millerlp / solarlib

Arduino solar position calculator

Currently this library only compiles under the Teensy modified version of Arduino provided by PJRC.com (https://www.pjrc.com/teensy/td_download.html), and relies on the modified Time library that is packaged with the PJRC version of the Arduino software. See the PJRC forum for downloads: http://forum.pjrc.com/forums/6-Announcements

To install this library, place the folder containing all of the Solarlib files into the libraries directory in your Arduino folder.

Solarlib Luke Miller December 2012

Released into the public domain (originally based on U.S. Govt. products) No warranty given or implied.

A library of functions for Arduino to calculate aspects of solar position in the sky using a time value, latitude, longitude, and time zone. Output includes estimates of current sun elevation and azimuth (position in the sky), sunrise, solar noon, and sunset times for the current day, and various other statistics for the sun at the given time. Results should be accurate for the years 1901 to 2099, for locations between +/- 72 latitude. Calculations are based on spreadsheet and information found at: http://www.esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html

Initialize the solar calculator using the initSolarCalc() function, which lets you specify:

• tzOffset - time zone Offset from Greenwich Mean Time (UTC). Time zones west of GMT should be given negative values. For example, Pacific Standard Time is -8

• lat - Latitude of the location you want to use. Values north of the equator are positive, given in decimal degrees.

• lon - Longitude of the location you want to use. Values in the western hemisphere have negative values (0 to -180), given in decimal degrees. For example: Monterey, California has lat/lon (36.62, -121.904)

After running initSolarCalc() in the Arduino setup loop, you can call any of the extractor functions to get the position of the sun and sunrise/sunset times. Each extractor function requires a Time value as input, from the Time library. The Time is given as seconds since 1970-1-1 00:00 (unix epoch).

Useful functions, supplied with a time value t as the sole argument:

•  `getSAA(t)`		- Solar Azimuth (degrees clockwise from North)
  

•  `getSEC_Corr(t)`	- Solar Elevation, corrected for diffraction (degrees)
  

•  `getSZA(t)`		- Solar Zenith angle (degrees below straight up)
  

•  `getSunriseTime(t)`	- Sunrise Time for the current day 	(Time object)
  

•  `getSunsetTime(t)`	- Sunset Time for the current day	(Time object)
  

•  `getSolarNoonTime(t)`	- Solar Noon for the current day	(Time object)
  

•  `getSunDuration(t)`	- Minutes of Sunlight for the current day	
  

WARNING

This has only tested on 32-bit ARM Teensy 3.0/3.1/3.5/3.6. This library will probably fail horribly on 8-bit AVR products such as the Arduino Uno due to limits in precision of double/float values. You can check your results against the NOAA calculator: http://www.esrl.noaa.gov/gmd/grad/solcalc/

Thanks to Raoul Smeets for catching a typo in the Approximate Atmospheric Refraction calculation (SE.AAR in calcSolar() function). 2021-04-19

solarlib's Issues

Elevation error shortly after sunrise

The example Solarlib_example.ino reveals an apparent error in the calculated solar elevation shortly after sunrise each morning (jumping to very negative values like -64 to -114 when they should be slightly positive). Elevation estimates before sunrise and an hour after sunrise appear to be, and agree with the ESRL predictions. Tested using the default settings (lat/lon and time) in the Solarlib_example.ino: 36.62, -121.9, 1352030400 starting time (2012-11-04 12:00).