the greatest of nature’s spectacles: a total solar eclipse.
Although total solar eclipses occur more often than total lunar ones, more people have seen the latter. The reason is simple: During a lunar eclipse, anyone on the nightside of our planet under a clear sky can see the Moon passing through Earth’s shadow.
Even at the Moon’s distance, Earth’s shadow is much larger than the lunar disk. If the Moon passes through the center of Earth’s shadow, totality can last as long as 106 minutes. It’s usually less, because the Moon often passes either above or below the shadow’s center.
On the other hand, the Moon’s shadow barely reaches Earth’s surface during solar eclipses. If you’re beneath the lighter outer region of the shadow, called the penumbra, you’ll see a partial eclipse. The lucky individuals under the dark inner shadow, or umbra, will experience a total solar eclipse.
But, although dramatic, the event is brief. Solar totality lasts a maximum of 7 minutes 29 seconds. Unfortunately, the next eclipse sporting that duration won’t occur until July 16, 2186.
The Moon’s umbral shadow during the total solar eclipse on December 14, 2020, will race across the same two countries — Chile and Argentina — as it did during the eclipse of July 2, 2019. This year, the only other land off of the main mass of South America to experience totality will be Isla Mocha, a Chilean island. All told, only about 5 percent of this eclipse’s totality path falls on land.
The finer details
The Moon’s penumbra first touches Earth at 13h33m48s UT and last contacts it at 18h53m00s UT. The total phase of the eclipse begins 28 seconds after sunrise, which occurs at 14h32m UT, 466 miles (750 kilometers) east of Nuku Hiva, the largest of the Marquesas Islands, at longitude 132°49’39” west and latitude 7°46’22” south. The umbra remains in contact with Earth’s surface for 3 hours 21 minutes 45 seconds, until 17h54m13s UT, when it vanishes 217 miles (350 km) off the Namibian coast at longitude 11°01’22” east and latitude 23°37’28” south. The total path length is 9,239 miles (14,869 km).
This eclipse is total because at the time of the event, the Sun’s apparent diameter is 32’30”, while the Moon’s apparent span is slightly larger at 32’48”. Greatest eclipse occurs at 16h13m23s UT at longitude 67°56′ west and latitude 40°21′ south. The maximum duration of totality is 2 minutes 10 seconds.
The sky during the eclipse
In December, the Sun and Moon lie in Ophiuchus the Serpent-bearer, one of the southernmost constellations the Sun passes through during the year. At the moment of greatest eclipse, the Sun’s right ascension will be 17h30m06s and its declination will be –23°15’32”. As a result, the Sun’s altitude from any land location will be at least 71.4° at mid-totality.
Observers who enjoy favorable conditions will be in for a nice planetary show. During totality, the planet nearest the Sun will be Mercury. The innermost world will lie a bit more than 3° west-southwest of our daytime star. Sharp-eyed observers using binoculars will certainly spot Mercury, glowing at magnitude –1.0 during totality. Depending on the sky’s transparency at your observing site, it might even become visible to the naked eye.
The visibility of Mercury might be in question, but there’s no doubt about Venus, which will lie nearly 25° west of the Sun. Shining at magnitude –4.0, this planet may be visible to even casual observers up to 15 minutes before totality. Knowing where to look should let you pinpoint it through binoculars even earlier. Be extremely careful not to point any unfiltered optical device near the Sun.
With our star obscured by the Moon, it also should be possible to spot two giant outer planets, which lie within 1° of each other. Jupiter will shine at magnitude –2.0, and Saturn will glow at magnitude 0.6. Both will lie 36° east of the Sun. Jupiter, three and a half times brighter than Saturn, shouldn’t pose a problem for naked-eye observers. Once you find it, use binoculars to locate nearby Saturn. One thing to remember if you’re going to search for these planets is that the real spectacle is the Sun’s corona, so don’t devote many seconds to the endeavor.
As far as stars go, the only bright one in the area is Antares (Alpha [α] Scorpii). It glows at magnitude 1.1, which means it’s only 60 percent as bright as Saturn. Still, if you’d like to try to snag it, aim your binoculars 14° west of the Sun.
The eclipse in Chile
Because the umbra initially contacts Earth in the center of the Pacific Ocean, it travels more than 3,000 miles (4,830 km) before it touches land. That first territory, Isla Mocha, belongs to the Republic of Chile. The Moon’s inner shadow reaches the southwestern tip of this island at 15h58m55s UT. There, totality will last 1 minute 19 seconds, with the Sun 71° high.
