10 Strange Facts About Total Solar Eclipses

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The upcoming total solar eclipse on Aug. 21, 2017 is not just a big event for astronomers. The public is also excited about this rare occurrence, the first total solar eclipse in the contiguous U.S. since 1979. While most of North America will witness a partial eclipse, a 70-mile-wide path of the Moon’s shadow (or umbra) will enter the U.S. along the Oregon coast and depart over the Atlantic coast of South Carolina 92 minutes later. Within that path, many hotels and campgrounds have been sold out for months, as the public clamors to get a view of the full eclipse (the maximum full eclipse lasts 2 minutes, 40 seconds near Carbondale, Ill.). In the meantime, here are 10 strange but true facts about total solar eclipses.

Solar eclipses are the result of an odd coincidence.

10. Don’t Stare at the Sun! You Really Can Go Blind

You’ll hear plenty about this leading up to the event. A recent Space.com article even shows images of patients who stared at solar eclipses and had tiny crescent suns (!) burned in the backs of their eyes. It is imperative that all safety precautions are taken during the partial phases of a solar eclipse. Even 1 percent of the Sun is surprisingly bright, enough to cause temporary or permanent blindness. Use only eclipse glasses (not sunglasses!) or telescope filters specifically designed to observe the Sun. Don’t forget to remove those glasses, however, during totality, or you’ll miss the best part of the eclipse. That is the only brief time it is OK to look at the Sun, but be careful to put those safety glasses before the first flash of sunlight appears around the edge of the Moon.


9. Eclipses Only Occur During a New or Full Moon

An elementary fact, for sure, but one that popular media often gets wrong. The Moon orbits the Earth once every 29.5 days, and a total solar eclipse occurs when the Moon passes between the Earth and the Sun, and total lunar eclipses happen when the Earth is between the Sun and the Moon. Eclipses don’t happen every month because the Moon’s orbit is tilted slightly (5 degrees) relative to the orbit of the Earth around the Sun. When the crossing of the orbits (known as the ascending and descending nodes) line up between the position of the Earth and Sun (known as a syzygy), an eclipse can occur. Two to three eclipse seasons occur a year, and a given year can experience a minimum of four (two solar and two lunar) eclipses to a maximum of seven.


8. Solar Eclipses Occur Because of an Odd Coincidence

We live in a fortunate place in time and space, in terms of eclipses. Total solar eclipses can occur because the Sun is 400 times larger than the Moon … but it’s also 400 times farther away. Not as precise a fit as imagined, but pretty darned close. But this situation isn’t perfect, and is continually changing (more on that in a bit). Moreover, the orbits of the Earth and Moon aren’t circular, and often, what is known as an annular eclipse occurs when the Moon passes between the Earth and Sun but is visually too small to cover the Sun. In fact, looking at NASA’s catalog of solar eclipses over a 5,000-year span from 2000 BC to 3000 AD reveals 33.2 percent of solar eclipses are annular, versus 26.7 percent totals (the remainder are partials/hybrids).


7. Total Solar Eclipses Won’t Occur in Millions of Years

Here’s another reason we live in a special epoch in terms of eclipses. Hundreds of millions of years from now, total solar eclipses will no longer occur. This is because the Moon is slowly receding from the Earth. We know this is happening to the tune of a current rate 3.78 cm per year because of laser reflectors placed on the Moon by Apollo astronauts. Likewise, the first annular eclipse must’ve occurred about a billion years ago.


6. Jupiter’s Moons Get Eclipses Similar to Earth

Eclipses aren’t exclusive to the Earth. When we see a lunar eclipse of the Moon, for example, the surface of the Moon sees a solar eclipse, something no lunar astronaut has yet to witness. Rovers on the surface of Mars have witnessed oddly shaped partial solar eclipses as the tiny moons Phobos and Deimos glide in front of the Sun. Again, this refutes the idea that Earth-Moon eclipses are “rare” or “perfect.” But the surface of the large Jovian moons might be of special interest to future astronauts. The moons of Jupiter get similar solar eclipses, with the occulting body the same size as the Sun as the moons pass one in front of the other. These events are faster than eclipses on Earth, though, with totality lasting under a minute.


