calculations
Limitations
  • SolarPlots.info is NOT for engineering, scientific, or navigational applications
  • Calculations are approximate and are intended for non-precision applications such as photography
  • Time is based on standards in the 48 contiguous United States. Entering coordinates in Canada or Mexico will return times for United States standard and daylight saving time.
  • Localized time zone or daylight saving exceptions (such as certain counties in Indiana) are not accounted for.
Accuracy

The results returned by SolarPlots.info have been compared to available benchmark data including sunrise, sunset, and solar noon. Based on comparisons to available data the accuracy is ESTIMATED to be the following
sunrise, sunset
±5 minutes
solar noon
±2 minutes
Angle for a specific time
±2°

SolarPlots.info uses equations published in standard references. Due to the numerous corrections and adjustments required in the calculation and appropriate simplifications for a non-precision application, the results are approximate. Error is larger as the sun is near the horizon due to the approximate atmospheric refraction correction.


Reference

Reference texts published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) provide equations for calculation of the sun's position. These equations are approximate and not intended for precise astronomical use.
1999 ASHRAE Applications Handbook    Page 1    Page 2    Page 3
2001 ASHRAE Fundamentals Handbook    Page 1

These equations break down in certain instances and this web site from the USC Architecture Department presents a modification of the above equations which always works. These equations were used in the computation.

The book Practical Astronomy With Your Calculator by Peter Duffett-Smith (1979) presents astronomical-quality equations for computing the position of astronomical bodies.

The equations for atmospheric refraction were used in the calculation for SolarPlots.info Page 1-2
Sun Position by John Clark Craig presents equations and programming code for making solar calculations. This book was published after I launched SolarPlots.info and I have not seen its contents, but it looks like a great resource for anyone looking to dig more deeply into solar calculations.