A mercurial freckle will cross the sun’s face Monday morning. It will be another 30 years before anyone in North America will see it again.

Which is one reason why people should try to see the crossing — the Transit of Mercury. It’s a rare chance to see another planet as it orbits the sun.

“These transits are really neat,” said Diana Hannikainen, observing editor for Sky and Telescope Magazine.

Ordinarily, Hannikainen said, we see planets in the night sky as shimmering star-like objects, illuminated by the sun’s light reflecting off them. But we have no idea what they might look like, or their relative size and shape.

“They could be triangles,” she said.

On Monday, we get to see Mercury — which is a rocky, crater-ridden, atmosphere-less uninhabitable planet with temperatures ranging from 800 degrees to minus-250 degrees — as an actual thing.

But because it is the smallest planet in the solar system — a third the size of Earth, 1-200th the size of the sun — and because in order to see it, you have to look directly at the Sun’s face, you’ll need serious optics with solar filters on them to watch Mercury pass.

Luckily, three area observatories — in New Milford, Redding and New Haven — will have public viewings of the transit.

In New Milford, the John J. McCarthy Observatory at New Milford High School will be open from 7:30 a.m. to 1 p.m. The telescope at New Pond Farm in Redding will focus in on the event from 7:15 a.m. to 10 a.m. And the Leitner Family Observatory and Planetarium at Yale University in New Haven will do the same from 10 a.m. to noon.

If you want to watch from your computer, the website Virtual Telescope will be watching as well.

Cliff Watley, who leads the astronomy nights at New Pond Farm and the Discovery Center in Ridgefield, said even if it’s cloudy, there’s a chance to see the transit in sunny breaks.

“We watched the Transit of Mercury in 2012 in Ridgefield,” he said. “We had 50 people there. There was a 45-minute window of clear skies and everybody got to see it.”

From Earth, we can see only the two planets closer to the sun than us — Mercury and Venus — in transit. Because the orbits of the three planets are on different planes, it doesn’t happen very often.

Transits of Mercury happen 13 to 14 times in a century. The last one was 2016. The next two won’t be visible from North America. So the next good look at Mercury for us will be in 2049.

Transits of Venus are far rarer — two a century, paired up in eight-year intervals. The last two happened in 2004 and 2012. The next ones will be in 2117 and 2125.

Michael Faison, director of the Leitner Observatory at Yale, said because we can see Mercury crossing the sun’s face more often, and because it’s just a dot, it’s not a showy event.

“It’s not like a total solar eclipse or a really good meteor shower or the Transit of Venus,” he said.

But Monty Robson, director of the McCarthy Observatory in New Milford, points out that seeing it connects us to the history of astronomy.

In 1677, the great English astronomer Edmond Halley — of Halley’s Comet fame — watched the Transit of Mercury from the island of St. Helena in the South Atlantic.

He realized astronomers, following the laws of planetary motion established by 17th century German astronomer Johannes Kepler, could use these transits to calculate the true distance between planets and the sun and then, in turn, between all other planets.

At his urging, there was an international effort to observe the Transits of Venus in 1761 and 1769. Astronomers spread out across the globe — to Siberia, Tahiti, Newfoundland, Baja California, Madagascar and South Africa — to make observations.

Their work made it possible to create a fairly accurate measure of the astronomical unit — the distance between the earth and sun. Modern technology has now set that as 92,995,807 miles.

At the McCarthy Observatory in New Milford, there will be an extra treat — the chance to see the transit through its beautiful 19th century telescope made by John Benjamin Dancer, an English inventor and maker of optics.

Bill Cloutier, one of the leaders of the McCarthy Observatory, said it’s taken a while to get the telescope working up to snuff. It is, now.

“Its images are spectacular,” he said.

Connecticut Media Group