Catching the Mercury transit from Seattle

The weather gods smiled on West Seattle Monday and provided relatively clear skies that allowed us to catch much of the first transit of Mercury across the disk of the Sun in ten years. The event served as a reminder of how hyper-local the weather can be, as many other locations around the area did not fare so well.

After about 8:30 a.m. May 9 

the clouds parted and we had 

excellent viewing of the transit of 

Mercury. Spencer (left) and Ryan 

take a peek through Spencer’s 

homemade Dobsonian. 

Photo: Greg Scheiderer.

We thought we may have jinxed the weather when we wrote on Sunday in our weekly calendar post that, “(I)t’s pretty unlikely that we’ll see the transit constantly from sun-up to finish, but also looks pretty unlikely that we’ll get skunked.” I arrived with telescope in hand at Seacrest Marina Park just before 6 a.m. on Monday, and the clouds were solid. I was soon joined by Ryan “Tortuga” Carpenter and a young man named Spencer who brought his home-made Dobsonian telescope to the party. For a while we just watched the clouds roll by.

It was well after 7 a.m. before we got our first glimpse of the Sun, and Mercury in transit—a fleeting look that lasted less than a minute. For the next hour or so we had several similar quick peeks at the transit when the Sun found a hole in the clouds.

We finally got some longer looks after 8 a.m., long enough to actually snap photographs of the transit. Then, right about 8:30, we suddenly had clear, blue skies. We had a few interruptions from clouds after that, but these were brief and we had close to constant viewing of the transit until it ended around 11:40 a.m.

Other areas didn’t have so much luck, especially those sites east of the city. The Seattle Astronomical Society had a transit-viewing event scheduled from one of its preferred observing sites at Snoqualmie Point Park, but had already canceled it by Sunday night because of inclement weather in the forecast. One member went there anyway and reported only brief views of the transit. Others reporting to the society’s Google forum, fittingly titled “Through the Clouds,” also noted limited success from Kent, Ellensburg, and Bellevue. The Green Lake neighborhood had decent weather and observers there reported more lengthy looks at the transit.

If you click on this photo to see the larger version you can see
Mercury just to the left of the center of the disk of the Sun, and
a sunspot cluster to the right. Taken with a Canon
PowerShot A530 through an 8-inch Dob at 48
power. Photo by Greg Scheiderer.

Our trio in West Seattle tried to do a little science, or at least figuring, at the end of the transit. We each clocked the time between third and fourth contact of the transit. Interestingly enough, our observations varied by about 15 seconds. Parallax doesn’t explain that; our telescopes were all set up within about 10 feet of each other! I would guess that the variation could be explained by differences in visual acuity, quality of telescope optics, and ability to find the start/stop button on the smartphone stopwatch. Carpenter did the math and said we were in the ballpark for determining the size of Mercury based on the length of time between the two contacts.

Mostly we just had fun seeing this rare celestial event, and sharing it with quite a number of interested passers-by. I chose the site because a lot of people are typically there, from those catching the West Seattle Water Taxi into the city to those just strolling in the park. Great weather was an unexpected bonus.

While there are only, on average, 13 transits of Mercury in a century, our next one is relatively soon: November 11, 2019. After that we’ll have to wait 13 years, until 2032, for another.