Some years ago, I was invited to go to the beach the following morning, to observe the sunrise.
. I preferred getting there well before sunrise, but I didn’t know how early she was willing to start, and so I suggested that we meet at a time that would get us there a little before Civil Twilight. . She said, “That late?” . Good, we agreed then. So I suggested something better: A meeting time that would get us to the beach a little before Nautical Twilight. She said that sounded better, and we agreed on that meeting-time. I’d written-down the time at which Civil Twilight would start. But, somewhat before that time, it was clear that Dawn, Civil Twilight, was *beginning* to arrive. The beginning of the arrival of Civil Twilight was definitely clearly there. That interested me. . Though the arrival of full Civil Twilight is practical for a number of reasons, the *beginning *of its arrival seemed more significant, more beautiful, to me. . So I wrote the time down, at that time. . It was a time when the altitude of the sun was about -9.37 degrees. . (Though I rounded to the nearest hundredth of a degree, I can’t guarantee that all the inputs were accurate enough to justify that precision.) . As I said, that beginning, having a special beauty, seemed more significant to me than the arrival of full Civil Twilight. . Later, I read that evidently that time in the morning was recognized in Roman and Medieval times, and was given a name. . In those days, when people were living closer to nature, getting up earlier, and without pervasive night-time artificial light, they recognized several significant times in the morning: . Sunrise: Self explanatory Dawn: . This was a distinct time, before Sunrise. Surely it referred to the beginning of Civil Twilight, the time when it’s first fully light enough to look and feel like daytime. …when it’s first light enough to read or do daytime activities, or to go safely. . Aurora: . This was the beginning of the arrival of Dawn. …named after the Roman goddess of Dawn. . That’s what I’d noticed, and recorded the time of, at the beach! . So: Unless someone else suggests a different time, I suggest that Aurora is the time when the Sun’s altitude is -9.37 degrees. . Check it out, next time you’re up early. . By the way, if you aren’t up at least a little before Nautical Twilight, then you aren’t really up early. . p.s. . On another topic: I wasn’t going to bother you about this in a separate posting, but, since I’m posting anyway, I might as well include it: . In my previous post, I was talking about sundials that have the best overall readability, in terms of time-of-year, time-of-day, and viewing-direction. . After posting that, it occurred to me that I’d left something out: A 2-sided translucent equatorial dial with an equatorial disk, and also an equatorial band. …all translucent. . The equatorial band could be affixed to the equatorial disk, via tabs. …like the Band-Equatorial dial described in Teacher’s Corner, at the NASS website. . That article made the useful suggestion of constructing a Band-Equatorial by affixing a flexible band to a circular edge--in that instance, the edge of a circular cut in a piece of cardboard—via tabs. . But that circular edge could also be the edge of a circular disk. For a table dial, viewed from above, the circular cut is best. . But, for a high-mounted dial, viewed from below, a disk is best. And so it might as well be a translucent equatorial band affixed (by tabs) to a 2-sided translucent Equatorial Dial. …actually, one such band affixed to each side of that disk. . Such a dial would be readable edge-on (in the plane of the disk). . If the band is narrow, then it won’t significantly interfere with the viewing of the disk, or the opposite side of the band (for someone to whom their side of the band isn’t readable because it’s morning and s/he’s east of the dial, nearly in the plane of the disk). . Surely such a dial would have the best overall readability in terms of time-of-year, time-of-day, and viewing-direction. ...with the Equatorial's added advantage of simplicity of explanation. . …but would take longer to build than a plain 2-sided translucent Equatorial, or Vertical Declining, or Reclining-Declining dial. . Michael Ossipoff
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