8:00 AM | **The "Great American" solar eclipse**

Simulated IR satellite image for this afternoon from the 06Z GFS Monday computer forecast model 

Overview
On August 21st, the US will experience its first “coast-to-coast” total solar eclipse since 1918. On that day, the moon will pass between the sun and Earth and the result will be a 67-mile wide shadow that will cross the country from Oregon to South Carolina.  In the DC, Philly and New York City metro regions, this eclipse will be partial with anywhere from about 81% of the sun being covered by the moon in DC to about 72% in New York City.  In the "totality zone" where the sun will become covered in its entirety (and of course depending on the local cloud conditions), the sky gets deep twilight blue, temperatures drop noticeably, the winds should die down, bright stars and planets come out, animals and birds may behave strangely like it’s the end of the day, and it can become strangely silent. A total solar eclipse provides a great opportunity for scientists to learn more about the sun's atmosphere as well as our own.

General Discussion
From all first-hand accounts, there is a major difference in the impact of viewing a partial solar eclipse versus witnessing a total solar eclipse.  A partial solar eclipse (which is what this will be in the DC, Philly and NYC metro regions) has been described as a "run-of-the-mill" astronomical event whereas a total solar eclipse is referred to as a "must see" and a "once-in-a-lifetime" opportunity.

Total solar eclipses occur somewhere on Earth every year or so, but generally cast their shadows over oceans or remote land masses. The last time a part of the contiguous US saw a total solar eclipse was in 1979 and the next total solar eclipse on US soil will take place in April 2024. The last total solar eclipse in the contiguous United States, 38 years ago, only clipped the northwestern United States, mostly rural areas of Washington, Oregon, Idaho, Montana and North Dakota. And on that particular day, Feb. 26, 1979, it was cold and dreary in the Northwest, and most people in the path of totality did not even see the eclipse due to clouds and rain.

On August 21, 2017, the moon will pass between Earth and the sun in a total solar eclipse that will be visible on a path from Oregon to South Carolina across the continental United States. Map courtesy NASA

On August 21st, the moon will move in between the Earth and the sun causing the moon to cast an umbral shadow over the planet and millions of Americans will witness a total solar eclipse. Everyone in the continental US outside of the totality zone will witness a partial eclipse of at least 48% caused by the moon's penumbral shadow. The 67-mile wide path of the moon’s umbral shadow in the totality zone will begin in the northern Pacific and cross the U.S. from west-to-east through parts of the following states: Oregon, Idaho, Montana, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, North Carolina, Georgia, and South Carolina. It will pass directly over cities such as Salem, Oregon, Idaho Falls, Lincoln, Nebraska, Kansas City, Nashville, and Columbia and Charleston, South Carolina. Places within a one- or two-hour drive of the eclipse include Portland, Oregon, Boise, Cheyenne, Rapid City, Omaha, Nebraska, Topeka, St. Louis, Louisville, Knoxville, Chattanooga, Atlanta and Charlotte. At any given location, the total eclipse will last for around 2 or 3 minutes turning day into a dark twilight and the total event will take about three hours from start to finish.

In the DC-to-Philly-to-New York City corridor, the eclipse will be partial and its duration will be about 2 hours and 40 minutes.  The beginning of the solar eclipse in this corridor will be around 1:20 PM (ET), the time of maximum coverage of the sun around 2:45 PM (ET), and the ending time around 4:00 PM (ET).  The percentage of totality will be roughly 81% in the DC metro region, 75% in and around Philly, and 72% in and around the NYC metro region. To get solar eclipse times in any part of the US on an easy-to-use interactive map from NASA click here.

In case you miss this total solar eclipse, the next one in the US is actually not that far away coming on April 8th, 2024.  Here are the “totality zone” tracks for the 2017 and 2024 solar eclipses (courtesy eclipse-maps.com)

Scientific value of a total solar eclipse
The total solar eclipse that takes place on August 21st across parts of the US will provide an opportunity for solar scientists to learn more about the sun’s atmosphere including its outermost layer called the corona.  In addition, if there is activity on the sun during the upcoming total solar eclipse such as solar prominences or coronal mass ejections, this will provide an opportunity for first-hand observations by solar scientists.  Atmospheric scientists and meteorologists will take this opportunity to closely monitor local weather conditions in the totality zone including such parameters as air temperature and surface winds – both of which typically drop off noticeably during the short totality time period.  In addition, during a total solar eclipse, the Earth’s upper atmospheric region known as the ionosphere behaves as if it is nighttime and this event will provide an opportunity for atmospheric scientists and meteorologists (and radio enthusiasts) to learn more about the ionization which is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons.  

