Block out your schedule for historic eclipse day

At the heart of a phenomenon that has alternately mystified, terrorized, entertained, transfixed and awed human beings for eons is an amazing and perhaps underappreciated cosmic coincidence from which millions of Americans will benefit come Monday afternoon.

For several minutes our humble moon will manage to block out the light of an object 400 times its size, as millions along a 100-mile-or-so corridor from the Mexican Pacific Coast to Newfoundland and slicing through western Pennsylvania, experience a total eclipse.

It so happens that on Monday the moon is making one of its closer approaches of the year, and will be positioned about 400 times nearer to Earth than the sun. In that alignment, from our perspective, both the sun and moon will appear to be about the same size in the sky.

As the moon passes between the Earth and sun, starting shortly after 2 p.m. in the Northeast, it eventually will be perfectly positioned to block out the sun’s surface entirely along the path of totality.

While Philly will miss out on the sudden nightfall and corona halo effects — and the potential traffic jams to the northwest — it will be getting quite the consolation prize, said NASA’s Gina A. DiBraccio, deputy director of the Heliophysics Science Division, and a Bucks County native.

As eclipso-mania events occur all over the region, about 90% of the sun will be covered by about 3:20 p.m. when the sun will look “like a cookie with a bite taken out of it,” said DiBraccio.

“Being able to experience any solar eclipse — partial, annular, or total — is sure to be an experience of a lifetime.”

» READ MORE: Six reasons to be excited about the eclipse

Said Amy Keesee, physics and astronomy professor at the University of New Hamphire, which is in the 95% zone, “It’s just a reality that not everyone can travel to the path of totality, but it will still be worth observing the partial eclipse.”

Philly will get its shot in 2079, just 600 years after that last happened in this nook of the planet, in 1478.

The skies won’t be completely clear, but at least partial sun is expected to be visible when the show starts, forecasters say. Plus, temperatures will be in the low and mid-60s, ideal for watch parties, just make sure you bring eclipse glasses or some other eye protection.

Temperatures can drop as much as 10 degrees during totality, said Stephen Morgan, a Fox Weather meteorologist, but it likely won’t change much here.

» READ MORE: Philadelphians appear to enjoy the partial eclipse in 2017

The August 2017 experience bodes well for this one. That summer, the partial eclipse received rave reviews from Philly spectators — and in that case the sun was only 75% obscured, and it was cloudy.

On Monday, the landscape may darken subtly, but nothing like it will in the totality zone. However, looking up, the path of the moon crossing the sun will be evident, with the peak darkness occurring around 3:23 p.m. That show will be over at 4:25 p.m.

» READ MORE: Here's what to expect on eclipse day

While this will be a celebratory occasion, it will be a bonanza for helio-physicists, the people who study the sun’s mysterious corona, which will be visible to the naked eye during totality. The fiery corona is millions of degrees hotter than the sun’s surface, and scientists would like to figure out why.

“The corona is the cause of all ‘space weather,’ ” said Keesee of the cosmic weather that affects the aurora, satellites, the power grid, and the communication systems used by the military and emergency responders.

“We eventually would like to forecast these types of events,” she said.

Said NASA’s DiBraccio, “Eclipses give us a unique chance to study the corona by performing experiments that we can’t do on a day-to-day basis.” She said NASA will be flying a high-altitude research jet to capture images of the corona during the eclipse.

The sun is approaching one of its 11-year peaks and has been quite active in recent weeks. Totality on Monday, she said, may represent “the best chance we could hope for to observe solar activity.”

Long before high-altitude jets and instrument packs, it is possible that the Pueblo people of New Mexico captured images of the corona during an active solar period on July 11, 1079, according to NASA.

It is known that a total eclipse occurred that day, and a petroglyph rock carving appears to depict a “coronal mass ejection” from the sun.

North American Indigenous peoples had many different interpretations of eclipses, some of them with profound religious significance.

Just how long the sun and the moon have engaged in their celestial ballet of crossing paths and magically getting in each other’s way is unclear, but presumably it has been happening since the birth of the moon billions of years ago. It just took a while for humans to write things down, or at least carve them into rocks.

NASA says the oldest available record appears to date to 3340 B.C. in petroglyphs discovered in Ireland.

The Mayans were known to maintain careful astronomical records that include eclipse documentations, and they have been credited with predicting the solar eclipse of July 1991.

Total eclipses occur regularly, about once every 18 months. However, totality occurs on only a relative sliver of the planet. For example, Monday’s path of totality from San Antonio, Texas, to Caribou, Maine, will cover about 0.1% of the planet.

Belgian astronomer Jean Meeus estimated that on average, a total solar eclipse occurs on the same point on Earth once every 375 years, says Harry J. Augensen, emeritus professor and director of the Widener University Observatory, in Chester.

The last one to cross the contiguous United States was the aforementioned Aug. 21, 2017, a partial eclipse in Philly.

Since the last total solar eclipse in what today we call “Philadelphia” occurred 546 years ago, you might say we’re overdue.

Set your clock for May 1, 2079.