A wonderful daytime celestial spectacle will occur on the morning of November 11th. when the planet Mercury will cross the face of the Sun—an event that can be seen with properly filtered binoculars and small telescopes (and possibly without them by keen-eyed observers as well). Transits of Mercury are infrequent events, so this month’s show is not to be missed if skies are clear!
What exactly is a “transit”? It’s when one of the innermost planets crosses in front of the Sun as seen from here on the Earth. Being the only two planets inward of us, just Mercury and Venus can transit it. Venus, being both larger and closer to us than Mercury, can actually be seen without optical aid (using proper safety precautions such as “Sun Shades” described below). Catching Mercury on the other hand with the unaided eye is a supreme test of visual acuity. Binoculars will definitely show the planet while even the smallest of telescopes will give a much better view. Again however, safety precautions are absolutely essential using any optical device when viewing the Sun (see below)!
What are the times of this event? “First contact” occurs after sunrise on the early morning of the 11th when the leading (right) edge of Mercury first touches the left (southeastern) limb of the Sun’s disk and is predicted for 7:35 a.m. EST. 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 10:20 a.m., at which time this swiftly moving planet will be halfway across the Sun. The transit continues until early afternoon, finally ending at 1:05 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 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 as round as the planet itself will appear. The Sun currently is at its sunspot minimum and will likely be spot free as it has been for much of this year.
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 “Sun Shades” similar in construction (only!) to the 3-D glasses being used in theaters, available from www.scientificsonline.com. These are ideal for looking at the Sun anytime to check its surface for any occasional large sunspots. However, in the case of binoculars and telescopes, proper solar filters are an absolute “must” to avoid serious eye damage or even 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 may 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 to eyes (and clothes)!
— 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.