"Mr. Eclipse" Explains Total Solar Eclipses and Why You Should See One

Aug 20, 2017
Originally published on August 18, 2017 9:01 pm

Retired NASA astrophysicist Fred Espenak has 'written the book' on eclipses - many, many books, actually. He has seen 27 total solar eclipses and at least one on every continent. Matt Markgraf speaks with Espenak about why you should see the total eclipse, what it is, how to predict one and what kind of gear he's using to take photos of "The Great American Eclipse" on Monday.

Why you should make an effort to see the total solar eclipse:

Fred Espenak said on the scale of the beauty of natural phenomena, a partial eclipse is a 3 and a total solar eclipse is a million. "There is absolutely nothing that compares to seeing a total eclipse. Even a 99 percent partial pales in comparison to seeing a total eclipse. If you happen to live near the path of the total eclipse but are just a little outside of it and you just decide not to bother on eclipse day because you're afraid there's too much traffic or you just don't want to bother, your neighbors and friends who go to the eclipse and watch the totality will be telling you about it and you will regret it. So make an effort to get to see the total eclipse if at all possible."

What is Mr. Eclipse doing for this eclipse?

Espenak has been a busy man traveling the country doing interviews ahead of the solar eclipse. He'll be in Casper, Wyoming as part of an astronomy conference, but may end up driving elsewhere depending on the forecast.

He'll be doing primarily photography with an SUV full of equipment. Two German equatorial tracking clock drives with astronomical mounts with on each two telescopes with DSLRs and a video camera. Surrounding him will be another 8 or 10 tripods with GoPros, wide-angle cameras, time lapse, etc.

His iconic photo featured on the U.S. postage stamp was taken in Libya in 2006. It was shot with one of his telescopes and is a composite photo of 20 exposures at different shutter speeds - high dynamic range imaging. He said the corona, while visible to the eye, is difficult to capture with a camera.

The sun's corona can be seen from certain spacecraft like NASA's SOHO using a large disc that can effectively create an eclipse, however it is very large an blocks some of the inner corona. From the ground, observers will be able to see this part effectively.

What is the difference between a solar eclipse and lunar eclipse?

Solar eclipses only happen during the new moon phase, when the moon is passing between the earth and sun. Most of the time, the Moon has two shadows: a faint outer shadow called the penumbral shadow where a partial eclipse takes place. The inner shadow is the umbral shadow where a total solar eclipse takes place (completely blocking the Sun's disc making the corona visible). By the time the umbral shadow reaches the Earth it's very small, between 50 and 100 miles wide. The path is long because it stretches across from the sunrise to sunset points. This summer's total solar eclipse path is about 10,000 miles long and 70 miles wide, but only encompass about a quarter of a percent of the Earth's surface.

Lunar eclipses only take place during full moon, at night. It's not the Moon's shadow, but rather the Moon pass into the Earth shadow, allowing the the nighttime hemisphere of Earth experiencing night can see the Moon. Most people have seen this.

Why do we need to wear the glasses?

Everyone knows it's dangerous to look at the Sun. It's even worse if you try to look at the sun through binoculars and telescopes. Human curiosity tempts people to take a look. Espenak said the eclipse isn't necessarily more dangerous to look at than on a normal, sunny day, it's just that people want to actually look at it.

The solar filters block out most of the sunlight, allowing only a small fraction to come through, enough to see the partial phases.

The glasses are only needed for the partial phases. The totality doesn't require the glasses because the Sun is then hidden by the Moon, revealing only the corona.

He said it's pretty obvious to know when it's safe and when it's not because when it's not safe, the sun is blindingly bright. During totality, there is an "eerie twilight" and the brightness of the sun vanishes.

If you don't have the glasses, you can take a piece of cardboard and punch a hole through a safety pin. Hold that in front of you and look at the shadow on the ground, on a white sheet of paper. The pinhole acts as a pinhole camera.

Underneath a shade tree, the patches of sunlight filtered down will create hundreds of crescents on the ground beneath the tree because the apertures on the leaves will act as pinhole cameras, same as a straw hat - creating little crescents on your face. He said you get a colander from the kitchen and hold it out to see how the shadows on the ground moves.

