Mercury Skims the Sun: The View from Florida

by Philip Steffey

 

 

At our October 1999 meeting I mentioned that a transit of the Sun by the planet Mercury would be visible here in the late afternoon of November 15 but would be a challenge to see well. The occurrence at altitudes under 15° dictated above-average clear sky transparency and the small angular distances the planet would be inside the Sun's north limb made good "seeing" essential too. The observing site had to have a low horizon in the Sun's direction, West-Southwest, and the Sun's reduced brightness might spoil views with small telescopes using eyepiece projection, for a large solar image would be needed to show the tiny (10 arc-second diameter) planet well. With direct viewing preferable and the transit lasting less than an hour, an observing group would need one telescope for three or at most four people. A current shortage of active and available observers among our members made the event unsuitable for a public showing.
 

The response from the attending club members was blank stares and clenched lips. Most didn't know what I was talking about. They were only slightly more interested in the Leonid meteor shower due in November. At the monthly meeting on Nov. 4 I announced my intention to observe the transit, weather permitting, probably from our dark viewing site some 20 miles northwest of Daytona Beach's old business district. This site, in an unfinished section of the Seminole Woods housing development in southern Flagler County, would provide a sufficiently low southwestern horizon and probably privacy as well. I hoped for company, but continued noninterest by most other CFAS members left me pessimistic. One member, Nancy Thomas, planned to observe the transit alone from a Daytona Beach site.

As it happened, a spell of mostly clear weather ushered in by Hurricane Irene in early October lasted through mid-November. (Go figure!) On the transit day a weak cold front moved in from Georgia, producing a few thin, low clouds that fortunately dissipated or drifted southward by mid-afternoon. By then, CFAS member Matt Allard had offered to join me to view the event and had telephoned some unaware members about it with uncertain results. The two of us drove to Seminole Woods together, equipped with my 3.25-inch f/6 refracting telescope and expecting to be alone or nearly so.
 

Arriving at the site shortly before 4 P.M., none too soon for an event due to start near 4:15 (EST), we were pleasantly surprised to see two other members, Tony Viviano and Darrel Carnell. Tony had his fine 8-inch f/6 Dobsonian reflector set up and already was viewing the Sun with a full-aperture filter. He remarked that there were some great sunspots.
I quickly put my refractor on its equatorial mount, installed a two-foot square heavy cardboard screen to block open sunlight, then my full-aperture filter in front of the objective lens. The Sun was soon located in a 50X eyepiece field and looked great. The sky transparency was fine and the seeing was good too. The Sun's low altitude proved a blessing in allowing straight-through viewing with the 'scope, for I detest having to use a zenith diagonal mirror or prism. A light, breezy wind brought by the weather front helped produce the fine sky conditions, so we didn't mind the occasional telescope jiggling it also produced. I switched to an 80X compound eyepiece--a 17mm Orthoscopic plus a 2.8X Klee Barlow. Roughly paralleling the Sun's equator, its spots including two magnificent groups directed my gaze to its north (celestial) limb.
 

At 4:12 P.M., almost catching me off guard, a slight "notch" appeared in that limb, and by 4:15 it was conspicuous. In the next five minutes the disc of Mercury became distinct, exhibiting only a weak "black drop" effect. By 4:25 the disc was cleanly inside the solar limb. We four observers took turns viewing in both telescopes. Tony drew the event as seen in his 'scope (see reproduction). Its superior resolution made the planet a constantly circular disc whereas my 'scope occasionally showed an ellipse or pointed disc approaching a square, presumably optical and atmospheric effects.
 

At 4:41, geocentric mid-event, Mercury's north limb appeared a full diameter inside the Sun's, more than expected, partly due to a relative parallax effect (Sun: 9 arcsec horizontal, Mercury: 12 arcsec), but perhaps also to irradiation at the solar limb. The tiny black disc remained well defined for another 10 minutes, its distance from the Sun's limb now shrinking, but by 5 P.M. the seeing was poor and "boiling" at the Sun's limb obscured the planet. Distant low trees also were interfering. Tony's last position record was at 5:01 and I last saw the planet as an indistinct blotch on the solar limb at 5:05.
 

We were very lucky to see most of the transit well. November '98's weather here had been cloudy and humid, the skies murky even when clear. It was the first such event seen by Matt and Darrel, I'm uncertain about Tony, not an ideal one but worth having observed due to rare visibility from one geographic location. It was my third but first since November 1960, when as a graduate student in Tucson, Arizona I assisted Gerard Kuiper in measuring Mercury's diameter with a Lyot double-image micrometer and timed the egress contacts as well. Our main regrets are that we have no photos of ourselves and our telescopes in use, and that more local astronomers weren't present. Thanks to Tony for providing a copy of his drawing, to which I have added key data, and to Matt for critiquing my text and helping put the report together. We have no information about Nancy Thomas' observations at this time.