Corona
The sun may be the Earth's closest star, but its very brightness makes it a challenge to observe. The light from the sun drowns out the fainter corona, the outer atmospheric layer of the star, a wispy region that becomes visible when the moon completely blocks the face of the sun during a total solar eclipse. Trying to understand the sun without seeing its corona would have been like trying to learn about Earth without ever seeing its atmosphere. The corona is a major part of the sun, though it is hidden from us on Earth's surface except for the glimpses we get during total solar eclipses.

The Sun's tenuous outer atmosphere is called the corona and it becomes visible during a total solar eclipse. The corona is not normally visible since the Sun's disk is so bright that the relatively faint light from the wispy corona is overwhelmed.

The first recorded observation of the solar corona occurred during the total eclipse of Dec. 22, 968. Byzantine historian Leo Diaconus observed the event from Constantinople. In the Annales Sangallenses maiores (annals compiled in St. Gallen, Switzerland covering the years 927 through to 1059), he wrote:
"...at the fourth hour of the day ... darkness covered the Earth and all the brightest stars shone forth. And it was possible to see the disk of the sun, dull and unlit, and a dim and feeble glow like a narrow band shining in a circle around the edge of the disk."

One of the most amazing findings about the corona is that it is much hotter than the surface of the sun itself which has puzzled scientists for many decades (nearly 3.6 million °F even though the surface of the sun is only about 11,000 °F.  It is analogous to the air around a bonfire being hotter than the bonfire itself.  Total solar eclipses provide an opportunity to view the corona and perhaps learn more about how it can get so hot.

Solar prominences and coronal mass ejections
Total solar eclipses can provide insights into solar flares and coronal mass ejections (CMEs), material from the sun that is spewed into space via enormous explosions. If a CME collides with Earth, the event can harm power and communication systems, as well as astronauts in space. With the sun blocked during a total solar eclipse, these clouds of charged particles could be spotted.

The first record of a CME in progress was made during the total solar eclipse of July 18, 1860. Solar prominences, the large accumulation of cooler gas held in place in the atmosphere by the sun's magnetic field, were also first recorded during a total solar eclipse. The University of Montreal's website "Great Moments in the History of Solar Physics" calls the description of the May 1, 1185, solar eclipse found in the Russian Chronicle of Novgorod "the first fairly unambiguous description of prominences."

Image of coronal mass ejection from the surface of the sun

The sun's magnetic fields drive CMEs and other forms of space weather. Although magnetic field lines can't be directly seen, the charged particles that make up the corona trace the magnetic field lines, making them visible. So rather than a diffuse gas, the corona appears to have twisting, moving jets and lines running through it. Observations of the corona can then help scientists better understand the primary drivers of the space weather that can affect Earth and missions to other worlds.

Probing Earth's own atmosphere
Total solar eclipses allow scientists to study the ionosphere in the Earth’s upper atmosphere and how sunlight impacts radio frequencies.  Ultraviolet light and X-rays from the sun can strip electrons from atoms in the Earth's upper atmosphere, in a process called ionization. Ionized particles tend to have an electric charge and one of the uppermost parts of the atmosphere, called the ionosphere, is defined by these ionized and electrically charged particles. The ionosphere requires a constant influx of X-rays and ultraviolet light to remain charged; otherwise, the atoms and electrons will recombine into neutral atoms and molecules. When the sun sets, the upper layers of the ionosphere are ionized by charged particles from space called cosmic rays. What this means is that some parts of the ionosphere — the lower parts — actively require the 'lights to be on' to exist. Hence, they are only there during daylight and disappear at night.

During the 1999 total solar eclipse that occurred over the United Kingdom, a project was organized to encourage eclipse watchers to track these changes during the eclipse.  All an observer needed was a radio. Many radio stations operate at frequencies that, during the day, are absorbed by the lower layers of the ionosphere.  At nighttime, when the number of charged particles in the lower layers drop, some of the radio waves can travel higher up into the ionosphere before they are absorbed. Instead of bouncing off the lower layers, which ultimately shortens the distance they travel, the signal can bounce off both lower and upper layers, allowing them to be transmitted across longer distances (while still being heard across short distances as well). That’s why some radio stations can be heard across great distances only at night, even though distant listeners cannot receive the station's signal during the daytime.

