On Tue, 15 Feb 2000, Gordon Uber wrote: > Third minutes (sixtieths of second minutes) are not in common > use, although I would note that the third minute of an hour is the period > of U.S. power main standard 60 Hz alternating current. Coincidence?
Hmm... Surprised I never noticed that! Perhaps no more of a coincidence than standard-gauge rail width. > The arc minute is so convenient for expressing human visual acuity because > the value of the latter is coincidentally close to 1 arc minute. For > practical calculations at small angles arc minutes and arc seconds are best > converted to radians, the arc second being approximately 5 microradians, > the arc minute about 17 milliradians. And, of course, the angular diameter > of the sun is approximately 10 milliradians. > > Gordon Whups! You meant a *degree* is ~17 mRadian, didn't you? An arcsec is very close to 0.3 mR or 300 uR... Still, 0.3 mR for a fine feature, or 1.5 mR for a character are still pretty convenient units for visual acuity. And the Tan function becomes trivial in that regime... Dave
