Transit of Mercury: An Infrequent Astronomical Event

May 2016  :  Dave Huestis

People have a fascination with records. No, I’m not talking about audiophiles and music on vinyl. I’m referring to statistics on practically everything in the universe. We keep copious records on sports achievements and precipitation events just to name a couple. And when it comes to the world of astronomy, many folks get excited about events that don’t happen every day, like lunar and solar eclipses, conjunctions of planets, and even record close encounters with Mars. Some of these events occur more frequently, but short sound bites in the media often infer their rarity.

A good example of a rare astronomical event was the pair of Venus transits in 2004 and 2012 (they occur in sets of two, eight years apart). Since Venus orbits between the Earth and the Sun it can be seen to pass directly in front of the solar disk and transit across the face of our star. This very unique astronomical phenomenon is so rare that in 2004 no one then alive had observed one. The immediate Venus transit predecessors occurred back in 1884 and 1882). And the next pair is not until 2117 and 2125! For a human lifespan, that is a rare event.

However, another planet can transit the Sun—Mercury. Though not as rare as Venus transits, transits of Mercury occur 12-13 times per century. The last one occurred on November 8, 2006, but unfortunately here in Southern New England we were clouded out. Luckily their increased frequency provides our next opportunity on Monday, May 9, when, weather permitting, we will be able to observe this fascinating event from start to finish.

Why don’t we experience a transit of Venus or Mercury every time they pass between the Earth and the Sun (called inferior conjunction)? It all has to do with the orbits of these planets and our ever changing viewing angle. Most of the time they pass above or below the solar disk as seen from the Earth. This concept is simply stated here, but it took the greatest astronomical minds of the past to solve this great mystery. The process took much observation, dedication and deduction to organize the solar system design and celestial mechanics we have all come to know.

Before I provide the local observing circumstances along with some guidance on how to experience this transit safely, I want to briefly review the historical record to understand why transits were once so important that expeditions were sent around the world to observe them.

In 1716, in trying to determine the distance to the Sun (called the astronomical unit - AU), Edmund Halley (1656-1742) suggested using transit timings from different locations on the Earth and applying simple geometry to determine that distance. Expeditions were sent around the globe to position observers to make precise measurements and timings to calculate the solar parallax. (This measurement is an apparent shift in the position of Venus’ transit across the disk of the Sun due to its being observed from different locations on the Earth’s surface.) Timing of the event was crucial, as was knowing one’s position to a great degree of accuracy. Unfortunately bad weather plagued many an expedition (some things never change). Also, a phenomenon called the “black drop” effect made getting precise timings nearly impossible.

When Venus appears to touch the limb (edge) of the solar disk, this event is called first contact. It will appear as a little notch or bite has been taken out of the Sun. As the planet moves farther onto the disk, just before Venus is fully in front of the Sun (called second contact), the “black drop” forms. It looks like a drip about to detach itself from a faucet, or like the shape of a teardrop, as a piece of the planet seems to elongate outward toward the blackness of space along the Sun’s limb. If an observer was situated in a location to experience the beginning and the end of the transit, you had two opportunities to conduct timings. For just before third contact when Venus would begin to exit the solar disk, one could conduct a second timing. The “black drop” affected those timings as well.

The “black drop” effect can last for several seconds, depending upon atmospheric conditions, thereby preventing astronomers from obtaining precise timings of the beginning (ingress) and ending (egress) of a transit. Observations differed greatly, thereby throwing calculations off by millions of miles. Using Mercury transits for timings was even more difficult to accomplish, since Mercury’s disk is about six times smaller than Venus. This means more magnification is required to time the event accurately, but increased magnification also increases distortion caused by the Earth’s atmosphere. See this website for an example of and explanation for the “black drop” effect.

Unfortunately, for all intents and purposes, the use of transits to determine the scale of the solar system proved fruitless. The expeditions to faraway lands did provide valuable scientific discoveries in other disciplines, not to mention the exploration of our world. For example, if you want to follow up on just one of these expeditions, read about Captain Cook’s voyage and his involvement with the transit of 1769.

