Don’t Miss Monday’s Rare Transit of Mercury

Amateurs around the world await the little black dot that will cross the Sun.

2003 Mercury transit

A montage of the 2003 Mercury transit photographed by the Solar and Heliospheric Observatory.
NASA / ESA

On Monday, November 11th, Mercury will pass directly in front of the Sun and look a tiny black dot silhouetted against the brilliant solar disk. The event, called a transit, occurs approximately 13 times a century. The last took place on May 9, 2016 and the next ones unfold in 2032 and 2039. But if you live in the United States, the Sun will be below the horizon for both, making this year's transit the last visible until May 7, 2049.

If you just added 30 years to your present age and realized Monday's transit could be the last of a lifetime you're not alone. Don't count on Venus for help, either. Its next sun-crossing is 98 years away. Such is the double-edged sword of astronomy's prowess at predicting the future.

Mercury intersects Earth's orbital plane

Mercury transits occur when the planet crosses one its two orbital nodes at the same time it's in inferior conjunction to the Sun.
ESO

Mercury's orbit is inclined 7° to the ecliptic, the plane of Earth's orbit, and intersects that plane at two points called nodes during each revolution around the Sun. The first intersection, called the descending node, occurs when the planet crosses south of the ecliptic. The second crossing takes place at the ascending node when Mercury intersects the plane moving north.

Transits happen when the planet crosses a node at the same time it reaches inferior conjunction, passing directly between the Sun and the Earth. In the current era, those crossings occur around May 8 (descending node) and November 10 (ascending node). During November transits, Mercury is near perihelion and therefore farther from the Earth, the reason for its smaller apparent size of 10″ compared to 12″ for transits when it's near aphelion in May.

Transit and sunspots

The last transit occurred on May 9, 2016. I used a 3-inch refractor at 27× and a solar filter to record Mercury's inky disk, which appears blacker than sunspot umbrae. As of November 6th the Sun remains spotless. Let's hope for some activity in the coming days. Sunspots help track the movement of the planet and add interest to the scene.
Bob King

I saw my first Mercury transit in 1970 when I hurried my 6-inch Edmund reflector through the woods into an open field to spy the perfect black dot shortly after sunrise before it fled the Sun. Four more transits followed and each involved some form of adventure and a tale worth telling. I wouldn't miss a one. Why? Because I know of no better way to appreciate the enormity of the star that illuminates and nurtures Earth than to see a fellow planet cross its face. Mercury outdoes Venus in this regard because it's considerably closer to the Sun with an apparent size nearer its true diameter, thereby providing a better visual comparison.

Mercury's path

Mercury's path across the Sun is shown for November 11th Eastern Standard Times. Click here for a UT version. Remember that the Sun will appear tilted counterclockwise ~45° to ~60° from this view at sunrise (depending on your latitude in the U.S.; value is 90° minus observer's latitude) and clockwise by roughly the same amount in the late afternoon.
Sky & Telescope

The Sun spans 1.4 million kilometers across yet matches the Moon in apparent size. Positively puny. But compared to Mercury it's a behemoth with the swagger of a true star. On the day of the transit Mercury will appear only 1194 as large and will lie some 47 million kilometers from the Sun, or one-third of the Earth-Sun distance. It tiptoes onto the stage at 12:35 UT (7:35 a.m. EST), reaches the midpoint of its act at 15:20 UT (10:20 a.m. EST) and exits the solar disk at 18:04 UT (1:04 p.m. EST). Mercury remains visible for 5½ hours, providing lots of time for a break in the clouds in uncertain weather. And there will be clouds. For much of the U.S. November is the cloudiest month of the year.

Contact times

Timetable of Mercury transit events.
Sky & Telescope

For a good idea of where the clouds are in North America and when and where it might be clear I use Clear Dark Sky. In the left-hand column click Find a Chart and then input your location. MyWeather2 is another useful site and provides cloud forecasts worldwide.