A total solar eclipse can act like a snapshot of night because charged particles from the sun no longer stream through the ionosphere, and as a result radio waves can bounce farther than they would during regular daytime hours. The transition from darkness to light happens faster during an eclipse than over the course of a typical sunset, and even though the sun may only be completely covered for a few minutes, the ions in the atmosphere lose their charge and reconnect during that time, making it possible for radio signals to cross long distances (just like they do during nighttime hours). The change in how far the radio signals can travel comes pretty well instantly in proportion to the loss of light and then returns to normal when the sun reappears in the sky.

Understanding the nuances of how the atmosphere is altered by the rising and setting of the sun can help radio broadcasters avoid pumping radio signals out with more power than needed, which unnecessarily pollutes radio bands far afield and creates interference. During a total solar eclipse, different parts of the United Kingdom receive different amounts of sunlight, which should also influence so the ionosphere, and hence, how far radio waves can travel. By tracking how far a known signal travels during a solar eclipse, scientists could learn about how the presence or absence of sunlight influences the different radio frequencies.

During the 1999 eclipse, this project requested that observers tune into a radio station in northern Spain that was detectable in the evenings in the United Kingdom, assuming the frequency wasn't obstructed by other local stations. Participants were asked to write down whether they could hear the station, and send in their responses along with their postcodes.
In a paper describing the project, it was stressed that the experiment was suitable for people of all ages, including the visually impaired, and didn't stop participants from viewing the eclipse. Even if local weather conditions prevented people from enjoying the eclipse by eye, rain and cloudy skies would not affect the radio waves. The project received 1,700 responses by mail, in addition to more-detailed measurements by radio amateurs, who had access to more sophisticated equipment. Wedged between the southern region that could always hear the station and a northern region that could hear it only during the eclipse, there was a stretch across the middle of England that couldn't hear the station during the eclipse. This revealed what has been called "a skip-distance effect." For the upcoming 2017 eclipse, some amateur radio clubs might be doing various experiments, and ionospheric researchers will be making special observations.

Other worlds
Total solar eclipses can reveal insights about other bodies in our solar system.
When the sun is covered during the August 21, 2017 eclipse, Mercury, Venus, Mars and Jupiter will be visible according to Earthsky.org. Venus will appear about half an hour before the sun disappears, while Mercury will be revealed from about 30 seconds before totality until about 30 seconds after it, when the moon totally covers the disk of the sun. During the 2017 eclipse, at least one set of astronomers will take the opportunity to take the temperature of Mercury while the star is hidden behind the moon.

Eclipses have also provided the opportunity to search for other worlds. In the 19th century, scientists noticed that Mercury wasn't traveling quite the way it was predicted to: the planet's closest point to the sun in its orbit slowly shifted over time, slowly moving around the star. Scientists assumed that an unseen planet, which they called Vulcan, sat between Mercury and the sun, with the unobserved planet's gravitational pull shifting Mercury's orbit. When various eclipses hid the sun, astronomers searched in vain for the missing world. During the total solar eclipse of July 29, 1878, astronomers flocked to the United States' Rocky Mountains to hunt for the hidden planet, but never found it.

It wasn't until November 1915 that a young Albert Einstein killed off Vulcan for good when he presented his theory of general relativity. A massive body can bend and distort "space-time" (Einstein's name for the fabric of reality, which weaves together three-dimensional space with time). As Mercury orbits the sun, the curvature of space caused by the sun's gravity would cause the planet's orbit to shift very slightly, Einstein's theory predicted. According to general relativity, the curvature of space-time could explain the strange wobbles of Mercury far better than a hidden planet could.

Einstein's theory was put to the test during the total solar eclipse of 1919 and this event provided observational evidence for his complicated theory. The findings made Einstein an overnight celebrity. 

In 1919, English astronomer Arthur Eddington traveled to the west coast of Africa to test another prediction that came from general relativity: that the path of light itself bends around a massive body. By studying how light from the stars behind the sun bent, Eddington and the eclipse provided observational evidence for Einstein's complicated theory. While for most people, a solar eclipse can be a once-in-a-lifetime, awe-inspiring sight, it is clear that these phenomena can also reveal a wealth of information about the universe.