During the transits of 1874 and 1882, photography was the new method of acquiring data to determine the solar parallax and to make other discoveries. However, simpler methods had already revised the value for the AU to unparalleled accuracy, and although scientific expeditions were still funded for the purpose of research, very little new information was forthcoming. The one thing that did arise from the 1882 transit was an increased interest and excitement by the general public.

The same is true today. No new scientific knowledge is expected from observing Venus or Mercury transits. Despite their historical significance, transits have become mere curiosities for the average citizen. Regardless, amateur and professional astronomers have been eagerly preparing to observe the May 9 Mercury transit with properly filtered telescopes. It’s a unique event to experience when you know exactly what is occurring.

Before I provide the times for specific key transit moments, I must express several very important words of caution. Do not attempt to observe this event unless you are an experienced solar observer. Mercury is so tiny that you won’t be able to detect it in transit with the naked eye anyway, so don’t be tempted to try. Number 14 welders glass will not show Mercury either. DO NOT use exposed film of any kind. This method is not safe under any circumstance. 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 because the projected solar disk is so tiny that Mercury’s even smaller 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. Experienced astronomers use special filters that prevent more than 99.99% of the light from even entering the telescope. That includes the dangerous infrared wavelength as well.

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

Our location on the Earth’s surface will allow us to observe this transit in its entirety. From start to finish the transit will last just under seven and a half hours. That’s a long duration event to allow interested individuals to be able to view even a few minutes of Mercury’s passage across the solar disk.

Please note that all times with this article are provided in Eastern Daylight Time and have been specifically calculated for Providence. (Times do vary slightly by geographic location, so if you are going to be outside of the Southern New England area, you may want to check online for specifics.)

Locally the transit begins bright and early at 7:13:32 a.m. with the Sun about 17 degrees above the horizon. This is the moment of 1st contact when tiny silhouette of Mercury will begin to appear along the lower left (east) edge of the Sun approximately at the eight o’clock position. Because the Sun arcs across the sky depending upon geographic location, and because we live on the surface of a sphere, the beginning and ending positions will differ greatly from the accompanying graphic. It’s all a matter of perspective. It will take two minutes for Mercury to emerge fully onto the solar disk.

Just before it does so, a keen eyed observer should notice the “black drop” effect. Many members of Skyscrapers observed this “black drop” during Venus’ transit in 2004. High magnification will be necessary to see this effect due to Mercury’s small size. When Mercury is seen fully in front of the solar disk is the time of 2nd contact at 7:16:44 a.m. The image of Mercury will be quite small and much darker and rounder than any sunspot.

Mercury’s motion will continue to carry it across the face of the Sun from east (left) to west (right). The mid-transit point will occur at 10:57:50 a.m. with the Sun 57 degrees above the southeast horizon. At 2:38:08 p.m. Mercury will reach the right edge of the Sun. This is 3rd contact. Just prior to this time an observer will once again have another opportunity to observe the “back drop” effect. Then at 2:41:19 p.m. Mercury will exit the solar disk completely. This event is called 4th contact. The Sun will then be about 55 degrees above the southwest horizon.

If you are not an experienced solar observer and wish to experience this transit, you may be able to do so at some of the local observatories. At the time of this writing during early to mid-April to meet deadlines, only Frosty Drew Observatory at Ninigret Park in Charlestown has an observing program scheduled. If and when any of the others offer Mercury transit observing opportunities, I will email the media with the details.

And heavens forbid the skies are cloudy here on May 9. I’m sure there will be many websites streaming the event live. I agree that watching online sites is not the same as experiencing the transit firsthand, but if the weather doesn’t cooperate, you may have no choice but to pull up a chair in front of your computer screen and watch the progress of the event. This course of action is also an option if you can’t observe it safely yourself or can’t travel to an organized observing program. Should the opportunity pass you by for any reason, you won’t have to wait long for the next transit of Mercury, which we will see here in its entirety as well on November 11, 2019.

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

And, just in case you’re wondering what the current value of the astronomical unit is, it’s 92,955,807.3 miles, plus or minus about 10 feet!