Warning: NEVER LOOK DIRECTLY AT THE SUN. This will cause permanent eye damage.
On August 21, 2017, people in North America will be able to see either a total or a partial solar eclipse, where the moon will either completely or partially cover the sun in the middle of the day from our perspective on Earth. Along the path of totality, a relatively narrow area that will stretch from Salem, Oregon to Charleston, South Carolina, the moon will completely cover the sun, stars will shine in a night-like sky and lucky observers will be able to see the sun's tenuous atmosphere, the corona, extending from the moon's border.
Louisiana isn't in the path of totality, but that doesn't mean we won't be treated to a spectacular event here at LSU. The moon will take a huge "bite" out of the sun, covering 80% of the sun over Louisiana. Don't forget to look down on August 21, 2017 during the eclipse - if you are under a tree, you'll see tiny crescent-shaped spots of light everywhere there normally would be circles of light!
We asked two of LSU's Physics and Astronomy professors, Distinguished Professor and Alumni Professor Brad Schaefer and Assistant Professor Tabby Boyajian, to tell us everything we need to know about the 2017 Solar Eclipse - see their responses below.
LSU College of Science: What causes a total solar eclipse?
Brad Schaefer: As the Moon (which is closer to Earth than the Sun) moves around the sky, sometimes it passes in front of the more distant Sun. When the Moon is in front of the Sun, we get no useable light from the dark side of the Moon, so the round and dark Moon occults a big circle out of the visible Sun. We are just seeing the Moon in silhouette.
For times and places where the Moon passes above or below the center of the Sun, we only get a partial eclipse, where the Moon only covers up part of the Sun. During a partial eclipse, the immensely bright solar disk is still blinding. If most of the Sun is covered, the visible part of the Sun will appear as a weird crescent shape. But for times and places where the Moon passes over the center of the Sun, the Moon can completely cover up *all* of the Sun. This is called a total solar eclipse. Totality is the awesome part. When the Moon blocks all of the blinding solar disk, we can see the Sun’s corona and the vast prominences arcing over the Sun’s edge. With totality, the whole sky and landscape goes eerily dark.
LSU College of Science: What is the difference between a solar eclipse and a lunar eclipse?
Brad Schaefer: A solar eclipse happens when we are standing in the shadow of the Moon, while a lunar eclipse is when the Moon is inside the shadow of the Earth. As the Moon orbits around our Earth, both the Moon and Earth have shadows from our Sun extending far out into space.
When the Moon passes between the Earth and Sun, its shadow can touch down on Earth, making us see a solar eclipse. When the Moon is on the far side of the Earth from the Sun, the Earth’s shadow can pass over the Moon, making a lunar eclipse.
Imagine that you were an astronaut on the Moon, during what we Earthlings would call a lunar eclipse. We Earthlings would see the Moon’s body pass into the Earth’s shadow. Well, from your (the astronaut’s) point of view, the Earth would be blocking out the Sun, and our home planet would appear only as a black silhouette covering part or all of the Sun. You (the astronaut on the Moon) would see a solar eclipse.
Eclipses are just shadows, and shadows that will soon pass. And who is afraid of shadows? Well, in ancient times (and up until c. 1600 outside the Eurasian milieu), people did not know this simple shadow explanation. They did not know to not be afraid of the eclipse shadows. Rather, many cultures took the events up in the sky as being evidence of the actions of the gods. Almost all ancient cultures had a Sun god near the top of their pantheon, so a solar eclipse looked like the death-of-the-Sun-god. Anything like this has to be a very, very bad sign! This is why many ancient cultures viewed eclipses as the worst of all omens.
Some scientific research can still only be done during solar and lunar eclipses. For example, I will be using off-the-shelf equipment to take pictures of the stars near the eclipsed Sun on August 21, trying to measure the small outward shift as predicted by Einstein’s Theory of General Relativity.
LSU College of Science: The May 29, 1919 solar eclipse helped researchers confirm Einstein’s theory of general relativity. What exactly happened?
Tabby Boyajian: The positions of the stars shifted due to the Sun’s mass. Stars are “fixed” in space, on what we call the celestial sphere. That is why when you give your telescope coordinates, it slews to the exact place your star should be.
But during an eclipse, the space along the Sun/Moon line of sight is distorted by the Sun’s mass. So when you look at the positions of the stars that are nearby, they will not be where we would expect them to be.
