Millions of people will tilt their heads skyward on Monday, marveling at a total solar eclipse.

The moon will cross the sun and block its light for a few fleeting moments, creating a communal celestial experience that will not again be so accessible to people in the United States, Canada or Mexico for decades.

The total solar eclipse's path - the expanse where the moon fully obscures the sun - stretches from Mexico's Pacific Coast to the fringes of Atlantic Canada, passing through dozens of major cities where authorities are preparing for an influx of visitors eager to experience what may be a once-in-a-lifetime opportunity.

In New York, signs along the Thruway urged travelers to "Arrive Early, Stay Late" to avoid the inevitable jams that will clog routes to and from prime viewing areas along the eclipse's path.

Closer to Niagara Falls, which is in the path of totality, the second half of the message switched to a more realistic "Expect Delays."

It will be the first total solar eclipse visible from the United States since 2017, and there will not be another visible in the lower 48 states until 2044.

On Monday, much of the country is expected to take in the view.

In 2017, a majority of American adults watched the eclipse in person, according to an estimate by Jon Miller, a research scientist at the University of Michigan.

The figure, 154 million, is far beyond the audience of even the most-watched Super Bowl (123.4 million this year).

And the path of totality for Monday's eclipse crosses over more than twice the number of people as did the 2017 event.