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copied!<p>If you want it to be physically accurate, you actually need to consider two offsets: a vertical (depending on the current date) and a horizontal (depending on the current time).</p> <p>The horizontal offset X may be calculated by looking at the current time on some fixed geographic location on earth. The shadow offset will be 0 at midnight and will increase by 1/86400 for each seconds after midnight. So the formular is</p> <pre><code>offsetX = (curHour*3600 + curMinute*60 + curSeconds)/86400 </code></pre> <p>The vertical offset will change between the <a href="http://en.wikipedia.org/wiki/Solstice" rel="nofollow">Solstices</a> on June 21st and Dec 22nd (if it's not a leap year, where the Solstices are on June 20th and Dec 21st). The maximum angles are 23.44° in both directions. We have 90° per hemisphere and 365/2 = 182.5 days between the two solstices, and we are working with a mapping of a circular motion, so a sin()-function has to be used. The wavelength of a sinus wave is 2pi, so we need pi for half the vertical offset Y of one year.</p> <p>Please note, that I did not take leap seconds into account, so the calculation might be a bit off in the distant past/future.</p> <pre><code>// current time $curHour = date("H"); $curMin = date("i"); $curSec = date("s"); // resulting offset X $offsetX = ($curHour*3600 + $curMin*60 + $curSec)/86400; echo "======== OFFSET X ==========\n"; echo "curHour: $curHour\n"; echo "curMin: $curMin\n"; echo "curSec: $curSec\n"; echo "offsetX: $offsetX\n\n"; // spring equinox date as day of year $equinox = date("z", mktime(0, 0, 0, 3, 20)); // current day of year // first line is for testing purposes //$curDay = date("z", mktime(0, 0, 0, 6, 21)); $curDay = date("z"); // Day offset, mapped on the equinox offset $offsetSin = ((365.25 - $equinox + $curDay)%365.25)/365.25; // sinus wave offset $offsetSinFactor = sin($offsetSin * 2 * pi()); // map onto angle $offsetY = $offsetSinFactor * 23.44; // optional: Mercator projection $degreesPerRadian = 180.0 / pi(); $offsetYmercator = atan(sinh($offsetY)) * $degreesPerRadian; // missing: mapping onto canvas height (it's currently // mapped on $offsetY = 90 as the total height of the // canvas. echo "========= OFFSET Y =========\n"; echo "equinox day: $equinox\n"; echo "curDay: $curDay\n"; echo "offsetSin: $offsetSin\n"; echo "offsetSinFac: $offsetSinFactor\n"; echo "offsetY: $offsetY\n"; echo "offsetYmerc: $offsetYmercator\n"; </code></pre> <p>You should be able to port this calculation to any language you want.</p>

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