Transit of Mercury

Do you remember the excitement we amateur astronomers experienced back in June 2004 when the planet Venus passed between the Earth and the Sun? That transit was indeed a rare event that we were privileged to observe. Well, on November 8, here in New England we will be able to watch Mercury perform the same feat. Though not as rare (last one occurred on May 7, 2003 and the next will occur on May 9, 2016) as a Venus transit, Mercury’s passage will be much more difficult to observe for the casual stargazer.

When the Venus transit was last visible in Rhode Island, the event was already in progress as the Sun rose. Filtered through our dense atmosphere and thick clouds that morning of June 8, Venus, a planet nearly the Earth’s twin in size and our nearest neighbor, was easily seen with a careful gaze at the Sun.

Well, Mercury is much smaller than Venus, and much closer to the Sun. Therefore it cannot be seen without binoculars or a telescope, and most importantly, those optics must be equipped with special solar filters. The image of Mercury will be quite small and darker and rounder than any sunspot has ever been. Plus, Mercury’s silhouette will not have a lighter penumbra surrounding it as do most sunspots.

Though we will not see the event in its entirety, we will have plenty of time to enjoy a portion of Mercury’s passage across the face of the Sun. The transit begins at 2:12 pm when tiny Mercury will “contact” the southeastern edge of the Sun (the edge of the Sun closest to the western horizon). It will take two minutes for Mercury to emerge fully onto the solar disk.

Just before it does so, a portion of the planet will seem to elongate outward toward the blackness of space along the Sun’s limb. It will look like an drip about to detach itself from a faucet, or like the shape of a teardrop. The effect can last for several seconds, depending upon atmospheric conditions. Many members of Skyscrapers observed this “black drop” during Venus’ transit in 2004. High magnification will be necessary to see this effect because of Mercury’s small size.

Mercury will continue to move across the face of the Sun (see the following web page for the path Mercury will take: http://sunearth.gsfc.nasa.gov/eclipse/OH/image1/TM2006Nov08-Fig1.GIF - your view may differ, depending upon what type of binocular or telescope you use), and at 4:41 pm the tiny dark spot will be halfway between the Sun’s southeastern and western edges. Unfortunately we will be unable to see this maximum, for the Sun sets around 4:30 pm locally. Observers further west will be fortunate to view a longer duration of the event. The west coast states will see the transit from start to finish.

Now, several words of caution are necessary to state here. Do not attempt to observe this transit unless you are an experienced solar observer. Mercury is so tiny that you won’t be able to detect it with the unaided eye anyway, so don’t be tempted to try. Number 14 welders glass will not show Mercury. DO NOT use exposed film of any kind. This method is not safe under any circumstances. In past columns I have instructed folks on how to build a solar eclipse viewer using a shoe box. This observing method also won’t work in this circumstance. The projected solar disk is so tiny that Mercury’s silhouette won’t be detectable.

If you have never observed the Sun before this event, don’t start now! Don’t risk your eyesight due to an oversight or an outright mistake. Even if you have one of those department store refractors that often come with small glass or plastic filters, do not be tempted to use them. They have been known to shatter when exposed to the Sun’s concentrated image. Many years ago, when I first started out in astronomy, I had one of those glass/plastic filters shatter during a partial solar eclipse. Luckily I wasn’t looking through the eyepiece at the time.

If you use the Sun projection method (using a telescope to project the Sun’s image on a white screen), remember to be very cautious if other folks, especially children are nearby. You don’t want anyone accidentally stepping up to an unguarded eyepiece to take a look. And regarding eyepieces, do not use cemented eyepieces. Use only those that are air-spaced. Eyepieces have been ruined when the cement has melted due to the concentrated light collected by a telescope.

Also, remember to block off your finder scope. I have seen observers singe their hair or clothes by failing to do so!!

(A quick note: Currently there are no plans to open Seagrave Observatory to the public for Mercury’s transit. However, should Skyscrapers decide to offer viewing of this event at the observatory or elsewhere, please look to the local media for details. Also, check with other local astronomical facilities to ascertain whether they will be scheduling any public viewing programs.)

I’m sure there will be many web sites providing live images of this event. I found this one in Hawaii that plans on providing near real-time images and time-lapse movies of the entire transit: http://www.astroday.net/MercTransit06.html.San Francisco’s Exploratorium (http://www.exploratorium.edu/)will also be offering a live webcast of the event from Kitt Peak in Arizona. Be sure to surf the web ahead of time to see which other web pages plan to feature this great event just in case these sites prove difficult to access.

I agree that watching online sites is not the same as experiencing the transit firsthand, but if you can’t observe it safely, don’t observe it at all. If all else fails, and I mean the weather, you may have no choice but to pull up a chair in front of your computer screen and watch the progress of the event.

Good luck in observing this interesting astronomical phenomenon, and remember to keep your eyes safe.