Great Circle Studio's Sun Map
Details and Credits

The SunMap utility is an Internet based CGI (Common Gateway Interface) application that modifies (on-the-fly) a PNG image of the Earth to display the current sub-solar position, day/night terminator and standard time zone values. When the solar declination value is greater than zero (the Sun is above the Equator) the standard time zone values are displayed (in red) along the top of the map and are positively offset by one hour. The time values displayed along the bottom of the map are in yellow and are not offset. The opposite is true when the sun is below the Equator, i.e., the positively offset time values (in red) are displayed along the bottom and the non-offset time values (in yellow) are displayed along the top. The Equator and Prime Meridian lines are drawn for reference. A table of current data values is also generated and includes: Julian Day number, Greenwich Civil Time, Greenwich Sidereal Time, Greenwich Solar Hour Angle, Equation of Time (TA-TM), Solar Declination, Right Ascension, Ecliptic Longitude and in the spirit of Y2K recognition, the number of days from Jan 1, 2000.

The World Map image is in equirectangular cylindrical projection i.e., Plate Carrée, with 80 km/pixel resolution. The image is used by permission of ARC Science Simulations. Their image series, Face of the Earth, both in digital and poster hard copy form, are well worth looking in to.

ARC Science Simulations                 phone: 800-759-1642
P.O. Box 1955                           fax:   970-667-1105
Loveland, CO 80539                      web:

All GIF image manipulation was done using Thomas Boutell's GD C library. Although there is no form processing I used Thomas Boutell's CGI C library to snatch a number of CGI environment variables. I have found both Boutell and the code he produces to be extremely helpful. Both my Sundial Generator and Solar Calculator use the Boutell CGI C library. Both CGIC and GD are freeware by the way. Pay a visit to Boutell.COM

Most all astronomical computation was done with the aid of Astronomical Algorithms by Jean Meeus.

I used the spherical transformation portion of the C function projillum(), within vplanet.c, from John Walker's Earth View. The transformation provides the map co-ordinates (lat/long) for the day/night terminator and is very similar to solving for the obliquity component of the Equation of Time, i.e., intersecting great circles.

If you are interested in more SunMap related sites visit Juerge Giesen's Sun Clock page.

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Last Update: Dec 2010
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