XSUNCLOCK(1)             USER COMMANDS               XSUNCLOCK(1)

NAME
     xsunclock - show illuminated portion of Earth

SYNOPSIS
     xsunclock

DESCRIPTION
     xsunclock is an OpenWindows application that displays a  map
     of  the  Earth  and indicates the illuminated portion of the
     globe by drawing sunlit areas dark on light, night areas  as
     light on dark.

     The program initially appears iconic, showing a small map in
     a  128  by 64 pixel icon with the local date and time at the
     bottom.  If opened, a 640 by  320  pixel  map  is  displayed
     which  shows  both the local and Universal (GMT) time to the
     second.  While the window is open, pressing the right  mouse
     button  displays  a  menu which lets you step the clock from
     the present time forward or backward in  increments  of  one
     hour,  day, week, month, or year.  If you select ``Animate''
     from this menu, the program steps  continuously  every  half
     second.   Try  selecting  ``Week'' then ``Animate'' to watch
     the seasons pass.  Select ``Year'' and ``Animate''  and  you
     can  see  the  (ever so subtle) precession of the equinoxes.
     Selecting ``Real time'' restores the current time display.

HOW IT WORKS
     xsunclock calculates the position of the Sun using the algo-
     rithm in chapter 18 of:

     Astronomical Formulae for Calculators by Jean  Meeus,  Third
     Edition, Richmond: Willmann-Bell, 1985.

     and Mercator projects the  illuminated  area  onto  the  map
     image.   The Sun's position is calculated to better than one
     arc-second in accuracy.

BUGS
     The illuminated area shown is the area which would be sunlit
     if the Earth had no atmosphere.  The actual illuminated area
     is larger because of atmospheric refraction and twilight.

     Only works for dates between 1970 and 2034  due  to  limited
     range of Unix time representation.

AUTHOR
          John Walker

     Permission  to  use,  copy,  modify,  and  distribute   this
     software  and  its documentation for any purpose and without
     fee is hereby granted, without any  conditions  or  restric-
     tions.   This software is provided ``as is'' without express
     or implied warranty.

NOTE
     This program, xsunclock, is not related in any  way  to  the
     Macintosh  and  Microsoft  Windows  product  "Sun Clock[tm]"
     marketed by Palo Alto Software.  If you're interested  in  a
     tool like xsunclock for Windows or the Macintosh, check out:

            http://www.pasware.com/sunclock.htm

     Palo Alto Software describes Sun Clock as follows:

         Ideal  Internet  utility,  puts a full-color map of
         the world on your computer  screen,  with  day  and
         night  showing  at  all times.  Click on a location
         and see what  time  it  is  there.   Set  clickable
         locations  that  give  you  exact time of day and a
         notepad  with  phone  numbers  (or  FTP  sites,  or
         whatever)  for  anywhere in the world.  SRP $39.95,
         Mac or Windows.  Published by Palo Alto Software.

     I  have no connection with Palo Alto Software; I'm including
     this note to avoid confusion between this program and  their
     product, which have very similar names and functionality.