A rare daytime celestial spectacle will occur on the morning of Monday, May 9th, when the planet Mercury will transit the face of the Sun—an event that can be seen with properly filtered binoculars and small telescopes. Transits of Mercury are infrequent events, the last one having been in 2006 and then not again after this one until 2019. So this month’s show is not to be missed if skies are clear!
What exactly is a “transit”? It’s when one solar system body (Mercury in this case) crosses in front of another one (in this case the Sun) as seen from the Earth. Being the only two planets inward of us, only Mercury and Venus can transit it. Transits are not to be confused with “occultations”—during which the Moon, for example, in its perpetual orbital motion eastward around the sky covers up a star, one of the planets, or even the Sun itself during a total solar eclipse, hiding it from our view.
What are the times of this event? “First contact” occurs after sunrise on the early morning of the 9th when the leading (right) edge of Mercury first touches the left (southeastern) limb of the Sun’s disk and is predicted for 7:13 a.m. EDT. It will require about two minutes for Mercury’s tiny disk to fully enter onto the Sun’s. The midpoint of the transit happens at 10:57 a.m., at which time this swiftly moving planet will be halfway across the Sun. The transit continues until early afternoon, finally ending at 2:42 p.m. (Note that all the times given may vary by a minute or two depending on exactly where you are within your time zone.) As Mercury exits the Sun’s southwestern western limb, it moves from the evening sky where it shone prior to the transit into the morning one where it will soon become visible before dawn.
What will the transit look like? Mercury will be a small jet-black dot slowly moving from lower left to lower right across the face of the Sun. Its appearance is often compared to that of a sunspot, but these magnetic storms are not as dark nor round as the planet. And as already mentioned, proper protection to filter the Sun’s intense glare is an absolute necessity while viewing it.. Which brings us to the next very important section.
Viewing precautions. And now we get serious! This month’s celestial spectacular is not to be missed. But you must exercise care in watching it. Looking at the Sun through even the darkest pair of sunglasses will not protect you. They may filter its visible light, but harmful ultraviolet and infrared rays will still get through. One traditional recommendation is using a #14 welder’s filter. Better is an inexpensive and safe pair of “Eclipse Shades” similar in construction (ONLY!) to the 3-D glasses being used in theaters. These are ideal for looking at the Sun anytime and the occasional large sunspot. But to see Mercury itself binoculars are required (unlike transits of Venus where the planet can be viewed directly without optical aid), and telescopes are needed for optimum visibility. However (and here especially), proper filters are an absolute must to avoid serious eye damage or blindness! And the filters must be of the type that fit over the front of a telescope or both binocular lenses and NOT over their eyepieces. Many inexpensive small telescopes (especially imported ones) have an eyepiece filter marked “Sun” on it. These are absolutely deadly and often crack due to the intense heat being focused by the telescope itself. Stopping most of the light and heat of the Sun before it enters an instrument is the only safe and sane way to view it directly. One extensive source for optical solar filters is Thousand Oaks Optical at: http://thousandoaksoptical.com/ (click on the “solar filters” icon). Another safe but not as effective way to view transits is to project the Sun’s image through a pinhole in a piece of cardboard and onto a white sheet of paper. The projected image will be quite small, but depending on the hole size and how far back the screen is placed it should reveal Mercury as a tiny speck. A similar scheme called “eyepiece projection” is sometimes used with telescopes themselves but has been known to crack eyepieces if prolonged too long. And here, as any time a telescope is being used for solar viewing, its finder scope must also be capped since it can also cause damage!
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.