The GCS Sundial Generator is an Internet based Common Gateway Interface (CGI) application that generates output of a fully corrected horizontal sundial in either the Portable Document Format (PDF) or Graphics Interchange Format (GIF). The sundial directly reads Standard Mean Time i.e., 'clock time' as well as the approximate date. The application is written in C and like the Solar Calculator, the Sundial Generator uses the Boutell CGI Library for its CGI interface. The application also makes use of The United States Naval Observatory Astronomical Almanac and the Société Astronomique de France - Commission des cadrans solaires publication " Calculation of a Planar Sundial", reprinted in English in Astronomical Algorithms by Jean Meeus.
Local Latitude and Longitude are entered in degrees, minutes and seconds using only whole numbers i.e., no decimal or negative values. Latitude is the angular distance measured north and south from the equator. Latitudes north of the equator are positive and southern latitudes are negative. Longitude is the angular distance measured east and west from the prime meridian at Greenwich, England. Longitudes east of the prime meridian are positive and western longitudes are negative. To avoid confusion however, radio buttons are used to indicate the sign of the entered latitude N(orth) or S(outh) and longitude E(ast) or W(est). Preceding a latitude or longitude with a negative sign will produce an error message.
To aid in finding the Latitude and Longitude information for your location, the following Internet resource links are provided.
Time Zone Offset is entered in hours and where applicable also in minutes using integers
i.e., positive or negative whole numbers. The time zone offset is the time difference between
the user's local zone time (LT) and
Universal Time (UT) i.e., (LT-UT). The entered time zone offset value should not include any
locally observed seasonal offset e.g., my location, in Pacific Grove, California USA, makes use of
Pacific Zone Time, therefore I would always enter -8 (hours) even though local civil time is only 7
hours behind Universal Time during Daylight Savings Time (DST).
Time zone meridians east of Greenwich have positive offsets and time zone meridians west of Greenwich have negative
offsets. The standard time zones are centered
on meridians spaced 15 degrees apart, resulting in 24 separate time zones. Standard time zone boundaries,
which in theory, are on longitudes 7.5 degrees east and west of a standard time zone meridian, in
practice meander to accommodate local centers of population and political borders. The stretching of
these boundaries can allow a region to use a zone time far different from the zone time that corresponds
to its longitude. Much of Spain, for example, which is west of Greenwich, uses the zone time whose meridian
is centered 15 degrees east of Greenwich. Lastly, not all areas make use of the standard time zones, instead
their time is offset some fractional part of an hour from a standard zone time. Parts of Australia, for
example, are offset one half hour from standard zone time. The minutes entry field is used to enter
irregular time zone offsets. The hour and minute time zone offset entries must be similar in sign.
To aid in finding the Time Zone information for your location, the following Internet resource links are provided.
|Latitude||Longitude||Time Zone Offset|
|degrees = 0-79||degrees = 0-179||hours = (+/-) 0-11|
|minutes = 0-59||minutes = 0-59||minutes = (+/-) 0-60|
|seconds = 0-60||seconds = 0-60|
Output Format is used to select the format of the Sundial Generator's output. Currently the two supported output formats are the Portable Document Format (PDF) and the Graphics Interchange Format (GIF). The generator can also be configured, using the radio buttons, to enable black and white B/W or color C(olor) output. When a hard copy of the sundial is to be generated only PDF should be chosen, if just a quick visualization of the dial is desired then the GIF image option can be used.
Sundial Motto Text is limited to a maximum of 17 characters. The entered text string is drawn inside the Zodiac border ring along the top of the sundial. Your name, location or a sundial motto are examples of what might be used.
Making a Printout:
As mentioned earlier in the Output Format section only PDF should be selected for the generator's output when the aim is to produce a hard copy of the sundial. Although Portable Document Format (PDF) files are not directly displayed by the browser itself, Adobe offer a freely available viewer to display PDF documents and print them to the printer configured on the system. If you don't already have the Acrobat reader, use the resource link below to get a copy.
The Sundial should be pinned or glued to a stiff backing then adjusted, using a spirit level, until it is perfectly flat. It is essential that the sundial be level and stable. A compass rose in the upper right corner of the sundial page is used for general orientation. Calibrate the sundial by rotating it at the beginning of the hour until the shadow is aligned to the correct analemma (figure-eight shaped hour line) and is on the portion of the analemma that corresponds to the current season. The hours are marked at the tops of the analemmas along the Summer Solstice date line from 8AM to 4PM. During Daylight Savings Time (DST) 'sun time' is an hour (or whatever offset is in effect) behind 'clock time', therefore when calibrating the sundial during DST rotate the dial as above but align the shadow to the hour analemma preceding the current hour (on your clock).
Reading the Date:
As well as reading time the sundial also displays the approximate date based on the length of the shadow cast. The shadow at its longest moves along the date line marking the Winter Solstice on December 21, at its shortest length it moves along the date line marking the Summer Solstice on June 21. The shadow in the course of one year will twice travel the distance separated by these two extremes, once from Winter Solstice to Summer Solstice and then again from Summer Solstice back to Winter Solstice. The remaining date lines divide these two half year periods into the twelve Zodiac periods each approximately thirty days in length and indicate the time when the Sun is positioned in front of the constellation. The symbols of the Zodiac signs, drawn along the outer border ring of the sundial, are centered between the date lines that mark their respective periods and are colorized to indicate which season they are in. The dates for each Zodiac period are listed as reference at the bottom of the sundial page. The date is read by interpolating the shadow position between the date lines of the current Zodiac period. The date reading is most accurate (to within a couple days) on dates close to the Equinoxes (when the declination rate is fastest) and the least accurate (to within several days) on dates close to the Solstices (when the declination rate is slowest).