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Transit of Mercury on November 11

On Monday, November 11, 2019, the planet Mercury does a marathon dash across the face of the sun. It takes awhile --about five and a half hours--for the tiny disk of Mercury to transit from one edge of the sun to the other. That's about how long Mike Karl can run, but he needs less. Out in space, Mercury is passing earth on the inside track, with the sun in the background.

Mercury in transit appears smaller than a sunspot near the solar limb.

Image: The circular dot Mercury transits the sun in 2006, a year with solar activity. A sunspot near the limb of the sun dwarfs the planet suspended in space between the sun and earth. The sun has been in a lull lately, with few sunspots visible during this low point of the 11-year solar cycle. Check for updates on solar activity.

Because the innermost planet appears relatively small within the disk of the sun, to observe the black silhouette of Mercury directly you need a magnified view through proper solar filters. That is, you essentially need a telescope with a solar filter. You can also project an image of the sun with optics. I like to use a Sun Funnel, too.


During the transit, Mercury appears as a tiny dot slowly gliding across the face of the sun. To witness this phenomenon, you need to view a magnified image with proper solar protection. Unlike Venus in transit, in which the planet is big enough for the human eye to discern without magnification, Mercury is smaller and more distant. On Nov. 11 the planet diameter appears a mere 10 arc-seconds across. For comparison, Venus is about 60 arc-seconds (one arc-minute) across, and the sun is about 30 arc-minutes (half a degree) across.

In 1677, at age 20, astronomer Edmond Halley witnessed a transit of Mercury from the island of St. Helena. Decades later, in 1716, Halley wrote of his new method using the transit of an inferior planet (i.e., Mercury or Venus) to measure the size of the solar system. Because Venus is closer and yields a larger parallax, Halley appealed for global expeditions to time the next transit of Venus.


I plan on setting up a telescope somewhere around South Bend, IN, just before sunrise. Join me wherever and drink some Black Drop Effect coffee from Victorian Pantry in Granger, IN. The bean commemorates the visual space oddity in which the circular disk of a transiting inferior planet (whether Mercury or Venus) appears to elongate at internal contacts. The planet looks like a stretched out droplet clinging to the darkness of space. The black drop relinquishes its grip on the solar edge and the planetary disk quickly resumes looking like a sphere.

That said, don't expect to see the black drop effect in most telescopes--it's just too small. A definitive paper on the black drop effect, using satellite imagery during the 1999 transit of Mercury, effectively dismisses previous suggestions of what causes the effect. See Poster on the 1999 transit of Mercury, by Jay Pasachoff, Glenn Schneider and Leon Golub; from the American Astronomical Society's Division of Planetary Science meeting in 2001. Limb darkening and the instrument's Point Spread Function seem to be the greatest culprits.


A few minutes after sunrise on 2019 Veterans Day, around 7:34 a.m. EST, Mercury begins to cross the edge of the sun. At least, it's rising as seen from South Bend, IN. (For circumstances at your site, please see Xavier Jubier's interactive map at With the sun so low and the planet so small, I don't fully expect to see the transiting planet until the sun rises a little higher out of the muck and turbulence near the horizon. Then again, the atmosphere can surprise you and perhaps amplify the experience. Either way, you won't ever see it if you don't look. So we look.

After awhile at one site, I may move to another location. After all, on Monday, November 11, 2019, we've got five and a half hours--until 1:04 p.m. EST--until the show ends.


If it's cloudy, come out anyway and together we'll make some solargraphs "under the hood" while supplies last. If you make a solargraph this November 11, you'll be ready to mount it and start the exposure at the upcoming December solstice.


The last (paragraph) shall be first (in importance). Though it goes without saying, I observe a planetary transit you are looking at the sun. Hence, you must wear proper eye protection to view the sun directly. A joint statement anticipating the 2017 solar eclipse explains some basics of solar safety that are applicable for the transit.

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