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Mercury crosses the sun on Monday, and it’s the last chance to see it until 2032

Mercury will come out of hiding for five hours. It will be a window onto how thousands of planets were discovered in the Milky Way.

The planet Mercury is seen in silhouette, lower third of image, as it transits across the face of the sun Monday, May 9, 2016, as viewed from Boyertown, Pa.
The planet Mercury is seen in silhouette, lower third of image, as it transits across the face of the sun Monday, May 9, 2016, as viewed from Boyertown, Pa.Read moreNASA/Bill Ingalls / (NASA/Bill Ingalls)

You won’t notice a thing, and it will have no effect on the Philadelphia temperature, but solar power will dim infinitesimally between 7:35 a.m. and about 1 p.m. Monday as Mercury passes between Earth and the sun.

The so-called transit of Mercury is visible to earthlings only once every eight or nine years, and the show requires optical devices — binoculars, telescope — with at least 50X magnification, equipped with solar filters, says Derrick Pitts, astronomer at the Franklin Institute. (No, never, ever look directly at the sun.)

If you don’t have access to such devices, or if it’s cloudy, you can watch it on YouTube, and NASA will be posting real-time images. Just don’t expect high drama.

“There’s no fire, no explosions, and not even any sunspots,” said Pitts, who will be conducting a program at the institute for museum visitors starting at 10 a.m. (Blame the sun-blocking Comcast tower for the delay.)

But you’ll be watching something of immense cosmological significance, astronomers say — a window onto how we know what we know about the universe.

Besides, “there’s a certain geek in all of us,” said Jim Napolitano, chair of the Temple University physics department, who will be hosting a transit-watching event at Temple’s Charles Library from 9 a.m. to 1:30 p.m. (That’s rain or what he hopes will be shine.)

If you miss this one, see you in November 2032.

What will sky-watchers see?

Mercury will look like a black dot sailing across the sun’s surface. That would be Mercury’s silhouette, Pitts says, not to be confused with “shadow.”

A silhouette is an object appearing dark against a light background, as opposed to a dark shape cast by light from behind.

This will not be a video game.

“I really do love watching astronomical events,” said Pitts. “Some are more exciting than others. I shouldn’t say this. I describe this as like watching paint dry.”

But Napolitano and Pitts point out that for a few hours, spectators will have a firsthand experience with a phenomenon that helped lead to the discovery of thousands of planets in our galaxy.

What does a flying dot have to do with the discovery of planets?

As NASA explains, a planet’s transit screens out tiny portions of the sun’s light that scientists can detect with sophisticated instruments.

By measuring the brightness of a far-off star, a slight drop in intensity would indicate that a planet was orbiting that particular sun.

NASA’s Kepler mission alone discovered more than 1,000 of these so-called exoplanets using that method. That mission, of course, was named for the great 17th century astronomer Johannes Kepler. Along with other early astronomers, he used that method to estimate the size of the sun and the planets and their distances from the sun.

Why are these transit sightings so infrequent?

This all gets very complicated.

The transit sightings are infrequent because Earth and Mercury are on slightly different orbital planes. Plus, appropriately for the winged messenger, for orbital speed Mercury, the closest planet to the sun, leaves Earth in the solar dust.

The earth travels about 67,000 mph on its 365-day trip around the sun; Mercury books it at 107,000 mph in less than three Earth months. In fairness, it has a shorter trip, about 225 million miles compared with 585 million.

Thus it’s not often that the planets align in such a way that earthlings can see Mercury’s silhouette across the sun.

How many of these exoplanets might ultimately be discovered?

The sky is not even the beginning of the limit in this case. Thousands and thousands have been identified.

“There’s Earth-like planets with maybe Earth-like people,” Napolitano said, adding only half-seriously: “Let’s hope they’re better than us.”