Here it is, right on the doorstep — the day that’s to bring a hint of sunset smack in the middle of the afternoon.

At a little past 1:20 p.m. Monday, as Long Islanders are working, shopping, running errands and cavorting on the beach, the moon is going to start slowly moving in front of the sun.

By around 2:45 p.m., about 70 percent of the sun will be covered by the moon, which then starts gradually moving away, until around 4 p.m. when the sun’s full brilliance is restored. That’s according to TimeandDate.com, a site that provides data on time, calendars and astronomical phenomena.

For Long Island it’s to be a partial eclipse of the sun, with the total blackout of this rare phenomenon to be viewed from a narrow strip 70 miles or so wide that runs diagonally from Oregon across to South Carolina. It’s to that area that hordes of eclipse chasers, plenty of Long Islanders included, have been flocking.

Astronomy buffs have been aquiver with anticipation, but “I’m most excited about seeing how excited other people are getting,” said Thomas C. Bruckner, a professor and chair of Nassau Community College’s physical sciences department. He’ll be giving a safe-viewing presentation to around 300 new students who will be attending orientation and taking a campus tour at eclipse time.

Just to give a sense of its novelty, the last time a total solar eclipse swept the country from coast to coast was in 1918, according to GreatAmericanEclipse.com, a site featuring eclipse maps and animations.

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Weather looks to be accommodating, with mostly sunny skies expected for eclipse time on Long Island, according to the late Saturday afternoon forecast from the National Weather Service.

If a few clouds were to arrive, they would be mostly of the fair-weather cumulus variety, nothing that would interfere with eclipse viewing on a “mostly sunny, nice day,” said Jim Connolly, weather service meteorologist based in Upton.

Sue Rose of East Meadow, a retired air traffic controller and president of the Amateur Observers’ Society of New York, reminds Long Islanders that viewing 30 percent of a really bright sun is still going to be bright.

That’s why special eclipse glasses — not sunglasses or binoculars and such — are necessary for viewing the eclipse from beginning to end, to protect eyes from damage, said Rose, who set out days ago for a drive to Wyoming to experience totality.

Indeed, even a quick glance could mean “permanent damage to the retina,” said Dr. John M. Alexander, retina specialist with the ophthalmology department at Nassau University Medical Center in East Meadow.

Any camera, telescope or pair of binoculars you might want to view through must also each have its own special filter, he said.

Still, those official eclipse glasses are hard to come by at this late date. The American Astronomical Society has a list of reputable vendors on its website, but as of last week most of those vendors were sold out.

Any glasses that you do come by should show verification of meeting the ISO 12312-2 safety standard, with ISO standing for International Organization for Standardization, according to the American Astronomical Society site.

Long Islanders looking for safe viewing options should check for events at places like museums, libraries and nature centers, but space could be limited.

The event will also be livestreamed, including by NASA on its website and smartphone app.

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Also, with an eye to the future, Long Islanders will want to mark April 8, 2024, on their calendars.

That’s the date of a solar eclipse that brings that narrow corridor of totality much closer to home — passing right over upstate New York.