Mercury Skims the Sun: The
View from Florida
Central
Florida Astronomical Society
(
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
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
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.
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