The world's most populous nations will gaze skywards on Wednesday as the longest total solar eclipse of the 21st century lays a carpet of darkness across India and China, from Mumbai to Shanghai.
The event is being hyped in the obscure world of eclipse-chasers as a once-in-a-lifetime opportunity which, due to its trajectory over some of the most densely inhabited areas on Earth, could end up being the most watched eclipse in history.
"This is a very important milestone. None of us will live long enough to see another one like it," said Federico Borgmeyer, the German-based manager of the specialist travel agency Eclipse City.
The American astrophysicist and acclaimed eclipse expert Fred Espenak has simply labelled the July 22 event "a monster."
Total solar eclipses occur when the moon comes between the Earth and the sun, completely obscuring the sun.
The excitement this time around is largely due to the unusually long duration of the instant of greatest eclipse, or "totality" -- when the sun is wholly covered.
At its maximum, this will last six minutes and 39 seconds -- a duration that will not be matched until the year 2132.
The up to 258-kilometre- (161-mile-) wide shadow cast along the "path of totality" will first make landfall on the western Indian state of Gujarat shortly before 6:30am (0100 GMT).
It then races across India, blacking out the holy city of Varanasi on the banks of the Ganges, squeezing between the northern and southern tips of Bangladesh and Nepal before engulfing most of Bhutan, traversing the Chinese mainland and slipping back out to sea off Shanghai.
Its next landfall is Japan's southern Ryukyu Islands, after which it curves southeast through the Pacific Ocean where the maximum duration of totality will occur.
For those in its direct path, the early morning eclipse will effectively provide a double sunrise, as the moon takes the low-lying sun hostage and then slowly releases it again.
The total transit will obscure the sun by 50 percent or more for an estimated two billion people, from the salt flat farmers of Gujarat to herdsmen in the foothills of the Tibetan Himalayas.
Some have seized on the natural phenomenon as a unique business opportunity.
Travel firm Cox and Kings has chartered a Boeing 737-700 aircraft that will take off from New Delhi before dawn, "intercept" the total eclipse at around 41,000 feet and then chase its path to the western state of Bihar.
All 21 sun-side, window seats have been sold at 79,000 rupees (1,600 dollars) a pop, with customers including amateur astronomers, photographers and a family of four planning a mid-air picnic in the shade.
In Shanghai, hotels offering special eclipse packages were booked out well in advance by guests from Japan, the United States and Europe.
Shanghai Sculpture Park, one of the best viewing locations in the city, has sold more than 2,000 tickets, which include special glasses and a commemorative T-shirt.
On a less commercial note, 1.5 million Hindu pilgrims are expected at the holy site of Kurukshetra in northern India, where bathing in the waters during a solar eclipse is believed to further the attainment of spiritual freedom.
In India and China, tradition views eclipses with a mixture of wonder and foreboding.
Raj Kumar Sharma, an astrologer in Mumbai, described July 22 as a "very dangerous moment in the universe," with the blocking of the sun's rays akin to a disease or virus.
"If the sun, the leader of the stars, is sick, then definitely there's going to be some bigger problems happening in the world," he told AFP.
In ancient China, eclipses were often associated with natural disasters or the death of an emperor, and some similar superstitions persist.
There are also less otherworldly concerns -- the potentially massive audience for this eclipse has raised fears that many could end up with permanent retina damage from viewing the event with the naked eye.
Delhi-based eye surgeon Rituraj Barauh said the most dangerous phase was immediately after totality when the sun re-emerged.
"It's this quick shift from darkness to very bright light that does the damage," he said.
Indian and Chinese authorities have used the media to educate people on the importance of wearing protective lenses, while others recommend live web casts.
Catch all the Latest Tech News, Mobile News, Laptop News, Gaming news, Wearables News , How To News, also keep up with us on Whatsapp channel,Twitter, Facebook, Google News, and Instagram. For our latest videos, subscribe to our YouTube channel.