Amateurs around the world await the little black dot that will cross the Sun.
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'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.
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.
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 1⁄194 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.
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.
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.
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."
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.
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.