(I like to capitalize the kinds of whatever I’m discussing, for clarity, even if it isn’t really correct. So I capitalize names of kinds of sundials.)
Of course the Horizontal Dial meets all but one of those requirements, and that’s a reason for its popularity (along with its ease of building). But it isn’t horizontally-readable. It can only be read from above. That means that it can’t be read if it’s mounted above people’s viewing-position—and sometimes it’s necessary to mount a dial somewhat high, to avoid shading, or to make it visible over an obstruction. Anyway, at whatever level, horizontal viewability makes a dial readable from farther away. *Translucent Two-Sided Flat Dial:* Of course Equatorial Dials are always or nearly always two-sided. But any Flat Dial, in any orientation, could benefit from being two-sided, if construction-conditions permit. And if a Two-Sided Flat Dial is translucent, then it would meet the requirements in the subject-line, because, if the the side of the dial away from you is receiving sunshine, then the shadow of that side’s gnomon will be visible to you, on the translucent dial-face. For such a dial, a Vertical Declining orientation, facing in the most-viewed direction, seems the most convenient. Actually, if the dial is mounted high, and most viewers will be nearby on one side, then readability could be improved a bit by tipping the dial-face down a bit on that side. But one wouldn’t want to tip it far enough northward to make it close to being a Polar Dial, with that dial’s hours-coverage limitation, and its especially non-uniform shadow movement rate. Also, of course, vertical mounting is at least a little simpler to build. If the 2-sided Translucent Flat Dial is Equatorial, instead of Vertical Declining, that would confer explanatory simplicity, at the cost of somewhat less dial-reading convenience, and somewhat more laborious mounting. A North-South Vertical Dial would be a compromise, with more explanatory simplicity than a Vertical Declining Dial, and more convenient reading than an Equatorial Dial. *Double Conical Equatorial Dial, with Circumference-Hole Style:* Another possibility would be two Conical Equatorial Dials mounted back-to-back. I mean, the dials would be like Equatorial Dials, but with cones instead of flat disks. But they probably shouldn’t be *directly* back-to-back, because, depending on their shape, one cone could shade the other cone’s circumference-hole. So they probably should be mounted a little apart. Or, of course, maybe just mounted entirely separately. With suitable cone-shape and hole-positioning, the light-spot could be on the cone throughout the year. But, on the north side, if the latitude is high, and the dial is mounted high, and the viewing position is close, then the lower inside surface of the cone might not be easily readable, unless the cone is sufficiently shallow or open (large angle at apex). But a too-open cone could result in the summer solstice sunshine shining on the north side of the cone-surface, resulting in no circumference-hole light-spot. In that case, the cone-dial, on that side, would need an axial gnomon, in addition to the circumference-hole. The axial gnomon’s shadow would be read when the solar declination is so high that there’s no light-spot. Then, the dial, on that side, would need two concentric hour-scales—one for the circumference-hole, and one for the axial gnomon. The south-cone wouldn’t need to be open (shallow) for *that* reason, but, even on that side, open-ness would result in dial-face visibility from a wider range of azimuths. The Double-Cone Dial would maybe gain some aesthetic appeal, because of not needing a special-properties (transluclent) material not easily-available in ancient times. Also, it has more explanatory simplicity than a Vertical Flat Dial. But those gains over the 2-sided Translucent Flat Dial come at the cost of a more limited range of viewing-angles. …and probably at least slightly more difficult or time-consuming building and mounting *Cube Dial:* Of course, though a Cube Dial is readable from all directions, and readable all day all year, it isn’t readable all day all year from every direction. So it doesn’t really fully meet the requirements in this posting’s subject-line. But I feel that it’s more aesthetically-appealing than the other dials I’ve mentioned so far. I’d probably choose it instead for that reason. …except that it requires more, and more time-consuming, building work. Maybe, if I wanted a high-mounted public-readable yard-dial put up in a short time, I’d use the 2-Sided Translucent Flat Dial, and then leisurely make the Cube Dial. *Reflecting Equatorial Dial:* I don’t know if a Reflecting Equatorial Dial could meet the requirements in this posting’s subject-line. Probably not, I guess. And, even if it could, it would probably require a novel, unfamiliar, complicated reading-method—unsuitable for a public dial. Michael Ossipoff
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