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Why this total solar eclipse is our best chance to discover mysteries of the sun

A woman watches an eclipse of the sun.
A woman watches an eclipse of the sun. Photograph: Simon Cooper/PA

Around 9am on Monday, observers standing on the coast of Oregon will notice a small black spot that will appear on one side of the sun. As the morning progresses, this inky imperfection will grow until, by about 10.15am, it covers the entire solar disc. The sun will be blotted out from the sky and night will descend on the Pacific state. Only the fiery, wispy filaments of the solar corona – the sun’s atmosphere – will be visible.

For just under three minutes observers there will experience a total solar eclipse, which occurs when the moon passes between the sun and the Earth and in the process turns day into night. According to the late astronomer Patrick Moore, the event is simply “the most magnificent in all nature”.

After that, the eclipse will cross the United States, watched by tens of millions of people who will witness not just one of Earth’s greatest wonders but one of the entire solar system’s most extraordinary spectacles. Our moon is 400 times smaller than the sun, but is 390 times closer to Earth, making the size of their images almost exactly the same. Mercury and Venus have no moons. Mars has only tiny Diemos and Phobos, minuscule spots when set against the solar disc while, observed from Jupiter or Saturn, the sun would appear so small it would be simply obliterated by a moon going round either of the gas giants. Only Earth has the right stuff for a total solar eclipse.

Eclipse graphic

And it is a phenomenon that scientists have learned to exploit over the centuries, having made numerous major breakthroughs – from the discovery of helium to confirmation of Einstein’s general theory of relativity – during eclipses. Crucially, astronomers are hoping to maintain this success rate on Monday.

“Despite all the space telescopes and satellites we have in orbit, we still have a lot to learn from total solar eclipses,” said astronomer Professor Robert Walsh, of the University of Central Lancashire. “We can make observations of the region that is just above the surface of the sun that you cannot study using space telescopes. That is why these events are so important.”

Total solar eclipses occur twice every three years on average and either sweep over oceans or pass over uninhabited regions such as Antarctica or Siberia. But not Monday’s – and that makes it particularly important. It will track 2,500 miles across the inhabited land of central US stretching from Oregon in the west to South Carolina in the east. About 12 million people live along that path and that number will no doubt be swelled as visitors flock to witness the event.

“If you watch a total solar eclipse you only get a few minutes to observe it before it passes,” said Matt Penn, an astronomer at the National Solar Observatory in Tucson, Arizona. “This time we have a huge path over inhabited land that is easily accessible to the public.” Penn intends to exploit this accessibility as part of the citizen science project called Citizen Continental-America Telescopic Eclipse or Citizen Cate.

Eclipse graphic

“We are going to place 68 volunteer observers along the path of the eclipse as it crosses America. When the eclipse ends for one person, the next person along the path will already have started observing it. We will be able to make a continuous film that should last 93 minutes – depending on viewing conditions,” Penn said. “That is really special.”

Each volunteer taking part in Citizen Cate will have their own 80mm diameter telescope, provided by corporate and federal sponsors, which they will use to study the eclipse and focus their attention on a region known as the inner solar corona. “We know a great deal about the outer corona thanks to space telescopes but even satellite observatories cannot get enough detail of the part of the corona that is nearest to the sun,” Penn said. “We aim to get around that problem with Citizen Cate.”

The corona, which means crown in Latin, is the aura of plasma that surrounds the sun and extends for millions of miles into space. “We are particularly interested in the solar wind, the stream of particles that pours from the sun and which batters the top of our atmosphere. It is the phenomenon that connects us with the sun.

“But how do these particles get accelerated so that they fly into space away from sun? We have a poor understanding of that but we do know that this process goes on inside the inner corona. This region, which is very close to the sun’s surface, is very difficult to study. However, by making a very long, continuous study of the inner corona through Citizen Cate we hope to get some answers.”

The sun’s corona will also be targeted by scientists at the National Center for Atmospheric Research at Boulder, Colorado. “This eclipse gives us a chance to test newly developed equipment for measuring the corona using infrared techniques that will give us a more accurate way to assess its magnetic field, which is crucial to understanding its behaviour,” said astronomer Scott McIntosh. “These new infrared instruments could form the next generation of corona telescopes and we will test them on Monday by flying them on aircraft that will track the eclipse.”

In the end, scientists believe the eclipse will give them a chance to make major improvements in understanding the solar wind and how it affects Earth. However, there are other effects closer to home that will also be studied – and not just those that concern humans. Over the centuries volumes of evidence have been amassed about animal reactions to the sudden darkening of the daytime sky as an eclipse takes place: birds being silenced, squirrels freezing in mid-stride and bats and other nocturnal creatures making appearances as the darkness increases.

“A total eclipse is purely a celestial event but it can have profound impacts on wildlife on Earth,” said Elise Ricard, of the California Academy of Sciences, in San Francisco. “We want to record those events.”

Ricard and her colleagues have developed an app called iNaturalist which allows users to join the Life Responds project. Volunteers are being asked to record animal activity before and during the eclipse. Other apps will allow them to video those activities.

“When an eclipse happens the sky darkens but not in the way that it does when the sun goes down at night, ” Ricard said. “An eclipse is a sunset in every direction and that can be disturbing. We want to observe how that affects living creatures and understand their behaviour a little more in the process.”

In addition, the sheer drama of the event is likely to stimulate interest in science, researchers hope. “The Apollo moonshots gave a great boost to the popularity of astronomy and I think this eclipse and the citizen science projects that are involved with it could have the same effect,” Penn said.

Certainly, this total solar eclipse is a great opportunity but we should note that these events will not always be with us. As Walsh pointed out:“The moon is slowly moving away from Earth and one day it will be so far away that its disc will be too small to completely eclipse the sun. The latter’s disc will still shine from behind the moon but it will be a far less spectacular event.”

However, the moon is moving away from the Earth at a rate of only a few centimetres a year – and so will take several tens of millions of years before it is too far from the Earth to affect the eclipse of the sun. So there is no need to panic yet.

“The crucial point is that we happen to be in the right place at the right time to enjoy these events,” Walsh added. “They happen to be the greatest show on Earth and we should certainly make the most of them while we can.”