On 9th May 2016, a transit of Mercury across the Sun was expected to take place. It had been quite a while (thirteen years) since the previous one, which took place back in May 2003. Therefore, it was one neat chance to observe this event which, though less spectacular than other types of phenomena, is anyway quite interesting. Furthermore, the observing conditions were quite favorable for Italy and the whole of Europe (first contact at 1:12 pm local time, last contact around sunset, at 8:40 pm local time).
The transit of 2003 had been perfectly visible, since it had taken place in the beginning of a summer season which later went on to be one of the hottest and longest ones in decades. Unfortunately, the situation was completely different for this year, since the month of May started with very changeable and unreliable weather. The forecast for the 9th of May left no hope for Northern Italy and for most of Central and Southern Europe.
However, this was not enough to discourage my good friend and fellow amateur astronomer Lorenzo Comolli and myself. We were willing to try our luck and travel some hundreds km away to observe the event. Over the previous days we frantically consulted all the weather forecast and models available, and the day before (Sunday 8th May), some models were indicating a chance of good weather in Tuscany, on the coast south of Livorno (Leghorn).
Therefore, in the wee hours of 9th May we set out for our observing trip. While Lorenzo left from his hometown (Tradate), I was already in Liguria, so I was a good 45 minutes ahead of him. The weather was cloudy all along the route: we were keeping in constant touch over the phone. At some point I suggested we stop somewhere around Piombino (LI). Since I got there first around 11:30, I found a suitable observing location at a meadow next to an olive tree grove. Of course, I asked the owners’ permission, and fortunately they were very nice and kind: not only did they allow us to set up there, but also showed some interest in our activity and the transit itself.
Lorenzo got at the chosen spot shortly thereafter. It was still partly cloudy when we were setting up, but – as predicted by the models – it cleared up by the time we were ready, just minutes before the transit! So we were treated with good visibility and we managed to image the first contact in its entirety, the only problem being a strong breeze which caused our telescopes to wobble and increased turbulence.
Since for practical reasons I could not bring along my h-alpha telescope (a modified Coronado PST), I just had to make do with white-light imaging, first in high-res during the transit’s initial phases, and then in a wider field for the rest of the phenomenon. In particular, here’s my setup:
A TS 130/900 APO refractor + a Thousand Oaks full-aperture solar filter, sitting on a Losmandy G11 mount
A PGR Chameleon camera at the prime focus for high-res work
A Canon 500D DSLR shooting multiple frame bursts every 5 minutes.
Unfortunately, the sky stayed clear only for a couple of hours. Later, some cirrus clouds started rolling in, and they gradually became thicker until they blocked the Sun almost completely. Lorenzo left around 5 pm, I stayed about one hour longer, but my last frames were so poor quality I had to throw them away.
Cloud cover forecast map of Europe showing the poor prospects on most of Italy and Central and Southern Europe. However, a break in the cloud is expected along the shores of the Tyrrhenian, which is where we managed to observe the transit from (Courtesy GFS)
Second contact. PGR Chameleon camera at the prime focus of my 130/900 refractor. 10-s snippet of a longer movie imaged with Firecapture. 90 frames out of 180 were added and processed in Autostakkert, Registax and Photoshop.
Mercury (transit on the Sun)
Solar disk at the prime focus. 12-frame bursts taken every 5 minutes from 1:49 pm to 5:10 pm CEST. The best 6-7 frames of each set were aligned and stacked. Exposure times ranging from 1/160 s to 1/40 s. Processed in Autostakkert and Photoshop. The blurring on the sunspots clearly shows the Sun’s rotation even within a few hours.
Mercury (transit on the Sun)
Time-lapse movie containing all the images. Viewing in full-screen mode and with earphones/loudspeakers is advisable.
I do have two regrets. First, for not shooting wide field from the very beginning, as my hi-res sequence was ruined by bad seeing. Second, and even more important, for not using my h-alpha telescope. Anyway in the end we did manage to observe a good half of the transit, which is not bad considering the situation.
The next transit of Mercury will take place on 11th November 2019. The observing conditions won’t be so favorable then, but nevertheless, let’s hope the weather will cooperate!