Your Guide To Monday’s Transit Of Mercury

I took this photo during Mercury’s last transit on May 9, 2016 with a 3-inch refracting telescope at 27x. While there are currently no sunspots, if any appear during the upcoming transit, compare their dark interiors — called umbrae — with the pitch blackness of Mercury’s diminutive disk. Bob King

On Veterans Day, Monday, Nov. 11, you can see the littlest planet pass directly in front of the sun. The event is called a transit, and they’re fairly rare. Mercury transits the sun about 13 times a century. What makes this event unique at least for North America is that we won’t see it happen again for almost 30 years — the next transit happens on May 7, 2049.

Additional transits occur in 2032 and 2039 but they’re visible from other parts of the planet. The sun won’t be up during those times for the U.S. You know what that means — don’t miss this one! Given my current age, Monday’s event will likely be the last of my lifetime. Astronomical prediction can sting like that, but it comes with the territory.

Mercury’s orbit is tilted 7° with respect to the plane of Earth’s orbit. Transits only occur when the planet cuts through that plane at one of two points called nodes at the same time as the planet is in inferior conjunction with the sun. Inferior conjunction occurs when Mercury passes between the Earth and sun. During most conjunctions Mercury is not at a node and misses the sun, passing a short distance north or south of it. ESO

During a transit, Mercury appears as a perfectly circular, inky-black dot a little smaller than a BB. As it marches along its orbit the planet slowly moves across the face of the sun. To see it you’ll need at minimum a pair of binoculars magnifying 10x capped by a safe solar filter. Unlike Venus, which transited the sun in 2004 and 2012, Mercury is too small to see without optical aid. One further caution, if you have a scope or binoculars but lack a proper solar filter to place over the end of either instrument, do not look through either using eclipse glasses. The unfiltered, concentrated light  coming through the telescope will instantly melt the plastic and fry your eyes!

Closeup of Mercury during the Nov. 8, 2006 transit. Hinode JAXA/NASA/PPARC

I’ve seen the solar system’s fastest planet parade in front of the sun at least five times and wouldn’t miss a one. Each has been an adventure. My first was May 9, 1970 at age 16. I remember toting my telescope through the forest to an open field to catch a minutes-long glimpse of Mercury shortly after sunrise. Monday’s transit will seem like easy-street in comparison with hours and hours of viewing time. That depends of course on where you live. If that’s South America, the east coast of the U.S., eastern Canada, Europe and western Africa, you’ll see the entire 5½ hour-long event. The further west the less time you’ll have. The transit will not be visible from western Alaska, China, India and Australia.

You can mount binoculars on a tripod, cover one lens with a lens cap and project the sun’s image safely onto a sheet of white cardboard to view the transit. Bob King

For example, in the Midwest, the sun will rise with Mercury already in view and 5 hours of transit time in store. That dwindles to around 4 hours in the mountain states and three hours along the West Coast. From my observing perch in Duluth, Minn. observers can start watching from the moment the sun clears the trees until the transit ends at 12:04 p.m. Consult the timetable for details for your time zone.

Each provides an opportunity to appreciate the true giganticness of our star, the sun. In our daily lives there’s little with which to compare it. All we see is a bright, shiny disk the same apparent size as the moon, a rocky orb a mere 2,160 miles across. Pfft!

The sun is some 864,000 miles (1.4 million km) across or 194 times the diameter of Mercury. During a transit it looks enormous compared to the tiny planet and offers a truer glimpse of its real size. All transits occur in four acts called contacts. At first contact at 12:35 Greenwich Time (6:35 CST), the leading edge of Mercury’s disk kisses the sun’s outer edge (called the limb). One minute and 41 seconds later, the planet’s trailing edge fully enters the sun’s disk at second contact, touching the inner edge.

Babak Tafreshi captured the black drop effect (right) when Venus last transited the Sun in 2004. Several factors contribute to the phenomenon including diffraction. To reproduce it, slowly bring your thumb and index finger together until they almost touch. Diffraction, the bending and interference of light waves as they pass through a narrow aperture, creates the dark bridge. Bob King (left); Babak Tafreshi (right)

If you have a telescope this is the time to look for the black drop effect, when the planet appears momentarily “stuck” to the sun’s limb by a taffy-like, dark filament. What you’re seeing is a combination of effects that include diffraction (the bending and interference of light waves as they squeeze through a narrow opening) and limb darkening. Even in a small, filtered telescope the center of the sun’s disk appears brighter than its edge. Square-on, we face directly into its brilliant, incandescent gases and it looks brightest. Along the edge we can’t peer as deeply so the material looks darker in comparison.

This diagram shows Mercury’s path across the sun from lower left (southeast) at transit start to upper right (northwest) at transit end. Contacts I-IV are labeled with Roman numerals. Times are Central Standard. Fred Espenak / www.Eclipsewise.com with additions by the author

After second contact Mercury spends hours crossing the solar disk until reaching third contact at the opposite inner edge and providing a final opportunity to witness the black drop. One minute 41 seconds later, it departs the disk at fourth contact, and the show is officially over. Or is it? Observers with hydrogen alpha (H-a) solar filters might see the planet both before and after the official transit times. The specialized filter reveals pink prominences sticking out from the edge of the sun normally only visible during a total solar eclipse. A well-positioned prominence would serve as a spectacular backdrop for Mercury’s black silhouette.

Transit viewing times across the time zones. Sky & Telescope

If you don’t have a safe solar filter you can safely project the sun’s image onto a white sheet of paper or a wall using either a pair of binoculars or a telescope. Take care to never look directly at the sun. Or you can contact your local astronomical society or university astronomy club to see if they’re holding a public viewing event. In Duluth, Minn., the Arrowhead Astronomical Society and staff at the University of Minnesota-Duluth planetarium will combine forces and set up telescopes outside the campus planetarium from 9 till noon on Monday, Nov. 11. Come on by! Clouds or not, you can watch the transit via live-streaming.

Member of the Arrowhead Astronomical Society in Duluth share the May 2016 Mercury transit with passersby. We’ll be out again to share this rare event on Monday, Nov. 11 weather-permitting in front of the Marshall W. Alworth Planetarium on the University of Minnesota-Duluth campus. Bob King

You can also catch the transit live on your phone or computer at Gianluca Masi’s Virtual Telescope Project site starting at 12:30 Greenwich (6:30 a.m. CST) as well as at SLOOH’s Livestream, also beginning at 6:30 a.m. CST. We’ve got you covered!

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