August's total solar eclipse in the US will almost certainly be the most watched such event in history.
More than 12 million people - from Oregon to South Carolina - live on the path of darkness that the Moon will cut as it sweeps in front of the Sun.
Nearly four times that many live within a two-hour's drive. And then there are all the tourists who will flock to America to witness the spectacle.
It makes the eclipse a wonderful citizen science opportunity.
"By going out and looking at the Sun we take part in this time-honoured tradition of citizen science," says astronomer and artist Prof Tyler Nordgren from the University of Redlands in California.
"Edmund Halley during an eclipse in 1715 in London asked people to go outside, look up and see if they could see the total solar eclipse and measure the length of totality, and by that he was able to help refine the orbit of the Moon," he told BBC News.
You might think that with all the space telescopes trained on the Sun these days there is little the citizen or even the keen amateur can contribute. But total solar eclipses are special because they afford particularly favourable conditions to study the tenuous outer atmosphere of the Sun called the corona.
It is in this superheated "gas" of charged particles that the solar wind originates, and from which billions of tonnes of matter can occasionally burst towards the Earth to disrupt satellites, communications and even electricity grids.
The corona is outshone by the Sun's surface, its photosphere. And satellites will block out this glare using devices called coronagraphs or occulters. But these are usually so wide that they also obstruct a doughnut of light immediately above the edge of the star.
"The spacecraft block out not only the Sun but also a whole lot of light around it, otherwise there would be scattering all over the image. And so we have that whole region uniquely to observe in white light from the ground at total solar eclipse," says Jay Pasachoff from Williams College, a veteran of 65 eclipses.
And he wants members of the public to get in on the act.
One key project in the planning is the Citizen Continental-America Telescopic Eclipse (CATE) project run by the National Solar Observatory.
It is making available 59 identical telescopes and digital cameras to universities, schools and astronomy clubs along the path of the eclipse (another 40 observing kits are available to purchase).
Participants are being trained to gather images of the corona from their locality that can then be spliced together with everyone else's to produce an uninterrupted 90-minute video.
Citizen CATE will rely on dedicated, calibrated equipment. But a similar venture plans to make use of the countless photos that will be taken on the day with general pocket cameras and smartphones.
It has a core band of photographers, but the public will be able to participate with the aid of an app that will offer advice on getting the best image quality and provide the means to upload pictures.
The 21 August event is the first coast-to-coast total solar eclipse in the US since 1918.
The Moon's shadow begins its journey across Earth's surface - the path of totality - out in the Pacific.
It makes landfall near Newport in Oregon at 10:16 local time (17:16 GMT; 18:16 BST); and leaves the continent close to the Atlantic coastal city of Charleston, South Carolina, at 14:49 local time (18:49 GMT; 19:49 BST).
The location that will experience full darkness for the "greatest duration" is just outside the town of Carbondale, Illinois. Totality there will last 2 minutes and 40.2 seconds.
So many people are expected to try to view the eclipse that the American Astronomical Society has set up a taskforce to advise urban and rural communities on how to prepare for the expected population surge.
Prof Nordgren works a lot with the National Parks Service: "I'm going to be at John Day Fossil Beds National Monument in eastern Oregon.
"They have six parking spaces and a porta-potty, and yet they're expecting maybe 20,000 people to come there on that day."
And taskforce colleague, Angela Speck from the University of Missouri, added: "We need to have communities ready for the influx of people that are coming, and that means things like emergency services, road traffic control, food and water. Especially water - the eclipse is in August."
Nordgren, Speck, and Pasachoff were speaking here in Boston at the Annual Meeting of the American Association for the Advancement of Science (AAAS).