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Catching Mercury’s transit across the sun

One astronomer’s quest to photograph Mercury as it eclipsed the sun

The Mercury transit of Nov. 11, 2019, featuring an aircraft, Mercury and our local star. The three were tens of millions of miles apart. Photo taken in Layton, Utah, by Ryan and Addison Cowley
Ryan and Addison Cowley

Editor’s note: A version of this has been previously published on the author’s website.

We like to think that Earth is a mighty large place — so big that we call it “the world” as if no other existed. And it is humongous for us humans, nearly 25,000 miles in circumference, making a typical round-the-world cruise last maybe half a year.

But do we have a feeling about its size from an unbiased perspective? Here’s how to get an idea, at least in terms of the solar system. With a diameter of 3,032 miles, Mercury is roughly a third the diameter of Earth’s 7,926 miles (being generous to the innermost planet). So a Mercury transit, when it crosses in from the sun as viewed from here, is a good time to make the calculation. Seeing Mercury at dusk or dawn won’t work because it simply has a starlike appearance through binoculars or telescope.

The only planets that transit the sun from our perspective are Mercury and Venus, those that orbit inside Earth’s track. During a transit, a planet shows up against the sun as a disk. If we mentally multiply whatever Mercury looks like by three, that will show about how Earth would appear if it were Mercury’s average 36 million miles from the Sun, as seen from our location of 93 million miles. That will give a notion of our planetary stature.

A fair sky and impressive telescopes with solar filters greeted visitors to the roof of the University of Utah South Physics Building. The U.’s Department of Physics and Astronomy offered views of the rare transit of Mercury across the Sun on Nov. 11, 2019.
Joe Bauman

At 7,521 miles diameter, Venus is close to Earth’s size, so observing the next transit of Venus would give a more precise sense of our world’s size in the solar system. But that won’t happen until 2117. However, the next transit of Mercury will be in 2032, according to “Eclipse Predictions by Fred Espenak, NASA/GSFC Emeritus.” In 13 years, those who wish may fly to Europe, Africa or Antarctica to watch it.

Nov. 11 was a mostly clear, sunny, comfortable day. The transit of Mercury had begun by sunrise, the planet’s orbital speed requiring about five and a half hours to complete the journey. That left plenty of time to photograph it.

Taping a solar filter onto a telephoto lens, I tried to get views of this rare event. Eventually I went into the backyard and, with my tripods out of commission, wrapped myself around an old volleyball pole in order to hold the camera still. I shot a series of images, getting the focus as sharp as I could, varying chip speed, f-stop and exposure times. The camera was hard to steady, even more difficult to aim, and the sun wasn’t always at the center of the viewfinder. I couldn’t see Mercury in front of the sun. Occasionally I enlarged an image on the camera screen. I took more than 50 pictures. Never did I glimpse Mercury.

However, the University of Utah Department of Physics and Astronomy was sponsoring a transit watch at the observatory on the roof of the South Physics Building, 125 S. 1400 East.

Two or three small telescopes with solar filters were set up on the sidewalk, offering passers-by a look at the transit, while more and larger scopes with filters were ready on the building’s roof. Groups of visitors stood talking and, from time to time, observing the slow progress of the transit.

Looking through the telescopes, it was immediately obvious why I couldn’t see Mercury with my telephoto: It was an incredibly small mote on the immense disk of the sun. During this part of the solar cycle, nothing much was happening on the sun’s surface — no sunspots were visible. The only exception was that little round planet. With one type of solar filter, the sun showed white; another, green, and with another, like the one on my telephoto, it was orange.

University of Utah astronomy major Hannah Blomgren, Salt Lake City, checks the transit.
Joe Bauman

“I saw a quarter of a ball and at like the bottom southwest there was this tiny little black dot,” said Ivy Kuemmerle, 8, of Salt Lake City. It was “totally” exciting, she exclaimed. The “quarter of a ball” referred to the sun, as, for a time only part of it showed because the telescope was a little off its aim until corrected. Fortunately, the part that showed was where the slow action was taking place.

Her father, Jim Kuemmerle, took a look and said, “Holy cow.” It was his first transit. “This is so cool.”

The pimple on Sol’s face did nothing to block the powerful light beaming onto us — it was the smallest possible solar eclipse. If you happened to face the sun, chances are you squinted.

Hannah Blomgren, of Salt Lake City, peered into one of the eyepieces. “It’s pretty cool. I’m an astronomy major and it’s always fun to see these things,” she said.

”I think it is very exciting. I’ve seen the Venus transit before. Seeing Mercury the first time, it’s just a fantastic experience,” said Connor Guile, of Riverton. “It’s nice to see so many people gathered around, interested in this.”

I had left my Nikon home and now attempted to photograph Mercury through one of the observatory’s telescopes, holding my iPhone to the eyepiece. I took 45 images. In none of them does Mercury show up.

Paul Ricketts, the South Physics Observatory director, said this is an unusual astronomical event for Utah, since the sky was beautifully clear. He explained that transits of Mercury are infrequent “because we’re not always on the same orbital tilt in our plane as Mercury is.” So our position doesn’t often line up with Mercury’s and the Sun’s. The next time such a transit will be visible from North America, including Utah, will be in 2049, he said.

“I’ll be 106 then,” I said.

“You’ll make it,” Ricketts assured. He said my interest in astronomy would keep me young. I told him I have a new goal.

Ryan Cowley, a member of the Utah Astronomy Club, was photographing the transit with his son, Addison, from their home in Layton. They used a Canon 6D camera attached to a Celestron eight-inch-diameter Schmidt-Cassegrain telescope, making a two-hour time-lapse movie of the event. The photos were taken every 30 seconds. Each image was exposed for 1/60 of a second through a solar filter.

They didn’t realize what they happened to capture until they were processing the frames into a movie. One picture was, in my opinion, the best photo of the transit. It shows in the foreground, an airliner that happened to be crossing; in the middle ground, Mercury; and the background, the sun.

I too happened to be lucky, though in a minor way. Out of more than 50 photos I took with the Nikon from our backyard, two did record Mercury.

Joe Bauman, a former Deseret News science reporter, writes an astronomy blog at the-nightly-news.com and is an avid amateur astronomer. His email is joe@the-nightly-news.com.