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Millions of people are expected to watch a total solar eclipse sweep across central and North America on Monday, offering rare views of the star that sustains life on Earth.
The alignment of Moon and Sun began to score a trail of darkness starting at Mexico’s Pacific coast at around 11am local time. It is expected to exit the continent 5,400km north-east at Newfoundland, Canada, just after 5pm local time, and head over the Atlantic Ocean.
Total eclipses occur about once every 18 months but this one is notable for travelling over the rotating Earth near many densely inhabited areas, at a time of high solar activity. The rolling shadow entered the US in Texas and will pass over Oklahoma, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire and Maine, according to Nasa, the US space agency.
“Part of the reason for the excitement is that it’s going over a populated area,” said Robert Massey, deputy executive director of the UK Royal Astronomical Society, who worked as an eclipse tour guide in Iran during the 1999 totality that swept up through Europe. “Eclipses have become much more public events because you have got mass transit and it’s easier for people to travel long distances to see them.”
Eclipses occur when the moon appears to viewers on Earth as being exactly in front of the sun, blocking out its light. The full effect is localised in a so-called “path of totality”, which is less than 200km wide this time.
The event will offer observers the primal experience of daytime darkness, with Nasa saying the effect will last up to four minutes 28 seconds in Torreón, northern Mexico.
It should afford spectacular views of the sun’s normally invisible outer atmosphere, comprising the reddish ring known as the chromosphere and the luminous envelope called the corona.
The sun is near the peak of its 11-year cycle of changing magnetic activity, which has driven an increase in disturbances such as solar flares and sunspots.
Eclipse viewers might be treated to the sight of prominences — massive loops of plasma comprising charged hydrogen and helium — bursting outward from the sun’s surface.
“There will be a lot of activity to see,” said Massey. “The corona will take on a radial appearance — and there is more chance of seeing these eruptions on the edge of the sun.”
It is unsafe to look at the sun without special protective eclipse glasses except during the brief total phase, Nasa warns.
This eclipse has — like its modern predecessors — stoked merchandising opportunities ranging from T-shirts to mugs bearing the Simon & Garfunkel lyric “Hello darkness, my old friend”.
There will not be another total solar eclipse over the US for more than 20 years — and it is predicted that the phenomenon will one day no longer be visible from anywhere on Earth. The moon is moving slowly away from the planet and will one day be unable to fully cover the sun’s sphere light.
That moment is some time away yet, though: scientists predict it will not happen for hundreds of millions of years.
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