When the Earth, Moon and Sun Align

By Kevin Matyi

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.

For more information on the eclipse, where to view it and how to view it safely (wear eye protection!), visit https://eclipse2017.nasa.gov/sun.

Make sure to wear eye protection when you go out to look at the eclipse!
Credit: NASA/Bill Ingalls

 

Heads Up, Earthlings! Watch the Skies Is Getting a Reboot.


By Kevin Matyi

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.

Fireball spotted northwest of Chicago, February 6, 1:25 AM CST

There was a very bright green fireball seen by hundreds of eyewitnesses surrounding Lake Michigan early this morning at 1:25:13 AM Central Time (February 6, 2017). The reports from these individuals and the video information from dash cameras and other cameras in the region indicate that the meteor originated 62 miles above West Bend, Wisconsin and moved northeast at about 38,000 miles per hour. It disrupted about 21 miles above Lake Michigan, approximately 9 miles east of the town of Newton. The explosive force of this disruption was recorded on an infrasound station in Manitoba, some 600 miles away – these data put the lower limit energy of the event at about 10 tons of TNT, which means we are dealing with a meteoroid – orbit indicates an asteroidal fragment – weighing at least 600 pounds and 2 feet in diameter. Doppler weather radar picked up fragments (meteorites) falling into Lake Michigan near the end point of the trajectory.
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Ground track and Doppler radar signature (done by Marc Fries at NASA Johnson Space Center); an animation of the orbit and approach of the meteoroid is being prepared and should be available soon. We will continue to look at data as it comes in and revise the calculations if necessary.

Links to videos of this event:

Lisle, IL Police Department
https://www.youtube.com/watch?v=cF0POBcZQRk

From Highway in Wisconsin:
https://twitter.com/KrazyPhukinFoo/status/828543708299657216
https://www.youtube.com/watch?v=-AozuKJZK_4

Chillicothe IL Police Department:
https://twitter.com/chillipd?ref_src=twsrc%5Etfw

Morton Grove Police Squad
https://twitter.com/NWSChicago/status/828532116300394496

Roof of Atmospheric, Oceanic & Space Sciences Building – University of Wisconsin https://www.youtube.com/watch?v=LHubXCtdEbo

EarthCams:

Looking over Lake Michigan, from Michigan Coast: (looking too north to see the meteor itself) http://www.earthcam.com/usa/michigan/grandhaven/lakemichigan/?cam=lakemichigan
Bright flash at 2:25:13

Bright Fireball Detected by 6 NASA All Sky Cameras

http://youtu.be/wGE7OQSCkyA[/embedyt]

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.

Ground track, still images from the cameras, and a movie from the NASA camera located in Cartersville, Georgia attached.

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Bright, Basketball Sized Meteor in Ontario

Researchers from Western University have released footage of a basketball-sized meteor that was almost as bright as the full moon.

The meteor lit up the skies of southwestern Ontario last week. Astronomers are hoping to enlist the help of local residents in recovering one or more possible meteorites that may have crashed in the area just north of St. Thomas, Ontario.

Meteorites may best be recognized by their dark and scalloped exterior, and are usually denser than normal rock and will often attract a fridge magnet due to their metal content. In this fall, meteorites may be found in a small hole produced by their dropping into soil. Meteorites are not dangerous, but any recovered meteorites should be placed in a clean plastic bag or container and be handled as little as possible to preserve their scientific information.

More details can be found at  http://meteor.uwo.ca/research/fireball/events/st_thomas/overview.html

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 Credit: The University of Western Ontario

First Observations of the 2013 Eta Aquarids

Despite interference from the moon and clouds (and rising sun!), this morning we snagged our first observations of the 2013 Eta Aquarids.  Here’s an image of one from the all sky camera in Tullahoma, Tennessee.  The Eta Aquarids peak in the pre-dawn hours on May 6 and are material from Halley’s comet.  They zoom around the solar system at speeds near 148,000 mph.  The one seen here burned up completely in our atmosphere over Nunnelly, Tennessee at a height of 58.7 miles above the ground.

(Credit: All Sky Camera Network)

Asteroid 2012 DA14 and the Eta Carinae Nebula

This image shows asteroid 2012 DA14 and the Eta Carinae Nebula, with the white box highlighting the asteroid’s path. The image was taken using a 3″ refractor equipped with a color CCD camera. The telescope is located at the Siding Spring Observatory in Australia and is maintained and owned by iTelescope.net.

Image credit: NASA/MSFC/Aaron Kingery

 

Look For The Harvest Moon This Weekend

Take a moment to gaze at the beautiful harvest moon this Saturday, September 29th.

(Image credit: NASA)

The harvest moon gets its name from agriculture. In the days before electric lights, farmers depended on bright moonlight to extend the workday beyond sunset. It was the only way they could gather their ripening crops in time for market. The full moon closest to the autumnal equinox became “the harvest moon,” and it was always a welcome sight.

Northern summer changed to fall last Saturday, Sept. 22nd, and is called the autumnal equinox. The word equinox comes from the Latin words for “equal night.” The fall and spring equinoxes are the only days of the year in which the Sun crosses the celestial equator.

Keep an eye on the moon as it creeps above the eastern skyline. The golden sphere may appear inflated. This is the moon illusion at work. This optical illusion is caused by the moon’s proximity to distant objects. A harvest moon inflated by the moon illusion is simply beautiful to us, but even more so to the farmers getting their crops in on those cool autumn evenings.

(Image credit: Stefano De Rosa)

Once in a Blue Moon


Image credit: NASA/MSFC

On Aug. 31, if the night sky is clear, you will be able to see the second full moon of the month, which is called a “blue moon.”

You may have heard the expression, “once in a blue moon,” meaning “almost never,” because having 13 full moons in a calendar year — instead of the usual 12 — is rare.

Once in a blue moon, an individual who embodies the spirit of an explorer crosses the horizon in our culture. Such was Neil Armstrong. It is appropriate that the farewell to Armstrong coincides with the appearance of a blue moon.

A blue moon occurs just seven times every 19 years. The next blue moon will be on July 31, 2015.

Usually months have only one full moon, but occasionally a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. This happens, on average, every two and a half years.

So, as we say Godspeed to Mr. Armstrong, take a moment tonight to observe the blue moon, and give it a wink in honor of the first human to set foot on the surface of the moon.