An illustration of the sun's corona during an eclipse in 1871.
An illustration of the sun’s corona during an eclipse in 1871. Wellcome Library, London/CC BY 4.0

A lot of space science relies on waiting for things to be just right—for planets to align, for the night sky to be dark enough. The solar eclipse that will be visible from the U.S. on August 21 is no different. Scientists have been planning experiments for years that can only be done during the brief period when the moon is completely blocking the sun.

The sun’s outer atmosphere, the planet Mercury, and even our own planet will all be studied by scientists who plan to position themselves along the path of totality. Here are some of the projects astronomers, physicists, and ordinary citizens will be working on later this month.

The Sun’s Corona

While we can see sunspots and other surface phenomena thanks to filters on telescopes and cameras, we still have a lot to learn about the nuclear fireball at the center of our solar system. The sun’s atmosphere, known as the corona, is particularly difficult to study because the sun is so bright. Scientists can create a sort of artificial eclipse to study the corona, but the total eclipse is a special opportunity because the innermost layers of the corona will be visible. Scientists have some unique experiments planned, and the data they collect could help predict future space weather that can affect us here on Earth.

The silver nose of this WB-57 jet contains a telescope.
The silver nose of this WB-57 jet contains a telescope. NASA’s Johnson Space Center/Norah Moran

Both satellites and scientists on Earth will be taking images of the sun’s corona during the eclipse, but they’ll be capturing more than just regular visible light. Scientists are interested in X-rays emitted by the sun, and images of a broad spectrum of light will show its magnetic field. Telescopes mounted on the noses of two of NASA’s WB-57 jets will try to capture small explosions, called nanoflares, that are believed to help heat up the corona.

The telescopes will also take the first thermal images of Mercury’s surface. In order to get the clearest images, the two jets will fly along the path of totality at a speed of 470 miles per hour, and an altitude of 50,000 feet. They’ll only be in the moon’s shadow for roughly eight minutes, but that’s enough time for the two instruments to collect valuable data. Another plane, a Gulfstream V owned by the National Science Foundation and the National Center for Atmospheric Research, will fly with the eclipse for about four minutes to learn more about the corona’s thermal structure.

The Size of the Sun

You’d think humanity would know exactly how big the sun is by now, but it turns out our measurements have been a little imprecise. Scientists have been able to measure its diameter by carefully observing transits, such as when Venus or Mercury pass in front of the sun, or by measuring images produced by satellites. Unfortunately, these methods aren’t perfectly accurate. Xavier Jubier, whose website models past and future eclipses, noticed that models of past eclipses, based on these measurements, didn’t quite match photographs unless he made the sun’s radius several hundred kilometers larger.

A precise measurement of the size of the sun doesn’t matter for most solar scientists, but for eclipse chasers (or even people new to eclipse viewing), an imprecise value can mean missing out on the path of totality. So during the big event, reports, scientists both in and out of the path will measure the exact size of the umbra on the ground. Using the known diameter of the moon, and the distances between Earth, the moon, and the sun, they’ll calculate the diameter of the sun that would be able to create that size of an umbra.

Earth’s Atmosphere

When the moon passes in front of the sun, light isn’t the only thing that’s blocked. For a brief moment, the sun suddenly won’t be bombarding a swath of planet with radiation, even though it just had been minutes before. That gives scientists a rare chance to observe one of the uppermost layers of our atmosphere, the ionosphere, as it rapidly switches between day and night conditions. Several experiments, including a crowdsourced project using cellphones, will use radio waves to observe changes in the ionosphere that could affect communications networks and GPS down on the ground. Another experiment will use 6,000 sensors on the ground to track gravity waves in the ionosphere that are triggered by the eclipse.

An eclipse in 2006, as seen from the International Space Station.
An eclipse in 2006, as seen from the International Space Station. NASA/Public Domain

Earth’s weather isn’t immune to changes caused by the eclipse. Temperatures will drop along the path of totality as the sun’s warming rays are blocked by the moon, and citizen scientists can contribute to a database of atmospheric conditions, including temperature and cloud conditions, using the GLOBE Observer app. A separate team of scientists will be measuring the amount of solar energy reaching Earth as the eclipse occurs. That data will help them refine something called a 3-D radiative transfer model, which helps scientists understand how energy from the sun affects climate. Measurements taken in Wyoming and Missouri will be combined with data from satellites, measuring the amount of energy Earth reflects back into space, to understand how solar energy passes through Earth’s atmosphere.

Proving Relativity Right, Again

Albert Einstein’s General Theory of Relativity says that a massive object like the sun will have such an effect on gravity that it will bend light passing around it. Arthur Eddington first tried to prove this, by measuring the change in positions of stars, during a 1919 eclipse that passed over Africa and Brazil. Clouds got in the way, and it wasn’t until 1922 that more definitive numbers, proving Einstein’s theory correct, were collected.

An image captured by Arthur Eddington's team of the 1919 eclipse.
An image captured by Arthur Eddington’s team of the 1919 eclipse. F. W. Dyson, A. S. Eddington, and C. Davidson/Public Domain

But the instruments of the time weren’t terribly precise. Don Bruns, an amateur astronomer, told LiveScience that Eddington’s numbers were off by about 10 percent. Bruns and several other groups will recreate the experiment on August 21, for the first time since 1973, and if you want to join them, you still have time to gather the necessary equipment.