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CHAMPAIGN — The planet Mercury s-l-o-w-l-y moves across the sun’s disk on Veterans Day morning.

You should know it’s a rare event.

A transit of Mercury was visible in Champaign in 2006 and in 2016. But the next one visible from Illinois will be in 2049.

Unlike the transit of Venus, Mercury’s transit will be very subtle, unless you have pretty good equipment.

“Mercury is difficult to see because it is so close to the sun,” said Erik Johnson, the director of the William M. Staerkel Planetarium.

It’s dramatic enough, though, that NASA had to quash prophecies adapted from the Bible that the transit portends the end of the world.

You need a telescope — one with special filters. A projector will also work safely. In our area, the dot will move across the sun from 6:39 a.m. until 12:07 p.m. on Monday.

“From our point of view, Mercury usually passes either above or below the sun as it passes between the Earth and sun,” said David Leake, the planetarium’s retired director. “Every now and then, Mercury appears to cross the face of the sun. There can be either 13 or 14 transits of Mercury in a century, and, given the inclination of Mercury’s orbit, they can occur in either May or November.”

The sun will be high enough to observe outside the planetarium starting at 8 a.m., said Johnson.

The Champaign–Urbana Astronomical Society will set up telescopes for the public throughout the morning near the planetarium. If weather permits, the transit will be visible from an open space near the planetarium.

This is a solar-observing event, not a planetarium show.

Johnson said there will be special telescopes that darken the sun —so fine-tuned that they're useless for ordinary star-gazing. There are also projectors that show the transit indirectly. Mylar, commonly used in balloons, also eliminates most of the sun’s intensity.

The astronomical society warns: “Your best bet is to project an image of the sun on a piece of paper; then you’re looking at the paper. If you use a solar filter on a telescope, be sure the filter goes on the end of the telescope facing the sun and does not thread into the eyepiece — those are dangerous.”

Two Coronado PSTs that Johnson has access to will be available. They block the sun’s signature glow from helium, for example.

“Hydrogen is the most common element on the sun. It converts to helium, creating huge amounts of energy. It’s the photosphere that blinds us,” the director said.

The photosphere is a star’s outer shell. The sun’s has a temperature up to 10,340 degrees Fahrenheit.

Size isn’t everything. The transit has been useful in well-established computations.

“The significance, beyond the rarity of the event itself, isn’t what it was in the past,” Leake said. “Using mathematics, we could figure out the relative distances to each of the planets, but to know the exact distances, we needed to know the distance from the Earth to the sun. Seems trivial! But it took careful observations of the transits of both Venus and Mercury to figure that out and refine the number.”

It’s like triangulation.

“It’s all geometry, really,” Leake said. “Observe a transit from two different spots on the Earth and, since the line-of-sight is a bit different, you can figure out the Earth’s distance to the planet, and then the Earth’s distance to the sun.”

It has other uses.

“Today the pros will look, as it gives us a chance to check out the radius of the sun (using Mercury as a sort of timer), looking at the mass of Venus and how its gravity affects Mercury’s orbit, and just to study Mercury itself,” Leake said. “Another thing is we are now observing transits of other planets orbiting stars outside our solar system. So it’s always good to take advantage of observing one in our solar system.”

The show isn’t over Monday.

“After the transit, Mercury moves into the morning sky where it has the second of two great pre-dawn views," the astronomical society said. "Mercury gets higher in the sky each morning until Nov. 28, when it is separated farthest from the sun. It rises at 5:15 a.m.

“A line drawn from (bright binary star) Spica through Mars will point to Mercury. Mercury’s descent toward the sun is slower, and it’ll be tough to see Mercury by the December holidays."

Reporter

Paul Wood is a reporter at The News-Gazette. His email is pwood@news-gazette.com, and you can follow him on Twitter (@pvawood).