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Eclipse Science: What They’ve Taught Us, What We Hope to Learn

Jessica Hall
Solar eclipses happen because of a fantastic cosmic coincidence. From our vantage point here on Earth, the Sun is about 400 times larger in diameter than the Moon. But the Moon is also about 400 times closer than the Sun. Because of this, the Sun and the Moon appear to be about the same size in our sky. This is what makes a total solar eclipse possible.
As a result, we on Earth’s surface get to see the Moon cover the sun completely, but leave its atmosphere — the corona — visible to our unprotected eyes during those precious few moments of totality. (ExtremeTech STRONGLY recommends that no one attempt to view an eclipse without appropriate eye protection -Ed).
Today’s eclipse is a total solar eclipse along its path of totality, and a partial solar eclipse in every other place it can be seen in North America. On the ground, people under the path of totality get to see some remarkable things, which we’ve rounded up below. As always, each image can be clicked to open it in a new window:
There are a few kinds of eclipse -- because the Moon's orbit is eccentric, sometimes it's a bit closer to the Sun than other times. When it's further away from us, the result can be a thing called an annular eclipse. In an annular eclipse, the edges of the sun are still visible, this is sometimes called a "ring of fire." It's not the kind of eclipse we're going to have, but it's still a very neat effect. Image by NASA.

 https://www.extremetech.com/wp-content/uploads/2017/08/Diamond-ring-640x360.jpgThe very last Baily's bead lasts for just an instant. It makes one bright, breath-taking flare, like a diamond catching the light. It's in a different place every time there's an eclipse, because the Moon's orbit is eccentric and its path weaves through space. This last tiny bit of sunlight is called the diamond ring. Image by Kubotake, courtesy of Wikimedia 
https://www.extremetech.com/wp-content/uploads/2017/08/Beads-438x246.jpg In the last few seconds before the Sun is completely blotted out, you may see the slender crescent of the Sun (still through your eclipse glasses!) suddenly break into small flares that look like a string of beads. These are called Baily's beads, for the guy who discovered them. They happen because the last few rays of light make it to Earth as they shine through valleys and low points along the Moon's horizon. Image by Arne Danielson
https://www.extremetech.com/wp-content/uploads/2017/08/Prominence-640x360.jpgDuring the few breathtaking minutes of totality, eclipse watchers can see flamelike projections from the still-visible corona of the sun. These are solar and they're part of what we call "space weather." Coronal mass ejections (CMEs) are present during a solar prominence erruption -- when one of them is pointed our way, we end up with a highly disruptive solar storm, like the Carrington Event. Numerous satellites and the ISS have all done research into managing this kind of space weather. Image by Nicholas Jones 
https://www.extremetech.com/wp-content/uploads/2017/08/ShadowBands-640x360.jpgShadow bands are wavering bands of light that race along the landscape during the last few minutes before totality. They happen because as the Sun becomes a thin crescent, the photons it's shedding are increasingly collimated (traveling in a parallel trajectory), so they produce bands of light and dark. Atmospheric perturbations distort and change the bands. Image by Dr. W. Strickling  

How do we know when and where we’ll see an eclipse?

Math! From the Babylonians until today, we’ve been using increasingly subtle math to calculate and predict the appearance of an eclipse. Ancient astronomers had no idea what an eclipse was, but they identified them as unusual occurences of enormous significance. An eclipse was typically viewed as a very bad omen, and a sign that the gods were angry with mortal men. In at least one case, however, an eclipse is credited with stopping a war between the Lydians and the Medes. Herotodus writes:
As, however, the balance had not inclined in favour of either nation, another combat took place in the sixth year, in the course of which, just as the battle was growing warm, day was on a sudden changed into night. This event had been foretold by Thales, the Milesian, who forewarned the Ionians of it, fixing for it the very year in which it actually took place. The Medes and Lydians, when they observed the change, ceased fighting, and were alike anxious to have terms of peace agreed on.
It is not known if this eclipse actually occurred or if it occurred in the manner described, but it’s a unique attribution to an eclipse in any case. More information on what ancient civilizations knew about eclipses is available here.
Over the millennia, we’ve learned a great deal about the Sun from studying various eclipses, including:
968: The first clear description of the sun’s corona during a solar eclipse was written by a chronicler in Constantinople.
1687: Newton publishes his Principia. For the first time, it becomes possible to predict eclipses accurately over long periods of time.
1715: Edmund Halley reports the phenomenon that would later be known as Baily’s Beads (discussed in the slideshow above). Haley also draws a map of the eclipse’s visibility across England.
1724: Jose Joaquin de Ferrer observes and names the corona, after observing it in a total eclipse. He also posits that it must belong to the sun rather than the moon, due to its overall size.
1868: The element of helium is detected through observation of a solar eclipse. Originally thought to be sodium, further research demonstrated that it was an element as-yet unkown on Earth. It’s named “helium” for Helios, the Greek god of the Sun.
1887: Proving that eclipses can be studied with balloons, Russian astronomer Dmitry Ivanovich Mendeleev ascends to 11,500 feet to observe an eclipse above the cloud cover. NASA later copies his idea in 2017 (no, not really).
1919: William Wallace Campbell and Robert Trumpler confirm Einstein’s general theory of relativity by observing the relativistic bending of starlight around the sun during an eclipse.
1973: Scientists use a Concorde prototype to extend the duration of a solar eclipse by 10x, spending well over an hour in the path of totality (74 minutes).

What’s NASA doing during the eclipse?

NASA has a ton of science experiments set up for the narrow physical and temporal window of the eclipse. Among others, the crew of the ISS will be watching the eclipse too; they should get to see a partial eclipse, and they also get to watch the moon’s shadow travel across the Earth’s surface from above. They’re doing an astrobiological experiment using bacteria sent aloft with balloons to study how terrestrial bacteria might respond to Mars-like conditions.
“The solar eclipse over the continental USA gives us a rare opportunity to study the stratosphere when it is even more Mars-like. Normally, stratospheric UVA & UVB are slightly higher than the surface of Mars. With the full solar eclipse on Aug. 21 and about 55 teams flying balloon payloads across 33 states at various points along the path of totality, we expect to obtain a modulated sunlight gradient in the upper atmosphere, more closely resembling Mars UVA & UVB levels. This will provide a high-fidelity analog environment, only available for a few hours, for studying microbial responses to Mars conditions,” NASA wrote in its 2017 Eclipse press kit (PDF).
ballon_launch_eclipse_map
A NASA infographic with details one of its balloon experiments.
“Moreover, it is almost inconceivable to perform a standardized, coordinated astrobiology experiment across 33 states simultaneously. The eclipse project offers a special opportunity for high statistical fidelity and extremely broad spatial coverage in the stratosphere.”
eclipse_epc_2016068_4
NASA’s Deep Space Climate Observatory (DSCOVR) will be in position to view the 2017 eclipse, the same way it observed the 2016 eclipse. The photo above is from that observation period.
NASA approved a variety of studies and investigations that focus on both the sun and the Earth. The University of Hawaii will be measuring the physics of coronal plasma, the University Corporation for Atmospheric Research will measure the infared solar corona, MIT will investigate how the solar eclipse changes the ionosphere over the continental United States, and DISCOVR and NISTAR will collaborate to perform a “3-D radiative transfer closure experiment.” A full list of experiments is available here.
Enjoy the show, everyone! Wear your glasses, don’t take stupid risks, and stay safe.
Eclipse Science: What They’ve Taught Us, What We Hope to Learn Reviewed by Chidinma C Amadi on 4:00 PM Rating: 5

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