On Monday, Aug. 21, for the first time in almost 100 years, all of North America will be treated to an eclipse of the sun. Those in the path of totality, running from Oregon to South Carolina, will experience one of nature’s most awe-inspiring events — a total solar eclipse.
Scientists, researchers and experts from NASA’s Marshall Space Flight Center in Huntsville, Alabama, will mobilize to experience the eclipse and share it with others. They will join participants from across the agency for a multi-hour broadcast, titled Eclipse Across America: Through the Eyes of NASA, to offer unprecedented live video of the celestial event, along with coverage of activities in parks, libraries, stadiums, festivals and museums across the nation, and on social media.
“It’s going to be a spectacular event,” said Marshall Chief Scientist James Spann. “We’ll be sharing our research and work with people and letting them know how to safely view the eclipse, not only at the events in the path of totality, but also worldwide online and on NASA Television. Excited doesn’t begin to describe how our team feels right now. It truly will be breath-taking, and we can’t wait.”
Marshall experts will be located at two of the broadcast’s 15 locations — Hopkinsville, Kentucky, and Austin Peay State University in Clarksville, Tennessee.
The motion of the moon is what causes eclipses, but the dramatic change in sunlight is what makes them so impressive to observers. But what exactly is happening when the moon passes in front of the sun?
The moon is blocking the sun’s light from reaching Earth, but there is more to the situation than just that. Their relative distance to Earth is one of the most important factors.
The sun is about 400 times farther from Earth than the moon and has a diameter about 400 times larger than the moon. As a result, both the sun and moon (near perigee) appear to be the same size in the sky, allowing the moon to perfectly block out the sun and cast a shadow on Earth during a total eclipse.
The shadow we see while in the path of totality is called the umbra, and the shadow of the surrounding partial eclipse is a penumbra. The shadow from an annular eclipse (when the moon appears smaller than the sun during an eclipse, and so a ring of light is visible around it) is called an anteumbra.
The physics of how each type of shadow is formed is difficult to explain but easy to visualize, so before I tell you about them, here is a picture (technically a ray diagram) of what happens during an eclipse:
Each of the three types of solar eclipse are caused by the moon blocking light from different parts of the sun. Credit: Wikimedia Cmglee
For a total eclipse, the moon has to block out all of the sun’s light. To put the moon in the best position, imagine that a person on Earth is standing under the exact middle of the moon, the centerline of a total solar eclipse.
In this case, light coming from the middle of the sun is clearly going to be blocked by the moon, since it is directly in the way and visible light cannot penetrate rock. The most difficult light to block will be coming from the top and bottom of the sun.
To figure out whether the light will be blocked, a bit of drawing can help. If the light is coming from the exact bottom of the sun and you are wondering if a person can see the light while under the exact center of the moon, draw a line between where the light starts and the person’s eyes.
Does the moon get in the way of the line? If yes, then the person is experiencing a total solar eclipse. None of the sun’s light can get past the moon, so the sun is fully blocked.
If the answer is no, but the person is still standing under the center of the moon, then they are in an annular eclipse. The moon is in the perfect position to block all of the sun’s light, but it still fails to do so. In this case, it will appear to be a large black circle with a ring of sunlight called an annulus around it.
A partial eclipse is the most difficult to explain, since it has the most variability. All but a sliver of the sun may be blocked, or the moon can barely cover any of the sun. In general though, a partial solar eclipse happens when the moon is not quite directly between the observer and sun, but is still in the way of some sunlight.
You can use the same process for determining whether a person is experiencing a total solar eclipse to figure out if they are in the penumbral shadow of the moon. A slight complication is that the moon is off center, so it matters more where the origin point of the light is.
If the person is standing a little north of the moon’s center, then the line from origin to person should start from the sun’s southernmost point, the bottom, since the northern light is less likely to be blocked due to the moon being a bit more to the south from the person’s perspective.
If any of the sun’s light is blocked by the moon, then the person is experiencing a partial solar eclipse. The limit of this blockage, where only the slightest amount of sunlight is blocked, is the edge of the penumbra shadow.
If the moon is not blocking any light, then the moon may be close to the sun but there is no eclipse happening on that spot of Earth.
On Aug. 21, 2017, a total solar eclipse will cross the full continental United States along a narrow, 70-mile-wide path from Oregon to South Carolina.
The last total eclipse in the U.S. was in 1979. And the last total solar eclipse that crossed the entire continental U.S. happened in 1918. But why? Why has it been 99 years, and why have the intervening partial and even total eclipses caught only parts of the country?
In short, celestial geometry is complicated but predictable. Much like many other aspects of the cosmos, it is cyclic.
Need a minute to catch up? Go ahead. We’ll wait. Credit: NASA
Eclipse cycles arise from a natural harmony between three motions of the moon’s orbit. We call them “months” due to their repetitive nature.
