Earth Viewer: Credits

The Earth Viewer was implemented by John Walker in December 1994. Most of the software that generates the various views of the Earth was adapted from Home Planet for Windows. See the details for additional implementation information.

The satellite tracking code is based upon the N3EMO Orbit Simulator: 

            N3EMO Orbit Simulator routines  v3.7

   Copyright © 1986,1987,1988,1989,1990 Robert W. Berger N3EMO
   May be freely distributed, provided this notice remains intact.
The GIF output file generation is based upon the ppmtogif module of Jef Poskanzer's pbmplus toolkit, of which many other components were used in creating the images you see here. 
   ppmtogif.c - read a portable pixmap and produce a GIF file
  
   Based on GIFENCOD by David Rowley [mgardi@watdscu.waterloo.edu].
   Lempel-Zim compression based on "compress".
  
   Modified by Marcel Wijkstra [wijkstra@fwi.uva.nl]
  
   Copyright © 1989 by Jef Poskanzer.
  
   Permission to use, copy, modify, and distribute this software and its
   documentation for any purpose and without fee is hereby granted, provided
   that the above copyright notice appear in all copies and that both that
   copyright notice and this permission notice appear in supporting
   documentation.  This software is provided "as is" without express or
   implied warranty.
  
   The Graphics Interchange Format© is the Copyright property of
   CompuServe Incorporated.  GIF(sm) is a Service Mark property of
   CompuServe Incorporated.
Steven Grimm's uncgi made the task of processing form arguments in the server immeasurably easier.

The global topographic map was developed by the Marine Geology and Geophysics Division of the National Geophysical Data Center operated by the United States Department of Commerce, National Oceanic and Atmospheric Administration.

The cloudy Earth image is generated on the fly, every three hours, from the whole-Earth weather satellite pictures made available by the University of Wisconsin-Madison Space Science and Engineering Center's Gopher server. I take this image, then transform it from the Mollweide projection in which it is published to the Mercator projection expected by Earth Viewer's image generator (thanks to code from the GCTPc package:
ftp://edcftp.cr.usgs.gov/pub/software/gctpc/gctpc.tar.Z) Then the image is contrast stretched, gamma corrected, and composited with a cartoon image of the Earth (too much detail in the terrain washes out the clouds) using tools from the pbmplus package described above.

All the images have been modified somewhat to align them with the map projection and to adjust the colour map for the night and day display.

The algorithms to calculate the position of the Sun and Moon are given in:

Meeus, Jean. Astronomical Algorithms. Richmond: Willmann-Bell, 1991. ISBN 0-943396-35-2.

Return to Earth viewer

by John Walker
kelvin@fourmilab.ch