You can take photos of the totality with your mobile phone, but the sun will show up as a blob during the partial phases. He said to get good photos of the partial phases, you'd need a telephoto lens with a special filter.

Why is the totality longer in some places?

Espenak said part of that has to do with the angle the Moon's shadow hits when it's moving across the Earth. When it's at a steep angle, its linear difference means it moves across the Earth faster. Near the middle of the path the rotation of the Earth is (to a small degree) counteracting the motion of the Moon's orbital motion making the eclipse longer at that point too. In addition, the eclipse is longer in the center of the path and drops off to zero at the edges.

If you're not in the path of totality you'll still see the penumbral shadow, which will be visible from all of North America. Most of the U.S. will see at least a 60% partial eclipse.

How do we know what the path of totality is going to be?

To calculate the path you need to know where the Sun, Moon and Earth are with respect to each other in the three dimensions of space. With that basic information, the Moon's shadow can be determined. Through spherical trigonometry the path can be determined.  

How did people predict eclipses before modern science?

Espenak said accurate calculations of the positions of the Earth, Moon and Sun came about around the time of Isaac Newton and his work The Principia, laws of universal gravitation. The accuracy of those predictions have since improved over time.

Before Newton, back to 2,000 years ago, people calculated them using the convenient fact that eclipses tended to repeat themselves every 18 years, 11 days and 8 hours - known as the Saros cycle - involving several motions of the moon in overlapping periods. Taking the eclipse on Monday, for instance, using the Saros cycle, there was another eclipse that looked similar, but passed through Europe, Turkey and India on August 11, 1999. He said Saros has some trouble in periods longer than 1,500 years.

Eclipses have been interpreted and used in unique ways over history. Espenak said in one eclipse that passed through ancient Persia occurred during a war between two factions. In the middle of a battle, an eclipse occurred. Both factions saw it as a sign from the gods that they were dissatisfied with the war. The people laid down their weapons and held a wedding uniting both sides.

A story involving Christopher Columbus and a lunar eclipse during his fourth voyage. He had problems with a ship and scuttled it on a beach in the Caribbean. He had to relying on the goodwill of the people for water and food until help arrived. As their goodwill was wearing out, Columbus used his almanac to learn there'd be an eclipse and told the chief of the tribe the gods were unhappy about this withholding. Sure enough, the Moon rose and turned red for the eclipse and the locals gave him food until he was rescued.

How do you encourage children to get into science after the eclipse?

Espenak said getting the public, especially kids, interested in science is a "great hope" of the eclipse. He said he's written a number books on the subject and his wife has written a children's book.

He said  personally got interested in astronomy when he was nine or ten and saw the moon in a neighbor's telescope. He later saw a partial eclipse in 1963 and learned there'd be a total eclipse in 1970 - when he was 18. He took the family car to the path of totality (in North Carolina) with a telescope and took some photos. He said it was "so incredible, so beautiful" that he had to see another. He later went to a total eclipse in Canada and then north Africa in the Sahara Desert. From there, he realized he had developed what would be a lifelong passion and has since seen 27 total solar eclipses and has seen at least one on every continent.

How did you get started with NASA?

The first 20 years working at NASA didn't involve eclipses, Espenak said. His initial work involved studying the atmospheres of the planets with an infrared spectrometer. But he said he was always interested in eclipse. By the early 1990s, the U.S. Naval Observatory had published information about eclipses but faced budget cuts and approached Espenak to write the information for the scientific community - but ended up writing the books due to a large amateur astronomer readership. He also started the NASA eclipse website on a computer in his office during the "Wild West days" of the Internet. By the time he retired, it covered maps and information for 5,000 years of eclipses in the past and future.

What should we know about the next total solar eclipse in the area in 2024?

This eclipse begins in the South Pacific into Mexico, through west Kentucky, Indiana and Ohio and into Canada. The Moon will be closer and will appear larger in the sky. The path of totality will be longer - closer to 4 and half minutes.

The bad part of this eclipse, however, is that it occurs in April - during a usual period of showers. He said because of this, there's a greater chance of seeing the eclipse under good weather conditions on Monday than the next one in seven years.

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