Transit visibility map

Map showing where the transit's visibility on November 11, 2019.
Sky & Telescope

All of South America along with parts of North America, Central America, and Africa will see the entire event. From the Midwest westward the Sun rises with Mercury already in tow, while farther east in Europe, Africa, and the Middle East the Sun will set with the planet still in transit. China, Australia, Indonesia, much of Asia, and the western half of Alaska won't see the show.

Projection method for solar viewing

If you don't have access to a telescope with a full-aperture solar filter, project the Sun's image onto a piece of white paper.
S&T: Sean Walker

To view the transit you'll need a small telescope or a pair of binoculars capped with a safe solar filter. Unlike Venus, Mercury is too small to see without optical aid. Most sites will tell you that a minimum of 30× to 50× magnification is required to spot it, but during the last transit, Roger Sinnott, one of S&T's senior contributing editors, discovered otherwise. He writes, and I quote from Kelly Beatty's 2016 transit story:

"I used three high-quality, prismatic optical systems: Nikon 7×21 binoculars, Bushnell 4× Xtra-Wide binoculars, and a 3× tank scope from World War II. For eye protection I held a Thousand Oaks solar filter in front of the objectives. At 7× Mercury's tiny speck was easy to see. At 4× it proved quite difficult, though once spotted I could hold it steadily in view. But at 3× the planet was never seen at all."

black drop effect 2004

Sky & Telescope contributing photographer Babak Tafreshi captured the black drop effect when Venus transited the Sun in 2004. Several factors produce and enhance the phenomenon, including the extreme darkening of the solar edge just inside Sun's limb, as well as the blurriness caused by the telescope and atmospheric turbulence.

Keep in mind that Sinnott performed the experiment when Mercury was near aphelion and closest to the Earth. The current transit occurs around perihelion when the planet will be about 17 percent smaller. Factoring that in, an eagle-eyed skywatcher under good conditions should be able to discern a speck of a disk relatively easily at 9×. I can't wait to try it, and I'm most eager to hear what you will see. Use a tripod for a rock-steady view to ensure success! For a more comfortable view and flexibility in magnification I recommend instead a small telescope in the 3–6 inch range at around 30×–80×.

Every transit plays out in four acts beginning with first contact, when Mercury's diminutive disk touches the Sun's eastern limb at 12:35:27 UT. Amateurs observing with hydrogen alpha (H-alpha) filters may see the planet silhouetted against a prominence or the tiny spicules of incandescent gas that fuzz the outer limb moments to minutes before it enters the solar disk.

Sharing the sky

Transits make for great public outreach opportunities. Visitors to the planetarium at the University of Minnesota-Duluth get a look at the May 9, 2016, Mercury transit in telescopes set up by the Arrowhead Astronomical Society. 
Bob King

One minute 41 seconds later, the planet's trailing edge moves fully inside the Sun's disk and touches the inner limb at second contact, Watch for the black drop effect at this time, a taffylike filament of darkness that appears to momentarily anchor the planet to the limb. Diffraction effects (above), atmospheric turbulence and limb darkening — the gradual darkening of the Sun as our line of sight approaches the limb — combine to create this most interesting sight.

It happens again just before third contact when the leading edge of Mercury touches the opposite limb. One minute 41 seconds later at 18:02:33 UT at fourth contact, the planet's trailing limb departs the solar border and Mercury rides like a cowboy into the sunset.

Transits are a wonderful way for local astronomy clubs or individuals to share the sky with the public. Daylight means you and your equipment really stand out, and it's easier for your audience to find their way around the telescope. As visitors marvel at the flyspeck, you can explain that astronomers use transits like this one to detect a majority of the currently known 4,100+ exoplanets. Contact your local news outlets and post an event on Facebook and Instagram to get the word out. Should weather interfere you can watch the transit live on Gianluca Masi's Virtual Telescope Project site starting at 12:30 UT (7:30 a.m. EST) as well as at SLOOH's Livestream, also beginning at 12:30 UT.

Will it be clear? Let's hope so. This one will have to tide me over into the afterlife.

