On 21 August a total solar eclipse (TSE) will be visible from a vast swath of the US. It is the first in the continental US since 1979, the first to cross the continent from ocean to ocean since 1918, and the first in centuries to be visible from only the US.
Angela Speck, an astronomer at the University of Missouri and author of this commentary, contemplates the heavens.
Angela Speck, an astronomer at the University of Missouri and author of this commentary, contemplates the heavens.
Although a TSE occurs almost every year, each eclipse is visible only from 0.1% of Earth’s surface. On a planetary scale, TSEs are common; on the scale of individual cities and even countries, they’re so rare that most people never see one. This month’s eclipse will be one of the most watched celestial events in history.
This particular TSE is special for other reasons as well. Up to 70% of Earth’s surface is ocean, so eclipses are more often visible from oceans than from land. This year’s TSE is over land for about 4800 kilometers; the path of totality runs from the Pacific coast of Oregon to the Atlantic coast of South Carolina. That remarkably accessible, 100-kilometer-wide path facilitates observing by both scientists and the general public. Scientists will be engaging in cutting-edge science that cannot be achieved any other way. Equally important, though, they will be harnessing the power of the US population through citizen science on an unprecedented scale. Two examples are Citizen CATE (http://eclipse2017.nso.edu/citizen-cate) and the Eclipse Megamovie (https://eclipsemega.movie).
In addition to the TSE, the entire country gets to see a significant partial eclipse. Locations close to the path of totality will get a 99% eclipse. As one moves away from the path, the fraction of the Sun that will be hidden decreases, but even northern Maine, as far from totality as one can get and still be in the continental US, will still see almost 50% of the Sun blocked by the Moon. Even Alaska and Hawaii will see more than a 20% eclipse. Everyone in the US gets an eclipse experience.
If you are lucky enough to live on the path of totality (definitely my heavy metal band name!), or if you can travel there, you will have a unique and memorable lifetime experience. From first contact, when the Moon’s disk first “touches” the edge of the Sun, to totality, when the Sun’s disk is completely eclipsed, will take about 90 minutes.
For the first hour, the sky will barely change. If you are using a pinhole projection or approved solar-viewing safety glasses, you will see the Sun as an increasingly skinny crescent. After an hour or so, enough of the Sun will be blocked that the sky will begin to darken, slowly at first, to a post-sunset, early-twilight blue instead of bright sky blue. Then, as the Moon is almost completely covering the Sun, you’ll see the “diamond ring”: Most of the Sun will be blocked out, with just a tiny portion visible through a deep lunar valley. And then totality hits. The sky goes dark, and the Sun’s corona shines out.
The corona, the Sun’s extended atmosphere, is only about a millionth of the brightness of the Sun’s main disk. It is always there, but it is impossible to see under normal sunlight conditions. During a TSE, the corona is a spectacular sight. In addition, viewers will see stars and planets in the darkened sky. The stars visually closest to the Sun and Moon during totality will be the constellation Leo, whose brightest star, Regulus, will be right next to the Sun. Harry Potter fans will recognize Regulus as the name of Sirius Black’s brother. Sirius will also be visible to the southwest. Farther west will be the constellation Orion, featuring another member of the Black family, Bellatrix, visible to some observers.
The planets that can be seen will depend on the viewer’s location. For the far west end of the path of totality, Venus will be clearly visible, with Mars and Mercury somewhat fainter. To the east along the path, Jupiter will rise above the southeastern horizon. During all this star-spotting, the sky will be darkest near the corona and will become gradually lighter farther away, until early-twilight colors are visible all around the horizon.
The duration of totality is also location dependent. The longest duration, 2 minutes and 42 seconds, will occur just south of Carbondale, Illinois. The shortest durations, only seconds, will be close to the edges of the path of totality.
During totality there is much to experience, and not just visually. The temperature drops as the Sun’s light diminishes. The change in temperature gives rise to an air pressure change, which in turn makes air currents flow. You will feel the eclipse on your skin. At the same time, animals and plants will react. Many animals will assume it is bedtime or suppertime and act accordingly. Nocturnal insects may start to chirp; birds may roost and stop singing; the cows may come home. Even plants will react, with some flowers or leaves closing up. We will see the eclipse, and we will see, hear, and feel its effects.
The TSE is a great opportunity for scientists to engage with the public. As the director of astronomy at the University of Missouri, a large land-grant institution, I have been traveling all over the state to talk to schools, chambers of commerce, service clubs, scouts and 4H groups, city managers, elected officials, and emergency management folks. I get to talk science to people who never knew they were interested in it.
Solar-eclipse tourism is big business. People travel all over the world to see these things. For this TSE there is nothing to stop US residents from traveling to the path of totality. If only 4% of the US population outside the path of totality travels to the path, that will double the population—and that does not include the influx of foreign visitors.
All these visitors will need travel services, accommodation, food, and safety advice for the eclipse. If clouds set in at any given location, some visitors will hit the road to find a clear viewing spot. Traffic volume will be on a scale we associate with large sporting events—but across a path that is 4800 km long and 100 km wide. It will be a hot day, so emergency services should be ready for casualties from the heat and from improper viewing of the Sun. Most locations don’t normally have enough beds, campsites, toilets, and food vendors to handle a doubling of the population.
With respect to viewing the Sun: During the total phase of the eclipse, it is safe to watch without any special equipment; the sky is approximately as dark as on a full-moon night, and the corona is only as bright as a full moon. It will not hurt our eyes. However, during the partial phases, the Sun is too bright to be viewed without protection. Since the entire US will see a good partial eclipse, safe viewing practices—solar-viewing glasses and other safe options—need to be known throughout the country.
In addition to the public viewing, many children will be starting back to school around the time of the eclipse. Teachers should have educational and safety materials so that they can make the most of the event.
The American Astronomical Society, with support from NSF, has developed a website for the eclipse: https://eclipse.aas.org. It includes information on events and activities, safety advice endorsed by eye doctors and astronomers, educational materials, and much more. Even if you cannot view the TSE from your location on 21 August, the event is a great opportunity to engage with the public.
This total solar eclipse has the potential to be the biggest public space-science event since the Moon landings! It will happen whether we prepare or not. Let’s be ready. But if you miss this one, you won’t have to travel the world to see another. The next TSE visible in the US will be in April 2024.