The synodic month governs the moon’s phases. It’s measured by the time it takes to go from one new moon to the next, which takes about 29 ½ days. In that time, the moon rotates once around its own axis and goes around Earth once.
From the perspective of a solar eclipse, the new moon phase is important. It’s the point in the moon’s orbit when it passes between Earth and the sun. A total solar eclipse can only happen at a new moon, and only when the other types of movement line up as well.
When the moon, on its orbit around Earth, reaches the point closest to the sun we can’t see the moon reflecting sunlight, so it appears dark. This is the new moon. Credit: NASA/Genna Duberstein
New moons happen once a month, but we don’t see eclipses every month because the moon’s orbit is tipped by about five degrees from Earth’s orbit around the sun. On most months, the new moon casts its shadow either above or below Earth, making a solar eclipse a rare treat.
The moon’s tilted orbit meets the sun-Earth plane at two points called nodes. A draconic month is the time it takes the moon to return to the same node. The moon’s orbital nodes drift over time, which is why a single location on Earth’s surface might wait hundreds of years between total eclipses.
As the moon orbits Earth, it also wobbles up and down, making total eclipses rarer than they otherwise would be. Credit: NASA
The moon’s path around Earth is not a perfect circle, which means the distance between us and the moon changes all the time. When the moon is closest to Earth in its orbit we call it perigee, and apogee when it’s farthest. This change in distance gives rise to the anomalistic month, the time from perigee to perigee.
The farther away the moon is from Earth, the smaller it appears. When the moon blocks all of the sun’s light, a total eclipse occurs, but when the moon is farther away — making it appear smaller from our vantage point on Earth — it blocks most, but not all of the sun. This is called an annular eclipse, which leaves a ring of the sun’s light still visible from around the moon. This alignment usually occurs every year or two, but is only visible from a small area on Earth.
When moon is too small to cover the entire sun’s disk, a ring or “annulus” of bright sunlight surrounds the moon. Credit: NASA/Cruikshank
A total solar eclipse requires the alignment of all three cycles — the synodic, anomalistic, and draconic months. This happens every 18 years 11 days and 8 hours, a period known as a saros.
One saros period after an eclipse, the sun, moon and Earth return to approximately the same relative geometry, a near straight line, and a nearly identical eclipse will occur. The moon will have the same phase and be at the same node and the same distance from Earth. Earth will be nearly the same distance from the sun, and tilted to it in nearly the same orientation.
The extra eight hours is the reason why successive eclipses in the same saros cycle happen over different parts of Earth. Earth rotates an extra third of the way around its axis. Each total solar eclipse track looks similar to the previous one, but it’s shifted 120 degrees westward.
Earth turning on its axis impacts where total solar eclipses occur. Credit: Espenak & Meeus
During this year’s total solar eclipse, anyone within the path of totality will be able to see one of nature’s most awe-inspiring sights. This path, where the moon will completely cover the sun and the sun’s tenuous atmosphere — the corona — can be seen, will stretch from Salem, Oregon to Charleston, South Carolina. Observers outside this path will still see a partial solar eclipse where the moon covers part of the sun’s disk. A total solar eclipse presents the rare opportunity to observe the corona and chromosphere, and eclipse observations are important for understanding why sun’s atmosphere is 1 million degrees hotter than its surface.
Want to find out more about this year’s total solar eclipse — like what totality means and why the path of totality is so much smaller than the overall eclipse? Wonder how long it takes photons from the sun to reach Earth? Curious about dark matter and what we know about it? All are possibilities in the newly revamped Watch the Skies blog.
Hello and welcome back! We will be posting content more regularly, although it will be somewhat changed from before. You can look forward to new articles explaining different astronomy topics, breaking down complex science and jargon in a way that people can understand.
We’ll kick things off this week with some solar eclipse science. So come back then to learn more about just how wonderful and strange our universe can get!
Credit: ESO/José Francisco Salgado
Kevin Matyi is a summer intern in the Office of Communications at NASA’s Marshall Space Flight Center.
On Saturday, Oct. 8, from 5:30 to 9 p.m. the public and media are invited to attend the 6th annual International Observe the Moon Night celebration, hosted by NASA’s Marshall Space Flight Center at the Davidson Center for Space Exploration at the U.S. Space & Rocket Center, both in Huntsville, Alabama.
The free event will include moon-related and solar system exhibits and hands-on activities for children and adults. Activities will include an out-of-this-world photo booth, airbrush tattoo station, and a meet and greet with Janet Ivey from “Janet’s Planet” on PBS. Live music will be provided by DJ Shell. Several large amateur telescopes will be set up to view the moon, stars, and other visible planets. Visitors can also take a virtual 3-D trip to the moon with the astronomy van, offering a magnified, command-module-like view of the lunar surface. The family movie, “Home,” will begin at dusk.
