Earth and Moon Viewer: Credits


The Earth Viewer was first 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. It was extended to allow viewing the Moon as well as the Earth in October 1996.

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. The Netpbm toolkit is a much-extended version of pbmplus; Earth and Moon Viewer was developed using only pbmplus components.

   ppmtogif.c - read a portable pixmap and produce a GIF file
  
   Based on GIFENCOD by David Rowley [mgardi@watdscu.waterloo.edu].
   Lempel-Ziv 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.

JPEG output file generation is based upon the tookit developed by the Independent JPEG Group. Their contribution of industrial-strength JPEG support to the public domain has enabled the development of innumerable software tools, Earth and Moon Viewer among them.

Steven Grimm's uncgi made the task of processing form arguments in the server immeasurably easier.

The Living Earth® image is a composite of Earth satellite imagery developed by The Living Earth, Inc.. This image is © Copyright 1996 The Living Earth, Inc./Earth Imaging, All Rights Reserved and is used here by permission of the publisher. For additional information about this and other images and products, contact The Living Earth via E-mail to contact@livingearth.com.

The original images of the Earth by day and night are 8640×4320 24-bit pixels: more than 110 megabytes each. To permit interactive access in Earth Viewer, the images were compressed to reduce the number of colours in the day side image to 1024 and the night side to 64. This reduces the image database to a large, but manageable, 74.6 megabytes. The ultimate resolution of the database is about 4.6 kilometres per pixel. The original images and products based upon them have additional fine detail which had to be sacrificed to allow quick response to Earth Viewer requests; please visit The Living Earth for further details and examples of the full-colour images.

The NASA Visible Earth image was assembled from imagery acquired by the MODIS instrument on the Terra satellite. Lights on the night side were imaged from Defense Meteorological Satellite Program spacecraft. Low resolution image requests use a day/night database of 8000×4000 pixels, providing resolution of about 5 kilometres per pixel. When you zoom in for close-up views, Earth and Moon Viewer automatically switches to a database with 1 km/pixel resolution (43200×21600 pixels). There is no night lights database with comparable resolution, so close-ups of the Earth's night side will appear coarse.

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 colour resolution in the original image has been reduced to allow rapid generation of day and night hemispheres. The topographic map is derived from the ETOPO2 global elevation database, which provides 2 minute of arc (2 nautical mile) resolution for most of the Earth.

The cloud cover, colour weather, and water vapour density images are generated on the fly, every three hours, from the whole-Earth weather satellite composites made available by the University of Wisconsin-Madison Space Science and Engineering Center. 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/) The images are postprocessed for use by Earth and Moon Viewer using tools from the pbmplus package described above.

The images of the Moon are generated based on a 1440×720 lunar relief and albedo databases created by the U.S. Geological Survey Flagstaff Field Center from imagery returned by the BMDO/NASA Clementine lunar orbiter. Albedo (reflectivity) is the intrinsic brightness of the terrain as opposed to its appearance from Earth or space, which depends also upon the angle at which the Sun illuminates the Moon at a given time. Except for high-latitude and polar regions, which are always illuminated obliquely due to the low inclination of the Moon's orbit with respect to the ecliptic, the albedo map represents the appearance of the Moon when the Sun is at the meridian for each longitudinal strip of the image.

Using an albedo map and ignoring the effects of elevation and sun angle leads to a washed-out appearance near the terminator (boundary between day and night), especially in the lunar highlands and most of the far side where there is little intrinsic albedo difference and most of observed detail results from terrain features. The relief map shows terrain but neglects albedo; choose whichever produces the most useful image for your purpose.

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

Meeus, Jean. Astronomical Algorithms . Richmond: Willmann-Bell, 1998. ISBN 0-943396-63-8.

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by John Walker