5. Total Eclipse is the Only Time Sun’s Corona is Visible

There’s one region of the Sun that is still poorly understood. On a sunny day, the region we see shining down on us is the dazzling photosphere, the roiling “surface” of the giant gas ball that is the Sun. This region sports temperatures in the 4,000 to 6,000 degrees Celsius range … but just above that is the solar corona, with temperatures measured in the millions of degrees Celsius. Why is this region so hot compared to the rest of the Sun? This is known as the “corona paradox,” and a total solar eclipse offers the only time that the corona of the Sun can be studied in detail. The corona is the pearly white glow you see around the Sun during totality, and it’s about as bright as a full Moon.


4. Scientists Once Hunted ‘Planet Vulcan’ During Eclipse

There once was a proposed planet inside the orbit of Mercury; scientists deduced that because of an anomaly in Mercury’s orbit. Astronomers in the 18th century even claimed to have observed this tiny world dubbed “Vulcan” transiting the disk of the Sun. Still, Vulcan proved to be elusive, due to its presumed close proximity to the Sun. One of the last great hunts for Vulcan occurred during a total solar eclipse over Wyoming territory on July 29, 1878. Though astronomers initially claimed success, reports of observations of Vulcan proved to be spurious, though it did make it into the list of solar system planets in textbooks of the day. The advent of Einstein’s general theory of relativity explained the anomalous motion of Mercury and did away with the need for Vulcan. The jury is still out, however, on the possibility that as yet undiscovered tiny asteroids may orbit near the Sun. Along with studies of the solar corona, NASA will continue the hunt for “Vulcanoid asteroids” near the Sun during the Aug. 21 solar eclipse.


3. Researchers Struck Out Hunting ‘Allais Effect’

Proposed by Nobel Prize winner in Economics Maurice Allais, the “Allais effect” was a strange gravitational effect claimed to occur during a total solar eclipse. Allais claimed to observe this motion on pendulums during total solar eclipses in the mid-20th century, and proposed that the Moon either had a “screening effect” on the Sun’s gravity, or that the laws of gravity needed to be modified. Allais’s work was never peer reviewed, and tests during subsequent eclipses failed to duplicate his results. It’s hard to see just how a total solar eclipse would make much of a difference compared to a garden-variety New Moon.


2. Any Given Spot on Earth Gets Total Eclipse Every 375 Years

Though partial solar eclipses cross a large swath of the Earth, the path of totality is relatively narrow, and is on average about 70 miles wide. Odds are, you’ll need to travel to see a total solar eclipse. Eclipses belong to saros cycles, and eclipses in the same saros occur 18 years, 11 days and 8 hours apart, and move 120 degrees westward. The Aug. 11, 1999 solar eclipse, for example, belongs to the same saros series 145 as the Aug. 21 eclipse. Several saros cycles are active at any given time. Keep in mind, the 375-year period is an average. .. if you live in the Carbondale, Ill., region, for example, you’ll get another total solar eclipse in just under seven years on April 8, 2024.


1. Strange Effects are Visible Just Before Total Eclipse

There are several effects to keep an eye out for leading up to and during totality. Watch for what’s known as the “Diamond Ring” effect or Baily’s Beads, as the final rays of sunlight shine through the lunar valleys along the edge of the Moon. Also watch for shadow bands tracing across the local landscape leading up to totality. Roosters may crow and nocturnal animals may stir to life, fooled by the false nightfall at midday. Then, after scant minutes of night at midday, the process reverses itself again. Ask yourself: What would you have made of this experience thousands of years ago, if it had occurred without warning?


One More: Power Grid Will Take a Temporary Hit

It’s strange but true: an eclipse impacts solar-power production. This is somewhat counterintuitive, as one wonders how the solar-power grid normally reacts to cloudy weather. It turns out, however, that the diminished solar output during a solar eclipse covers a much wider swath than localized cloud coverage, and the diminished output and the subsequent oncoming surge is a unique situation for the continual balancing act that power producers must play. California now produces about 10 percent of its energy using solar, and expects to see a 50 to 75 percent reduction in solar output during the partial phases of the Aug. 21 solar eclipse. According to the Washington Post, this means a loss of about 70 megawatts per minute during the start of the eclipse, with about 90 megawatts a minute coming quickly back online afterward.


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Written by

David Dickinson is a backyard astronomer, science educator and retired military veteran. He lives in Hudson, Fla., with his wife, Myscha, and their dog, Maggie. He blogs about astronomy, science and science fiction at www.astroguyz.com.