Comet Hyakutake continues to brighten. Tonight at about 0715 UT (on March 21) I was showing the comet to someone from the campus of San Jose (California) State University -- that's downtown in a big city -- in sky of limiting visual magnitude about 3. By reference to beta Leonis and Spica, I estimated a magnitude of about 1.9, and saw two or three degrees of tail, both naked-eye and in a 7x35 binocular. I stopped at a somewhat darker location on the way home (the Page Mill Road exit off US 280, in Palo Alto, California), and in sky of magnitude limit about 5, found the comet a little brighter -- it was higher then, at 0745 UT -- and saw some seven degrees of tail, both naked eye and with the 7x35. In the binocular, the coma appeared slightly darker, or perhaps even slightly notched, on the side away from the tail. This latter effect was subtle.
From my home in the flatlands of Palo Alto, with sky limit perhaps 4.5, at 0910 UT, Spica and the comet were at about the same altitude, and I estimated the comet as about magnitude 1.5, with reference to Spica. That is probably my most reliable estimate for tonight.
Then I hauled out my 90 mm refractor (Vixen fluorite) for a better look. I observed at magnifications of 25, 65 and 202, and it was hard to tell what magnification I was using -- the brightness of the coma varied with radius out from the center in such a way that the appearance was nearly constant -- more magnification spread out the light, but left me looking at a brighter part of the coma, and the two effects just about canceled. The appearance was that of a white disc -- the central part of the coma -- surrounded by a diffuse glowing cloud. There were hints of structure in the more diffuse parts of the coma -- slight variations in brightness, just on the threshold of detectability -- but nothing I could draw or report reliably. The motion of the comet against the background stars was very obvious over a minute or two, at 202 diameters.