Brad Schaefer: In 1915, Albert Einstein formulated the weird idea of General Relativity (GR). This astounded the science world. But science progresses not by putting forth speculation, but rather we do experiments to test the speculation. So Einstein made a unique prediction only explainable by GR, that stars near the eclipsed Sun will appear shifted slightly away from the Sun. For the May 1919 total solar eclipse, Sir Arthur Eddington led two expeditions. Observers took pictures of the stars near the eclipsed Sun, and saw that the stars were indeed slightly shifted, exactly as predicted by Einstein. Major newspapers worldwide ran the confirmation of Einstein’s prediction at the top center of the front page.
Read more: Why eclipses have inspired terror and awe
Here at LSU, we are a large part of the LIGO collaboration, with one of the two LIGO facilities nearby. Recently, LIGO made a great discovery with the long and hard sought testing of another unique prediction of Einstein’s GR. Einstein predicted distant moving massive bodies would produce an incredibly tiny shifting of bodies (called gravitational waves) on Earth, and LIGO was built to test this prediction. Even though scientists have good confidence in GR (for example due to the Eddington Experiment), it is the nature of scientists to push hard on predictions to test models and theories and idea. LIGO recently did discover gravitational waves from three separate events, all three produced by BlackHole-BlackHole binaries.
LSU College of Science: How does the moon “cover” the sun if it’s so much smaller than the sun?
Brad Schaefer: The small Moon covers the huge Sun simply because the Moon is much closer to Earth than the Sun is. You can recreate this same effect by walking outside on a sunny day, and holding your pinkie up to occult the Sun. Your pinkie is much much smaller than the Sun, but it is a LOT closer to you. Given the relative sizes and distances, it chances that the apparent angular size of the Sun and Moon from Earth are pretty close to being equal.
The Moon’s orbit around Earth is not perfectly circular, so the Moon sometimes swings a bit farther away than average and sometimes a bit closer than average. When the Moon is relatively close, it of course must appear a bit larger, and so it can completely cover up the entire solar disk. This is totality, as we’ll be getting to the north of Baton Rouge on 21 August. But if the Sun is a little bit farther away, then the Moon will appear a bit smaller in size, so the lunar disk won’t be able to perfectly cover up the entire solar disk. This is called an ‘annular eclipse’. In this case, if you are standing on the centerline, you will see the Sun as a very thin circle, or annulus, with the Moon centered in the middle and the bright solar disk shining around on all sides.
LSU College of Science: How do scientists know when an eclipse will occur?
Brad Schaefer: Astronomers for the last 2000 years have been able to chart the path of the Moon accurately enough to be able to predict eclipses with passable accuracy. Solar eclipses occur as the Moon passes in front of the Sun, so simply projecting the Moon’s future path in the sky will show when it will pass in front of the Sun. With lunar laser ranging (based on the retroreflectors placed on the Moon by the Apollo astronauts), modern astronomers know the position of the Moon to a relative accuracy of about one-inch for any given second of time, from long in the past up to many millennia in the future.
LSU College of Science: How long will the eclipse last (in Louisiana)?
Brad Schaefer: Three hours total. The eclipse starts at 11:54 a.m. on Monday, August 21, but the small "bite" out of the Sun will be very hard to spot at this time. The Moon will be covering the Sun at a maximum (in Louisiana) at 1:26 p.m. The eclipse ends at 2:54 p.m. With the partial phases near the beginning and end not being readily visible, the best time to look (using a solar viewer - never look directly at the Sun!) will be around 1 or 2 p.m.
LSU College of Science: Will it get dark in Louisiana?
Brad Schaefer: No. Indeed, with only 80% of the Sun covered, the ambient lighting will not be noticeably different from any other time. From historical records, the threshold for someone to even notice a solar eclipse (without already knowing about it) is 90% coverage. So if an out-of-touch friend is outside all day on August 21, the odds are good that they will not realize an eclipse is happening.
This is another way of saying that partial eclipses are 12th best as compared to totality. So if you can get from Baton Rouge to the centerline, you really need to go!
LSU College of Science: Are there any dangers about being exposed to a solar eclipse? What about looking at one?
Brad Schaefer: Well, you do NOT want to stare or look at the un-eclipsed Sun (or an eclipse in partial phase. (A brief glance will not cause permanent damage, but it might make you see ‘white spots’ in your eyes for a few hours.) During the time of totality (Note: this will NOT occur in Louisiana), it is perfectly safe and awesome to stare directly at the eclipsed Sun. But during the partial phases, it hurts to look at the Sun, and that is your body’s way of telling you loudly to not look. There is no need for fear, but use ordinary prudence, and don’t stare at the uneclipsed Sun.
