By Jim Melvin
CLEMSON, South Carolina – The sun, Earth and moon have been swirling together through the depths of space for billions of years, intertwined like cosmic dancers.
For almost countless millennia, solar eclipses have been a part of this dance.
Over the past couple of thousand years, recorded history has shown that the moon has passed between the Earth and sun two to three times each year, casting its shadow on the Earth and creating solar eclipses. Over the course of the 20th century, this occurred 239 times, according to records published by the National Aeronautics and Space Administration (NASA).
But not all solar eclipses are created equal. Of the ones that took place during 1900-1999, only 62 were total solar eclipses. The rest were either partial, in which the moon casts only the outer part of its shadow on the Earth; annular, in which the moon doesn’t cover the entire disk of the sun; or hybrid, in which the eclipse appears annular or total along different sections of its path.
On Aug. 21, 2017, a total solar eclipse will streak across the United States from coast to coast and pass directly over Clemson University.
Though the entire Continental U.S. will be able to witness portions of the eclipse, the total eclipse will only be visible on a narrow track stretching from Oregon to South Carolina. Clemson is located almost dead-center within this slender band. The eclipse will begin its pass over the Upstate at about 1:07 p.m. EDT and finish around 4:02 p.m. But the totality of the eclipse — the part that viewers will find the most fascinating — will begin around 2:37 p.m. and last less than three minutes.
Clemson University already is making plans to host its own eclipse viewing event that will include in-person appearances by a slew of College of Science experts. Anyone interested in attending the viewing party should go to clemson.edu/eclipse for more details.
If you are planning on taking part in the big event at Clemson on Aug. 21, here are five things to further pique your interest.
1) SIZE (AND DISTANCE) MATTERS
Given the disparity in size between the sun and the moon, how is the moon – such a relatively tiny piece of rock – able to entirely block out the blazing light of a mammoth nuclear inferno?
“The sun is massively huge – 109 Earths can fit across its diameter. People ask me all the time: ‘How can the moon cover this huge sun during a total solar eclipse?’ ” said Amber Porter, a lecturer in the department of physics and astronomy in the College of Science at Clemson University. “The answer is relatively simple. The sun is about 400 times larger than the moon, but it’s also about 400 times farther away. Because of this, the geometry works out so that, from our perspective, the moon covers the sun almost perfectly.”
2) WHEN, WHERE AND … HOW!
Astronomers are able to identify when and where eclipses will occur decades and even centuries in advance. In fact, the National Aeronautics and Space Administration (NASA) has mapped out all solar eclipses over a period of 5,000 years – from 1499 BC to the year 3,000. During this five-millennia stretch of time, Earth will have witnessed almost 12,000 solar eclipses. But how do scientists know when and where the eclipses will occur?
Rough eclipse predictions have been a mainstay of society for thousands of years. But today, computers make it easier and more accurate to predict eclipses than ever before. Still, to understand how it’s possible to predict an eclipse, you will first need to better understand what causes one.
It is important to know that the moon’s orbit is tilted about 5 degrees relative to Earth, causing the moon to pass through the Earth’s orbit twice each month at points called nodes. The moon crosses one node by passing down through Earth’s orbit and then, two weeks later, crosses up through the other. Therefore, each orbit contains two nodes, one on each side of the Earth. An imaginary line connecting the two nodes is referred to as the “line of nodes.” If the line of nodes is pointed toward the sun when the moon is crossing one of its nodes, an eclipse can occur. If the moon is between the Earth and the sun, a solar eclipse occurs. Lunar eclipses occur when the moon crosses the node on the far side of its orbit from the sun.
“Anytime the line of nodes is not pointed at the sun, the moon’s shadow will miss the Earth and go too far north above the planet or too far south,” Porter said. “The only time eclipses can occur is when the line of nodes is pointed at the sun – which occurs twice a year during a time known as ‘eclipse seasons.’ And astronomers use computers to crunch the numbers of precisely when and where the moon’s shadow will fall across the Earth for a total solar eclipse.”
3) RARE, MEDIUM OR WELL DONE?
If each year has two different eclipse seasons, why are total solar eclipses considered to be so rare? The answer is, they really aren’t that rare. In fact, they happen all over the world at an average of one every 18 months. Yet, the last total solar eclipse over Clemson occurred in 1778. Something doesn’t jibe.
Here’s the catch. When a sphere, such as the moon, blocks light cast from another sphere, such as the sun, the darkest part of the resulting shadow becomes shaped like a cone that narrows as it extends. The point of this cone touches the Earth and – because of the movement of the moon – slides across a stretch of land, sea or both for thousands of miles. But the point of this dark cone is less than 100 miles wide, which means that a total solar eclipse seen in Clemson won’t be seen in most other areas of the world. Some areas will see a partial eclipse, some nothing at all.
“It’s possible but very rare to see more than one total solar eclipse at the same place in one lifetime,” Porter said. “Total solar eclipses can sweep any part of the Earth and have even passed over the North and South poles. But there is no one place on our planet that is ideal. The average time between total solar eclipses in any given location is about 375 years.”
4) EIGHT EXTRA HOURS MAKE A BIG DIFFERENCE
Eclipses follow cycles. One of these is called the Saros Cycle. Astronomers have long known that every 18 years, 11 days and eight hours, an eclipse – with the same amount of totality, the same position and the same time of year – will repeat itself. Therefore, these cycles allow scientists to predict when and where eclipses are going to happen in the future. Scientists can also work backward and determine when and where eclipses happened in the past. There are currently 41 different active Saros cycles working their way up and down the planet.
“You will get an identical eclipse every Saros cycle, but not in the identical location,” Porter said. “The Earth rotates every 24 hours, so the eight additional hours of rotation at the end of each Saros Cycle will cause the eclipse to shift one-third of the way westward around the Earth. However, after three Saros cycles – about 54 years – the eclipse will return to its original spot, except for a slight shift to the north or south.”
5) WHY SHOULD ANYONE CARE?
Astronomers care because total solar eclipses provide an optimal opportunity to study the sun’s corona, which includes the solar wind that travels millions of miles from the solar surface. When the sun is not eclipsed, its sheer luminosity overwhelms anything beyond its surface, rendering the corona invisible. But during eclipse totality, the moon blocks this brightness, revealing the corona in exquisite detail. Because of this, researchers from all over the world will come to the U.S. on Aug. 21 and enter the narrow path of totality.
“The corona is gravitationally bound to the sun, but the strength of the sun’s gravity diminishes over distance. So we’ve found it difficult to explain the behavior of solar winds, which are composed of individual particles that stream free from the corona’s outer edges and travel throughout our solar system,” Porter said. “Solar wind is like steam rolling off a pot of boiling water, except that steam particles cool off and slow as they get farther from the water’s surface, while in the case of particles within the solar wind, their temperature and speed increase rather than decrease, as expected. Therefore, we believe there must be some continual input of energy throughout the corona to heat and increase the speed of the individual particles. Forming a better understanding of why this occurs is just one of the reasons it’s so important for us to study the activities of the corona during total solar eclipses.”
Meanwhile, casual viewers should care about total solar eclipses not just because of their rarity but because they are widely reputed to inspire amazement and awe. Only a tiny percentage of humankind has ever had to privilege to travel to outer space. The rest of us will spend the entirety of our lives on or near the ground. But even from this limited perch, we will still be able to witness the most extraordinary cosmic phenomenon of our lifetimes.