Mercury Transit

Battling Clouds

Mercury lines up in front of the Sun every 115.9 days as seen from Earth. During these alignments Mercury is usually above or below the Sun, but, sometimes, Mercury transits directly across the face of the Sun as it did on May 9th. There are 13 or 14 Mercury transits per century compared to only two for Venus. I've been lucky enough to see both Venus transits in 2004 and 2012, but I'm running out of time for transit observing. There will be no Venus transits for the remainder of my life. Although another Mercury transit will happen in 2019, I could be taking a dirt nap before the next transit occurs in 2032! So I was motivated to make extra effort to observe the May 9th transit.

One day before, on May 8th, I checked my equipment to see if everything was working properly. After confirming all was well, in spite of many clouds, I hastily captured one image of sunspot 2542 with nearby filaments. There was no time to take a flat field, so there are subtle vertical bands in this image. The Sun was finally showing remarkable surface features after many bland months. It would have been nice to take more images, but increasing clouds prevented this.

I was excited about transit day because, for once, my observing site was a favored location. The entire transit would be visible for 7.5 hours from 7:12 am to 2:42 pm EDT. All I needed was a clear day! Unfortunately, Williamsburg was in the midst of a long stretch of cloudy rainy weather, and May 9th was destined to be mostly cloudy. Nevertheless, I rose at 5:00 am, set up my equipment at dawn, and was ready to take advantage of any small bit of clear sky by 7:00 am. Temperature near dawn was a chilly 53 degrees, but rose rapidly in the next few hours.

The Sun broke through a cloud gap as the transit began. I hoped to capture Mercury's black disc crossing either a prominence or spicules on the Sun's eastern limb. Hopes were dashed because seeing was absolutely horrible with the Sun close to the horizon barely an hour after sunrise. I could clearly see Mercury crossing spicules very near a small prominence, but the solar limb was swimming around in a blurry mess. The first image below shows Mercury just crossing the limb at 7:15 am. The dreadful lack of detail in this image is caused by extremely unsteady air.

More than an hour later the Sun had risen higher and seeing improved. I patiently waited for an opening in the clouds and captured the next image through haze at 8:41 am. The small prominence in the upper left is where Mercury earlier entered the solar limb. It would have been great to capture Mercury's black disc silhouetted against this prominence, but, as I mentioned before, roiling air made this impossible.

The angular diameter of Mercury's disc was only 12 arc seconds, about one fifth the size of Venus' disc when Venus transits. Twelve arc seconds is 1/300th of a degree! Thirty minutes later I captured one last transit image at 9:21 am showing another prominence lower down on the rim.

Seeing conditions improved as time went on, but clouds thickened. I managed to capture two more images through thin clouds to make the following 2-panel mosaic of sunspot 2542 near the transit action.

Soon the temperature had risen to 77 degrees. By 10:00 am the sky was 90 percent cloudy and getting worse. Satellite images showed no prospect of future clearing, so I decided to quit at 11:45 am. Maybe conditions will be better for the next transit in 2019.