22 thoughts on “Don’t Miss Monday’s Rare Transit of Mercury

  1. RodRod

    Bob, nice job here 🙂 I plan to view the Mercury transit using my 90-mm refractor with white light solar filter. Mercury near 10″ angular size will be no problem viewing the black, planetary disk (40x – 80x views). I tested my ability to see the Sun recently doing some prep work 🙂 For Monday next week, the forecast is partly sunny skies here in Maryland at my location – not the best but doable. I had no trouble a few days ago viewing the Sun despite altocumulus and cirrus clouds. No timing measurements for me or effort at measuring the solar parallax – checking the distance between Earth and the Sun. I hope to enjoy the view though.

    1. Bob KingBob King Post author

      Hi Rod,
      The 90mm will be perfect. That’s almost the same size instrument I’ll be using. And like you, partly sunny skies are in the forecast. Good luck!

      1. Ben-Williams

        Enjoyed the transit from start to finish from central North Carolina. Mercury was easily visible in 7×50 Antares finderscope with Baader solar film. Superb views through 90mm Meade ETX at 56x where whole solar disk was visible, sharper than Meade 7″ mak which I found was more affected by the seeing.

    2. luigim

      Bob, currently I’m enjoying the transit in Southeastern PA. I was able (when the seeing and high clouds allowed) to see Mercury with my 6x Lunt solar binocular.

      Regards,
      Louis Marchesi
      New London Twp, PA

  2. Zubenelgenubi 61

    Not that it really matters, but I think the 2049 transit is on May 7 and not November 7. The chances of my being around and in good enough shape to see it then are not good.

  3. RodRod

    Okay folks, at the early part of Bob King’s report it says “…this year’s transit the last visible until May 7, 2049”. This is correct. I checked Starry Night Pro Plus 8, it shows the Mercury transit starting near 0557 EST, 07-May-2049 for my location in Maryland with the Sun and Mercury in Aries. However, I will be 30 years older – I am already an old man 🙂 I did check Stellarium 0.19.2, did not get this result or MS WWT application for 07-May-2049. For my location in Maryland, the forecast now is *mostly sunny* day for the Mercury transit. I will wait until Saturday and Sunday before I place high confidence in this nice looking forecast for viewing the Mercury transit at my location 🙂 Perhaps I may witness the entire event here.

    1. RodRod

      Okay Bob et al. Stellarium 0.19.2 does show the Mercury transit for 07-May-2049, correctly. I needed to run the phenomena calculation for Mercury and solar system. In May 2049, it will be EDT too. I get confused at times over EDT vs. EST at my age now 🙂

  4. jessicasunmoon

    For those of you doing outreach, you can register your event to be shown on the Astronomers Without Borders World Map and you’ll automatically be entered to win a fabulous Globe of Mercury, courtesy of Sky&Tel. You won’t get it in time for the Transit, but it’ll still be cool to have a Mercury Globe.

    https://astronomerswithoutborders.org/awb-programs/community-based-programs/mercury-transit-2019.html

    wishing you clear skies,
    Jessica

  5. Irukandji

    I understand that Mercury is currently in “retrograde”. What effect will that have, if any, on the appearance of the transit? Will it shorten or lengthen the duration of the transit?

    Jeff

    1. Corey RueckheimCorey Rueckheim

      Jeff, in order to say that Mercury’s retrograde motion shortens or lengthens the duration of the transit implies comparison to the duration of a transit when Mercury is NOT in retrograde motion. However, this is impossible because due to the nature of a transit, Mercury is ALWAYS in retrograde during a transit.

    2. Bob KingBob King Post author

      Hi Irukandji,
      No, it does not affect the duration since Mercury is always in retrograde when passing in front of the sun during inferior conjunction as noted by Corey (below). It can only move one direction during inferior conjunction — from celestial east to west — since it is transitioning from the evening sky to the morning sky. Venus does the same.

  6. RodRod

    Bob King et al. Rod in Maryland is enjoying the show. I will be viewing throughout much of the day until egress at 1304 or so EST. At 0713 EST, the Sun rose above a tree line in the farm field I was viewing from. I am using my 90-mm refractor with 25-mm plossl and 14-mm eyepieces for 40x and 71x views, true field of views, 1.3 and 1.0 degrees. Morning temps near 5C with dew and light fog in the fields. Scattered cirrus clouds too but sunny day. At 0737 EST, I could distinctly see Mercury’s black disk on the limb of Sol, I think this was 2nd contact at ingress. I am going back out now to continue observing with my white light solar filter and eyepieces. Mercury is moving across Sol. Much fun, hope others get a view too. My telescope uses an alt-azimuth tripod with slow motion control cables.