The island’s northern edge lies at the northern limit of totality. Here, and for 16 miles (25 km) south along the coast, is where climate statistics say observers will likely find the best weather in Chile. Conveniently, it’s neither difficult nor expensive to travel to the island.
The umbra first encounters the Chilean mainland at Champulli, which lies 50 miles (80 km) west of Temuco. As a city with some 300,000 residents and corresponding amenities, Temuco will undoubtedly be the destination of choice for many eclipse chasers. From Temuco, it’s a quick 30-mile (50 km) drive down Highway 5 to either Gorbea or El Liuco, both of which will enjoy nearly 2 minutes 9 seconds of totality.
One spot that might offer an intriguing photographic opportunity is the southern side of Villarrica, one of Chile’s most active volcanoes. There, the eclipse will last 2 minutes 6 seconds, with the Sun 72° high in the north-northeast.
Before leaving Chile, the Moon’s umbra quickly encounters two other volcanoes within Villarrica National Park: Quetrupillan and Lanin, which sit on the country’s border with Argentina.
Chile is a long country, but it’s not wide. The total length of the umbra’s path through it, therefore, is only about 100 miles (160 km).
According to Canadian meteorologist Jay Anderson, the acknowledged expert when it comes to predicting weather for eclipses, the coastal region west of Teodoro Schmidt offers the most promising weather prospects in mainland Chile. The lower cloudiness (an average of 45 percent) occurs because cool air from the Pacific suppresses the formation of convective clouds. He writes on his website, eclipsophile.com, “The best cloud prospects on the coast are even better on the south side of the track where the terrain is roughest, but the benefit is only about 5 percent.”
After Teodoro Schmidt, average cloud cover goes up a bit, peaking at 52 percent east of Gorbea before declining to 45 percent at Villarrica. This decrease is due partly to the distance from the ocean and partly to the cooling effects of Lake Villarrica, which suppresses convection.
The eclipse in Argentina
The second country the umbra touches offers better weather and the longest totality. In fact, every point along the center line in Argentina offers a duration of totality within one second of the maximum.
As the Moon’s shadow crosses from Chile into Argentina, it encounters a vast stretch of small villages and rough roads. One possible destination for eclipse watchers is Los Menucos, a town of some 3,000 residents, which lies just outside the southern limit of totality. The midpoint of the eclipse lies 37 miles (60 km) north of Los Menucos and experiences 2 minutes 10 seconds of totality, but no roads go to it. Travelers who drive 53 miles (85 km) to the northeast along Highway 23 to the small village of Ministro Ramos Mexia can get the equivalent length of totality.
Another destination to consider is Valcheta, a town of nearly 4,000, which lies just north of the center line (and also on Highway 23). From there, totality will last 2 minutes 9 seconds, with the Sun 73° high in the north at mideclipse.
Better accommodations, but less favorable weather prospects, will be found at the coastal towns of San Antonio Oeste, San Antonio Este, and Las Grutas. And although Las Grutas’ population is just 8,000, some 400,000 tourists visit it during the summer, which, in Argentina, begins a week after the eclipse. The center line, with its 2 minutes 9 seconds of totality, lies only 6 miles (10 km) south of this town. Observers in the northern sections of San Antonio Oeste will lose nearly 20 seconds from that duration.
According to Anderson, weather prospects throughout Argentina are best on the western side of the country, where the sunniest and driest areas lie along the eastern edge of the Andes Mountains. Eclipse chasers may want to head for the village of Piedra del Aguila, which offers the best weather prospects. Don’t go too far west, though, because cloudiness is highest near the Chilean border.
East of Junín de los Andes, where totality will last 1 minute 1 second, the mean cloud cover drops below 40 percent until the track reaches the Atlantic coast at San Antonio Oeste. Afternoon clouds are less frequent in Argentina than in Chile, but the savvy eclipse chaser will stay mobile enough to make a last-minute change of location if necessary.
After leaving Argentina, the umbra travels another 3,000 miles (4,830 km), nearly the width of the Atlantic Ocean, before the eclipse ends a scant 155 miles (250 km) short of the Namibian coast.
Unfortunately, as I write this story (at the beginning of June), restrictions on international travel are high due to the COVID-19 threat, so it’s unclear whether amateur astronomers outside of Chile or Argentina will be able to view totality. Hopefully, a vaccine or treatment will soon open up our world and we’ll once again be able to experience the spectacle of darkness at midday.