> At 07:14 AM 9/1/99 -0400, Mac Oglesby wrote: > > > >I wonder what it would take to make it a sundial showing hours until dark. > >That is, how would the time of "dark" (relative to sunset) be determined? > ------------------------------------------------------------------------
Hello Mac, in astronomy the definition of "dark" is connected to the definition of the twilight. Twilight is caused by the scattering of sunlight from upper layers of the Earth's atmosphere. In the "Explanatory Supplement to the Astronomical Almanac"- 1992 - p. 492 - the twilights are defined in the following way : - The Astronomical Twilight begins at sunset and is conventionally taken to end when the center of the Sun reaches an altitude of - 18 deg. The indirect illumination from the Sun on a horizontal surface is about 0.0006 lux, rather less then the contribution from starlight and airglow (about 0.002 lux) The illumination received from full Moon is about 0.1 lux - The Nautical Twilight is conventionally taken to end when the center of the Sun reaches an altitude of - 12 deg : the sea horizon is in general not visible and is too dark for the observation of altitudes with reference the horizon. The indirect illumination from the Sun on a horizontal surface is about 0.008 - 0.01 lux - The Civil Twilight is conventionally taken to end when the center of the Sun reaches an altitude of - 6 deg. After Civil Twilight outdoor activities that depend on natural lighting require artificial illumination. The brightest stars are visible and the sea horizon is clearly defined. The indirect illumination from the Sun on a horizontal surface is about 1-3 lux With my program SUND98P distributed by NASS in September 1998, it's possible to calculate and to draw sundials that mark the hours to the twilight (Civil , Nautical or Astronomical) and therefore the hours of light. Sundials of this type can be used in open places , ski fields, camp sites , etc. The hour lines are not straight lines but curves and they have the shape of those in the attached image (horizontal Sundial with Lat. = 50 deg - Civil Twilight) Best Gianni Ferrari ------------------------------------------ Ing. Gianni Ferrari Via Valdrighi, 135 41100 - MODENA ( ITALY ) EMail : [EMAIL PROTECTED] http://members.tripod.com/meridiane/index.htm Attachment converted: MAC Hard Disk:Twi1.jpg (JPEG/JVWR) (0001A7F9)