A panel discussion titled: “Planets, Moons & Meteorites Oh My!” will begin at 7:15 p.m. in the National Geographic Theater and will feature Marshall speakers Mitzi Adams, solar physicist; Dr. Barbara Cohen, planetary sceintist; Dr. Bill Cooke, manager of the Meteoroid Environments Office; and Dr. Renee Weber, planetary scientist.
The U.S. Space & Rocket Center is the official visitor center for NASA’s Marshall Space Flight Center.
Make plans now to stay up late or set the alarm early next week to see a cosmic display of “shooting stars” light up the night sky. Known for it’s fast and bright meteors, the annual Perseid meteor shower is anticipated to be one of the best potential meteor viewing opportunities this year.
The Perseids show up every year in August when Earth ventures through trails of debris left behind by an ancient comet. This year, Earth may be in for a closer encounter than usual with the comet trails that result in meteor shower, setting the stage for a spectacular display.
“Forecasters are predicting a Perseid outburst this year with double normal rates on the night of Aug. 11-12,” said Bill Cooke with NASA’s Meteoroid Environments Office in Huntsville, Alabama. “Under perfect conditions, rates could soar to 200 meteors per hour.”
An outburst is a meteor shower with more meteors than usual. The last Perseid outburst occurred in 2009.
How to Watch the Perseids
The best way to see the Perseids is to go outside between midnight and dawn on the morning of Aug. 12. Allow about 45 minutes for your eyes to adjust to the dark. Lie on your back and look straight up. Increased activity may also be seen on Aug. 12-13.
For stargazers experiencing cloudy or light-polluted skies, a live broadcast of the Perseid meteor shower will be available via Ustream overnight on Aug. 11-12 and Aug. 13-14, beginning at 10 p.m. EDT.
An outburst of Perseid meteors lights up the sky in August 2009 in this time-lapse image. Stargazers expect a similar outburst during next week’s Perseid meteor shower, which will be visible overnight on Aug. 11 and 12. Credits: NASA/JPL
We observed a fireball the morning of May 4 around 12:50am EDT, traveling southwest at about 77,000 mph over the Nantahala National Forest on the Tennessee/North Carolina state line. At its brightest point, it rivaled the full moon. According to Dr. Bill Cooke in NASA’s Meteoroid Environment Office at NASA’s Marshall Space Flight Center in Huntsville, Ala. , “The fireball was bright enough to be seen through clouds, which is an attention getter. In Chickamauga, Ga., one would have thought it was a flash of lightning lighting up the clouds beneath.”
NASA, in partnership with the Exploratorium Science Center in San Francisco, will host activities around the March 8 total solar eclipse, including opportunities to talk with solar scientists and live coverage of the eclipse originating from Woleai island in Micronesia.
At 1 p.m. on Tuesday, March 8, solar scientists from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, will participate in a Reddit Ask Me Anything.
NASA Television will begin coverage at 8 p.m. on March 8. The period of total eclipse, called totality, will occur from 8:38 to 8:42 p.m.
Twitter, Google+ and Facebook users will be able to join the conversation and ask questions using the hashtag #eclipse2016. The NASA Twitter account for the eclipse is @NASASunEarth. The public will be able to tag and share their images of the solar eclipse on the NASA Flickr group at: https://www.flickr.com/groups/eclipse2016/.
The total eclipse will be visible in parts of South East Asia and a partial eclipse will be visible in parts of Alaska, Hawaii, Guam, and America Samoa. An eclipse occurs when the moon passes directly between Earth and the sun. When the moon’s shadow falls on Earth, observers within that shadow see the moon block a portion of the sun’s light.
Information about eclipses is available online at:
NASA’s All Sky cameras captured this image of a Quadrantid meteor overnight, the meteor was moving at 93,000 miles per hour. This 1 inch diameter member of the Quadrantid meteor shower leaves a brilliant streak in the north Georgia skies before burning up 44 miles above Earth’s surface. The Quadrantid meteor shower peaks overnight on January 4.
We have received numerous reports concerning a bright fireball that occurred over Georgia at 5:33:55 PM CST (6:33:55 PM EST). All 6 NASA all sky meteor cameras in the Southeast picked up the meteor at an altitude of 50 miles above the town of Georgia (SE of Atlanta). From its brightness, it is estimated that this piece of an asteroid weighed at least 150 pounds and was over 16 inches in diameter. It entered the atmosphere at a steep angle and moved almost due south at a speed of 29,000 miles per hour. The NASA cameras tracked it to an altitude of 17 miles above the town of Locust Grove, where it had slowed to a speed of 9000 miles per hour, at which point the meteor ceased producing light by burning up. It is possible that fragments of this object survived to reach the ground as meteorites.
A more detailed analysis will be performed tomorrow and further details will follow if this analysis still indicates the possibility of a meteorite fall.