My interest was aroused, while driving near my home on May 26, by a naked-eye view of Mercury in the west-northwest as twilight faded; and another view from home turf on the 28th indicated that telescopic observation would be possible from there. I had last seen the planet sunlit at the CFAS Astrofest in early April 1997 with my 3.25-inch refractor, which at 90X showed only the half-sunlit shape. My 8-inch reflector, with a motor drive and providing up to 400X, was available for the May-June 2000 apparition, and I was curious about what it would show.
 

About fifteen minutes after true sunset on May 30 I spotted Mercury in binoculars and a few minutes later located it in the viewfinder of the 8-inch. In the 'scope at 200X it was a small gibbous disc, dazzling on a bright sky background. The "seeing" was lousy, hopeless for seeing any surface detail except slight shading toward the terminator. But I made a pencil sketch and later a colored copy with Claris Paint on my computer (see Figure 1). Two days later the planet's phase was down to almost half, and better seeing permitted observation with 300X which revealed definite terminator shading. Somewhat reduced surface brightness helped too. Alas, for my next view, on June 6, the seeing was again lousy and I could barely distinguish the fat crescent shape well enough for accurate drawing. On the 9th the seeing was even worse and I didn't attempt a drawing. In the evening of June 10 I attempted observation with the 14-inch Schmidt-Cassegrain telescope at Bethune-Cookman College, but was too late. When the planet was acquired the 'scope's field of view was half obstructed by the bottom edge of the dome slit, and the image was so bad that the relatively large crescent, 8.5 arc-seconds from cusp to cusp, was not reliably seen. Thereafter, cloudy evening skies prevented any more observations.

Although my telescopic observations during the May-June apparition were technically disappointing, I enjoyed the reunion with an old friend that was better understood than when we used to meet regularly. From the photographs of Mercury obtained by the Mariner 10 spacecraft during a flyby in 1974, scanned electronically and sent to Earth by radio, we know the surface is similar to our Moon's (see Note 1 and Fig. 2). Impact craters, rilles and scarps near the sunlit limb cast no visible shadows whereas those near the terminator cast long ones, greatly reducing the average surface brightness there. Hence the disc shading toward the terminator that I saw without resolving a topographic feature. There are also volcanic plains like the Moon's, but their albedo is higher so they only weakly modify the shadow effect.

However, the large, low-contrast spots and strips seen by many visual observers since the 19th Century, and even occasionally photographed, probably were such plains ringed by heavily cratered areas. The 800 mile-diameter Caloris Planitia (or Basin), centered at longitude 190°, latitude 30°N, is such a feature. Unfortunately, Mercury's visible longitudes for my May-June 2000 views were outside the range photographed by Mariner 10, so the contributions of shadows and volcanic plains to my barely seen terminator shadings is indeterminate.

The last time I observed Mercury telescopically more than once or twice in a single apparition was long ago: March 1958, when I lived in Kalamazoo, Michigan, and had a 6-inch f/11 reflector. Previously I had viewed the planet for five years almost exclusively with a 3.5-inch Skyscope reflector, mostly during morning apparitions when it could be followed till sunrise or later. The small 'scope provided a maximum magnification of only 190, but about 10% of my views showed coarse surface details. Mercury had displaced Jupiter as my third favorite planet, after Mars and Saturn. Why? Because seeing any surface detail with a small telescope was a huge challenge, and indirect evidence for a Moonlike surface offered the prospect of relating markings that were seen to real detail when future observation at higher resolution would reveal it (Note 2). Intensive observation of other planets, especially Mars during its favorable opposition year 1954, was great practice for Mercury.
 

In early 1956, after three years in development, the 6-inch reflector was fully functional, as described in my Starlight Memories ,5. But its mounting was fixed on a 3-inch pipe-pier in my backyard that didn't allow predawn viewing of Mercury, and I never finished a satisfactory portable one, so two years passed before the planet appeared in the western evening sky where this 'scope could reach it and the weather also cooperated.
 

The March '58 apparition was ushered in by a close conjunction, on the 21st, of the planet with a 2%-illuminated Moon that could only be seen in a telescope due to thin cirrus clouds. In the 6-inch at 54 and 100X the gibbous Mercury, appearing yellowish-orange, and deep yellow 'sliver' of Moon, was the most spectacular such phenomenon I had yet witnessed. On the 22nd the low western sky was clearer, but poor seeing limited the telescope to 200X which only showed that the planet's phase had diminished slightly. On the 25th I finally had a view that beat any with the Skyscope. At 200 and 300X, two gray markings were seen as well as disc brightness falloff from the sunlit limb to the terminator ( Fig. 3). Telescopic observation good enough for drawings was made in two more evenings. Little did I suspect that 42 years would pass before I would observe the little planet as often with a telescope in a single apparition.

Sometime in the 1960's I learned that only 1% of astronomers had ever seen and recognized Mercury with the naked eye, fewer still with a telescope. Since planetology is a larger discipline today, a larger fraction of professionals probably have seen the planet; but I doubt if amateurs are any more familiar, because I have never seen a photograph or CCD image from that community, which has produced thousands of Mars, some clearly showing detail when the angular diameter was in Mercury's range.