Local dramatic weather changes in the totality zone 
Total solar eclipses allow meteorologists to study unusual weather conditions (e.g., “eclipse wind”) only seen in totality regions. Using data from several networks of sensors, weather stations, satellites, weather balloons and members of the public, scientists found that the total solar eclipse of March 20, 2016 led to changes in the wind. Also, the temperatures across the UK, where the study was conducted and between 85 per cent and 97 per cent of the sun was obscured, dropped by an average of 1.5°F (0.83°C). In some places, temperatures dropped by as much as five or six degrees. Wind speeds fell by up to 2.3 mph (2 knots) while in areas with clear skies it changed direction by as much as 20 degrees.

All of these findings support the idea of what is known as an ‘eclipse wind’ and strange silences often detected during total solar eclipses. The strange silence can also be partially due to the fact that birds typically go silent as skies darken and they tend to go home to roost for the night. Researchers also took the opportunity to assess how computer forecast models are able to predict changes in the weather during the sudden loss of sunlight. They also found that temperature drops during the eclipse appear to vary depending on the given landscape. Temperatures fell less in coastal regions, mountainous areas and in those areas covered in vegetation.

Professor Giles Harrison, an atmospheric physicist at the University of Reading in the UK was one of the scientists involved in the studies, said “last year’s solar eclipse was essentially a chance to conduct a giant experiment with the atmosphere.  It allowed us to watch what happens to the atmosphere when there is a sudden drop in sunlight.” Together with colleague Professor Suzanne Gray, a meteorologist at the University of Reading, Professor Harrison examined data collected by Met Office weather stations and roadside weather sensors across the UK. They found that the wind change appears to be caused by variations to the ‘boundary layer’ in the atmosphere – the area of air that usually separates high-level winds from those on the ground. As temperatures dropped during the eclipse, it appears this boundary layer also fell closer to the surface, meaning the drag on the wind from the land was greater. It is similar to what occurs at sunset, but over a much shorter period of time. Professor Harrison said: This is perhaps the most complete explanation for reports of the “eclipse wind” that many people report experiencing.

‘As the sun disappears behind the moon the ground cools. This means warm air stops rising from the ground, causing a drop in wind speed and shift in its direction as the slowing of the air by the Earth’s surface changes.’  The results are published in a special edition of the world’s oldest scientific publication Philosophical Transactions of the Royal Society. It is published 301 years after Edmund Halley reported sensing the chill and damp that accompanied a solar eclipse in 1715 in the same journal. For more information on the scientific value of a total solar eclipse click here.

This map shows the path of totality overlaid on a statistical map of cloudiness for the month of August where the best places to be are shown in blue. Courtesy spaceweather.com and meteorologists Jay Anderson, Jennifer West

Best viewing places based on climatology
Based on historical data from NASA's Terra and Aqua satellites, the best place to view the total solar eclipse from a statistical viewpoint is in the interior western US (blue areas on map) where fractional cloud cover dips as low as 15%. In general, cloud cover becomes more of a problem east of the Mississippi River and it can be problematic along both immediate coastlines.  From a meteorological point of view, one thing to monitor as we approach August 21st will be tropical moisture to see if it could play a role in cloud cover across the southeastern US. The Atlantic Basin tropical season is now well underway and a tropical system could certainly be problematic in terms of cloud cover. 

Warning: Everyone in the US needs solar eclipse glasses for viewing! Never look at the sun without proper eye protection. Glasses specifically designed for solar eclipses are needed - whether one is in the totality zone or not - and watch out for fakes.  Looking directly at the sun during a partial eclipse, such as will be the case, for example, in DC, Philly, and New York City, can cause serious eye damage or blindness. In the totality zone (narrow dark path on map), it is said to be safe to look at the sun without glasses for those couple of minutes in which the sun is completely covered by the moon.  However, both before and after that short totality period of a couple of minutes, solar eclipse glasses are needed as it will effectively be a partial eclipse at those times.  One can do a search online to find proper solar eclipse glasses or check with your local library as some are giving away the glasses for free.  For more on eye safety click here.

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com
 

Extended video discussion on the solar eclipse: