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 default databases for generating images of the Earth are the NASA Blue Marble Next Generation cloudless Earth and Black Marble night lights images. Blue Marble images include global cloudless Earth data from the MODIS instrument on the Terra spacecraft, combined with global topography and bathymetry data, with a spatial resolution of 500 metres per pixel, resulting in images of 86400×43200 pixels. Twelve images are included in the data set, containing representative ice and snow cover and vegetation colour for each month of the year. The image is automatically selected based on the month being displayed. (Due to disc and memory space constraints on the Fourmilab server, the monthly images used by Earth and Moon Viewer are reduced in resolution to one kilometre per pixel, or 43200×21600 pixels.) The night portion of the Earth uses the Black Marble v2012 imagery with a resolution of 750 metres per pixel, or 54000×27000 pixels, derived from cloudless Earth images returned by the Suomi NPP spacecraft.

A higher resolution alternative to the Blue Marble Monthly images is the basic NASA Blue Marble and Black Marble night lights images. This data set is based upon the same sources as the Blue Marble Monthlies, but provides a a single image representing the Earth with only year-round ice and snow cover. Earth and Moon Viewer makes this image available at its full resolution, 86400×43200 pixels, or 500 metres per pixel, so more detail will be visible in close-up views of the day side of Earth than with the Blue Marble Monthlies.

The NASA Visible Earth image was assembled from imagery acquired by the MODIS instrument on the Terra spacecraft. 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 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 contact The Living Earth for further details and examples of the full-colour images.

The high-resolution topographic map is based upon the ETOPO1 Global Relief Model developed by the National Centers for Environmenal Information operated by the United States Department of Commerce, National Oceanic and Atmospheric Administration. The image has resolution of one minute of arc, or one nautical mile, and is thus 21600×10800 colour pixels. Reduced resolution versions of the image are used for wide area renderings, automatically switching to the full resolution image for close-ups. Permanent ice cover on Antarctica, Greenland, and arctic islands are shown, but sea ice is not, showing the topography of the ocean floor instead.

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 cylindrical projection expected by Earth Viewer's image generator (thanks to code from the GCTPc package. The images are postprocessed for use by Earth and Moon Viewer using tools from the pbmplus package described above.

The default database for generating images of the Moon is the LRO LROC-WAC Global Mosaic 100m produced by the Arizona State University Lunar Reconnaissance Orbiter Camera Team from imagery returned by the NASA Lunar Reconnaissance Orbiter spacecraft from orbit around the Moon. The image database consists of 109164×54582 grey scale pixels in a cylindrical projection compiled into a mosaic from imagery collected between November 2009 and February 2011, and provides 100 metre per pixel resolution for the entire lunar surface. The 5.6 gigabyte image is sub-sampled at several intermediate resolutions to improve performance when generating images where the full resolution is not required, while using the 100 metre per pixel data for views as close as one kilometre from the surface.

Images of the Moon can also be 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 978-0-943396-61-3.

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