LSU College of Science: Do our human eyes play any tricks on us when it comes to viewing a solar eclipse?
Brad Schaefer: Nope. The view with the unaided eye is awesome, especially if you get to see the corona and the weirdly-pink solar prominences on the edge of the eclipsed Sun.
LSU College of Science: Where can the eclipse be viewed?
Brad Schaefer: All of North America will see the eclipse, at least in partial phases. But seeing a partial solar eclipse not nearly the same as standing in the path of totality. On Monday, August 21, the path of totality cuts a hundred-mile-wide swath through the United States. It starts near Portland, Oregon, goes over the Tetons, passes over Casper, Wyoming (where my wife and I are going), then over Lincoln, Nebraska, Kansas City, St. Louis, Nashville, and Charlottesville, South Carolina.
The weather prospects are definitely much better from Nebraska to the west. Roughly, west of the Mississippi has a roughly 35% chance of clouds, while east of the Mississippi has a 65% chance of clouds. That is why everyone serious about seeing the eclipse is heading west, so as to improve the odds of getting clear skies. Actually, as amateur astronomers learned in the 1970s, being mobile is the best way to get out from under oncoming clouds. I will go to Casper, Wyoming the day before the eclipse, carefully check upcoming cloud predictions, and if need be, I will drive overnight to another location anywhere from Idaho to Nebraska. That way, I have a very good chance of getting clear skies.
LSU College of Science: How can you get the best picture of an eclipse?
Brad Schaefer: With a camera equipped with a solar filter. But I strongly recommend that you do NOT try to take pictures of the Sun. Your pictures will only turn out pathetically poor unless you really know what you are doing. However, taking a broad panorama with a teeny eclipsed Sun appearing up in the sky can be fun and beautiful. But even this is moderately hard to do well. I suggest that you NOT take any pictures, but rather simply enjoy the experience.
LSU College of Science: If you don’t have a solar viewer, is there still a way to safely view the eclipse?
Brad Schaefer: Do not look at the un-eclipsed Sun with the unprotected eye. But there are many ways to watch the partial phases of the eclipse with full safety.
- Special-made eclipse glasses with aluminized mylar. These can be bought or got many places.
- Welder’s glass #14 (ONLY). Buy at any welder’s supply store for about a dollar.
- Pinhole cameras cast a Sun-shaped image on the ground. Make your own pinhole with a cardboard sheet with a small hole cut in the center. Or use Nature’s own pinholes, perhaps created by a small chink of light filtering through a leafy tree onto the ground. (You can even use your hands to make a pinhole viewer - make a hashtag sign!)
LSU College of Science: How many solar eclipses will occur this century anywhere on Earth?
Brad Schaefer: An average is one solar eclipse per year, with the totality visible from somewhere on Earth. From any one location, the average for being able to see a total solar eclipse is about once every 300 years.
Lunar eclipses can be seen from an entire hemisphere of Earth, unlike a solar eclipse where totality is visible only from a narrow path of totality. So on average, a person anywhere on Earth can see a total lunar eclipse about once a year. Or well, with the clouds being ‘willing.’
LSU College of Science: Will the solar eclipse disrupt any concurrent measurements or observations, for example of Tabby’s Star?
Tabby Boyajian: Fortunately, no. An eclipse happens during the day. We can't observe most stars during the day (only our Sun). The Sun is on the opposite side of Earth when astronomers are taking data.
LSU College of Science: What are you most excited about seeing during the eclipse?
Tabby Boyajian: I have never seen one in person, yet! I'm just super excited to see it get as dark as night and see the stars in the middle of the day!
LSU College of Science: Do you know any “eclipse chasers,” people who travel to view eclipses?
Brad Schaefer: Yes! I married one! Well, and she married one too. We’ve been to seven total eclipses now, including in the middle of the Gobi Dessert, Aruba, the Coral Sea south of New Guinea, and Hawaii. We’ve spent a lot of money and a lot of time and effort so as to be in the Moon’s deepest shadow for just a few minutes. But this is easily worth it. Around 50,000 people the world over feel the same way. Seeing a total solar eclipse will easily be one of the best views of your whole life. And for the 2017 eclipse, you don’t have to trek across deserts or spend a lot of money. Don’t let this awesome opportunity pass you by!
If you are on LSU's campus during the eclipse, come by the Parade Grounds in front of the LSU Student Union for a eclipse-watching party with us!