eclipse this past February, and this month’s partial lunar and total solar eclipse.

Eclipses always occur in pairs, or very rarely triplets with an alternating lunar-solar pattern. This is because the tilt of the Moon’s orbit is inclined five degrees relative to the ecliptic, the plane of the Earth’s orbit around the Sun. The Moon therefore misses the 30′ wide disk of the Sun and the 80′ – 85′ wide inner shadow of the Earth on most passes.

partial lunar eclipse
The partial lunar eclipse of April 26th, 2013. Image credit and copyright: Henna Khan

Fun fact: at the Moon’s 240,000 mile distance from the Earth, the ratio of the apparent size of the Moon and the shadow is approximately equivalent to a basketball and a hoop.

When celestial bodies come into alignment, however, things can get interesting. For an eclipse to occur, the nodes – the point where the Moon’s orbit intersects the ecliptic – need to align with the position of the Moon and the Sun. There are two nodes, one descending with the Moon crossing the ecliptic from north to south, and one ascending. The time it takes for the Moon to return to the same node (27.2 days) is a draconitic month. Moreover, the nodes are moving around the Earth due to drag on the Moon’s orbit mainly by the Sun, and move all the way around the zodiac once every 18.6 years.

Got all that? Let’s put it into practice with this month’s eclipses. First, the Moon crosses its descending node at 10:56 UT on August 8th, just over 16 hours after Monday’s partial eclipse. Two weeks later, however, the Moon crosses ascending node just under eight hours from the central conjunction with the Sun, and a total solar eclipse occurs.

Tales of the Saros

The August 7th lunar eclipse is member number 62 of the 83 lunar eclipses in saros series 119, which started on October 14th, 935 AD and will end with a final shallow penumbral eclipse on March 25th, 2396 AD. If you witnessed the lunar eclipse of July 28th, 1999, then you saw the last lunar eclipse in the same saros. Saros 119 produced its last total lunar eclipse on June 15th, 1927.

The next lunar eclipse, a total occurs on January 31st, 2018, favoring the Pacific rim regions.

 

Partial lunar eclipses have occasionally work their way into history, usually as bad omens. One famous example is the partial lunar eclipse of May 22nd, 1453 which preceded the Fall of Constantinople to the Ottoman Turks by a week. Apparently, a long standing legend claimed that a lunar eclipse would be the harbinger of the fall of Byzantium, and the partially eclipsed Moon rising over the besieged city ramparts seemed to fulfill the prophecy.

In our more enlightened age, we can simply enjoy Monday’s partial lunar eclipse as a fine celestial spectacle. You don’t need any special equipment to enjoy a lunar eclipse, just a view from the correct Moonward facing hemisphere of the Earth, and reasonably clear skies.

See the curve of the Earth’s shadow? This is one of the very few times that you can see that the Earth is indeed round (sorry, Flat Earthers) with your own eyes. And this curve is true for observers watching the Moon on the horizon, or high overhead near the zenith.

This month’s lunar eclipse occurs in the astronomical constellation of Capricornus. The Moon will also occult the +5th magnitude star 29 Capricorni for southern India, Madagascar and South Africa shortly after the eclipse.

The viewing footprint for the 29 Capricorni occultation shortly after the eclipse. Credit: Occult 4.2.

Finally, anyone out there planning on carrying the partial lunar eclipse live, let us know… curiously, even Slooh seems to be sitting this one out.

Update: we have one possible broadcast, via Shahrin Ahmad (@shahgazer on Twitter). Updates to follow!

The final eclipse season for 2017 is now underway, starting Monday night. Nothing is more certain in this Universe than death, taxes and celestial mechanics, as the path of the Moon now sends it headlong to its August 21st destiny and the Great American Total Solar Eclipse.

-We’ll be posting on Universe Today once more pre-total solar eclipse one week prior, with weather predictions, solar and sunspot activity and prospects for viewing the eclipse from Earth and space and more!

-Read more about this year’s eclipses in our 2017 Guide to 101 Astronomical Events.

-Eclipse… science fiction? Read our original eclipse-fueled tales Exeligmos, Shadowfall, Peak Season and more!