The Solar Eclipse of 2017-A (Protected) View From the Path of Totality.

Something is about to happen in Nashville, Tennessee, that has not happened since Andrew Johnson was president of the United States in 1865. The time before that was Christmas Day 1628: the year Charles I of England granted a land charter to the Massachusetts Bay Colony. And the next occurrence will not take place until August 2566, some 549 years hence. You get the picture! On Monday, August 21, 2017, at about 1:30 PM, Nashville residents will experience the rare privilege of witnessing a total eclipse of the sun for just under 2 minutes. For the first time since June 1918, the path of a total solar eclipse—the path of totality—will cross the continental United States from coast to coast, beginning near Eugene, Oregon, and ending in Charleston, South Carolina, some 90 minutes later. Although a total solar eclipse is observable from some place on Earth every 12 to 18 months, the chance occurs only once every 375 years for any particular location.1 The most recent opportunity to view a total eclipse in the United States was in 1991, and then only from Hawaii. Cities along the path of the eclipse across the United States, including Nashville, are celebrating because for most people, experiencing the path of totality is truly a once-in-a-lifetime event. A solar eclipse occurs when the moon passes between the Earth and the sun, casting a shadow over a limited portion of our planet. From our terrestrial perspective, this alignment is only possible during the new moonphase,whenthesun illuminates theopposingside of themoon. In contrast, a lunareclipseoccurswhen the Earth’s shadowcovers someportionof themoonwhen our planet passes between the sun and the full moon. Onemisconception is that solar eclipses are rare. In reality, solar eclipses occur regularly, on average about twice each year, whenever themoonblocks any part of the sun. Because the moon’s diameter is about 400 times smaller than the sun’s, the lunar distance from Earth needs to be just right for the moon to appear to be the same size as the sun. According to the National AeronauticsandSpaceAdministration(NASA),2 this special geometry happens every 18 months on average. However, themoon’s shadowvaries, depending on the distance fromEarth. Thus,mostof the time, viewersexperience apartial eclipse. Because thepathof totality is only about 112 km (70 miles),3 the rest of the United States and elsewhere will observe a partial eclipse of varying solar coverage. A second misconception is that an eclipse allows safe viewing of the sun. A total eclipse allows observation of the sun’s corona, which forms the outer atmosphere, and the lunar disk obscures the denser core. The corona is a super-heated collection of ionized gases that, during the eclipse, will appear as a spectacular iridescent crown radiating around the disk formed by the moon. The abject beauty of the corona is stunning and quite enticing, and therein lies the danger. Many sources maintain that the only safe time to view the sun directly with the naked eye is during a total eclipse. However, for most people, at least some portion of the sun’s core will be visible during the event. Even for those in the path of totality, as the moon’s shadow passes the sun quickly (at 2880 km/h [1800 miles/h]), most of the eclipse duration will be partial when viewing the sun without protection can lead to permanent injury of the retina. Confusion between viewing the corona vs the sun’s core is increasingly likely during the brief transition from partial to total eclipse and then back to partial eclipse. Core sunlight is intense at about 1350 W/m2 in irradiance. Irradiance is calculated in terms of incident surface area, which may not seem like much for the narrow fovea, with its diameter of only 1.5 mm. However, concentrated energy combined with the refractive power of our eye is why handheld lasers are dangerous to the retina. Although sunlight is more diffuse, when focused, the effect is similar. Damage to the fovea in solar retinopathy (sometimes called photic retinopathy or solar retinitis) occurs in 2 ways and by 2 distinct physical mechanisms. The spectrum of sunlight contains a significant fraction of near-infrared radiation (700-1500 nm), which can cause direct thermal injury (burns) via heat. Because we cannot see this light, and the retina lacks nociceptive receptors to signal pain, damage can occur without our knowledge.4 However, the more pressing concern when viewing a solar eclipse is for visible light, which in excess causes photochemical toxicity through rapid accumulation of reactive oxygen species and free radicals. This is especially damaging to the retinal pigment epithelium and choroid, which contain an abundance of photoactive materials rife for oxidation: heme proteins, melanosomes, lipofuscin, and the like. A prospective study5 after an August 1999 solar eclipse viewed in the United Kingdom supports the idea that following well-publicized safety concerns, photochemical toxicity is the more frequent cause of damage in solar retinopathy. Fortunately, in most cases, the evidence of injury ultimately resolves. Of 45 patients who attended a posteclipse clinic with concerns ranging from ocular discomfort to “black spots” near fixation, only 4 (8.9%) were symptomatic after 7 months.5 Typical clinical presentation of solar retinopathy evaluated with optical coherence tomography includes retinal pigment epithelium and photoreceptor atrophy in the fovea without evidence of VIEWPOINT