  7. Fabrice MoratFabrice Morat

    Hello Bob,

    Good conditions weather for the moment at La Palma island. No drop effect for me, the diurnal seeing was bad even at 45x with Miyauchi 45×141. So, i used my small finder, a special hunting zoom refractor, austrian optics : 1.1-4×24. Monocular and small diameter (24mm) for a “minuscule” transit ! Coupled with Astrosolar, this quality zoom option allowed me to try the detection of the tiny disc on the phosphere. I began at max, i.e. 4x with my director eye. Not easy, when Mercury was more detached from the edge, i ‘ve succeeded several times and some dustes on the focal plane helped me to keep my eye focused. It was funny to compare some larges dustes with the planet ! Then, i tried 3.5x, even more difficult and only in rare moments. At 3x, never seen. For me, the play stops at 3.5x with one eye. I think Roger Sinnott had better chance detection because he used the 2 eyes (small binoculars) but his performance is really excellent. Fabrice M.
    Nota : Perhaps my long black tube with perfect baffling was an help.

  8. RodRod

    Bob King et al. Rod is back in now. I enjoyed the entire transit today. Cirrus clouds were somewhat of a problem but by 1200 EST, most cleared out leaving large areas of blue sky. The last 60 minutes of the transit, some very good views using my 25-mm plossl and 14-mm eyepiece. At 71x and at 1302 EST, Mercury black disk near or on the solar limb, preparing to depart. By 1304 EST or a bit after, I could no longer see Mercury’s disk. My times are flip, cell phone time checks. No high tech for me. Two immature eagles were flying around in the morning too over the fields, they apparently have not departed the nesting grounds yet for winter or may winter over here in Maryland. My stargazing log is updated – a celestial event I will not forget (my first Mercury transit observations). Next Mercury transit at my location will be 07-May, 2049 🙂

    1. Scott

      Glad you enjoyed the transit. It was really cloudy today in the UK but I managed to catch first and second contact at my telescope eyepiece at x91 in my 4.5 inch Dobsonian. I later took an image to show friends and family and posted on Facebook. Thanks to Bob for the article, and especially the link to the YouTube video – my 5 year old son took that into school and his class watched some of the action after lunch as they are studying space at the moment!

    2. Bob KingBob King Post author

      Woo-hoo, Rod! You had fun. I had to travel so I set up at two gas stations along the way, took photos and shared the view with curious onlookers. My favorite view was near mid-transit with Mercury near the center of the solar disk and so overwhelmed by the sun.

  9. AB

    >sigh< Like every other major astronomical event in the last 15-20 years or so in this foggy corner of Canada, it was a battle with cloud 🙁 (That June day I drove around for ages trying to find a hole in the fog, for a 30-second glimpse of Venus…)
    I spent more time trying to find the sun through the solar filter, than actually able to see it. BUT, I DID get to see a few glimpses through the thinner cloud, so I have now seen Mercury for the first time 🙂 I love events like these that give you a sense of the clockwork motions of the cosmos 🙂
    I enjoy all the observing news and updates!

  10. Joe StieberJoe Stieber

    I was lucky for a change and saw my first November transit of Mercury. Previous attempts in November had been foiled by inclement weather. While transparency wasn’t ideal, it was still mostly cloud free so I was able to see the whole thing, including first and last contacts. On top of that, temperatures were quite pleasant, reaching >60°F. The next day, a cold front moved in, we had a brief snow shower and temperatures plunged.

    I wrote a report, which I posted at…

    http://wasociety.us/SJAstro/Documents/Report_11-Nov-2019.pdf

    1. RodRod

      Joe Stieber, I read your report. Very good. I viewed too in Maryland but had cirrus – I could see through the cirrus and later, mostly clear skies. A great Mercury transit 🙂

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