Library / C++ class for calculating sun rise/set events for Unix, Linux, Arduino.
Compute times of sunrise and sunset at a specified latitude and longitude.
Determine the nearest sun rise or set event previous, and the nearest sun rise or set event subsequent, to the specified time in seconds since the Unix epoch (January 1, 1970) and at the specified latitude and longitude in degrees.
This software minimizes computational work by performing the full calculation of the solar position three times, at the beginning, middle, and end of the period of interest. Three point interpolation is used to predict the position for each hour, and the arithmetic mean is used to predict the half-hour positions.
Rise and set events are found by checking if the sun has crossed the horizon at hourly intervals. The time of the crossing then is determined by interpolating between the calculated positions. In polar regions during periods when the solar day or night is shorter than an hour, sunrise or set events may be missed.
The algorithm's full computational burden is negligible on modern computers, but it is effective and still useful for small embedded systems.
While the number of calculations is manageable on small systems, it is unlikely that accurate results will be produced on systems with four-byte double precision floats (such as the older Arduinos using ATmega processors). Satisfactory results will be had on newer processors such as the ESP8266.
This software was originally adapted to javascript by Stephen R. Schmitt from a BASIC program from the 'Astronomical Computing' column of Sky & Telescope, April 1994, page 84, written by Roger W. Sinnott.
To use the SunRise library, include SunRise.h
#include <SunRise.h>
SunRise sr;
sr.calculate(double latitude, double longitude, time_t time);
latitude, longitude:
The location of interest, in decimal degrees. Latitude ranges
from -90 (south pole) to 90 (north pole). Longitude ranges
from -180 (west of Greenwich) to 180 (east of Greenwich).
time:
The time to search for events, in UTC seconds from the Unix
epoch (January 1, 1970). The closest sun rise/set event will
be found before and after this time. In polar regions there
may not be an event within the configurable search window, in
which case zero, one, or two events may all be found either
before or after *time*.
bool sr.isVisible; // If sun is visible at *time*.
bool sr.hasRise; // There was a sunrise event found in the search
// interval (default 48 hours, 24 hours before and
// after *time*).
bool sr.hasSet; // There was a sunset event found in the search interval.
float sr.riseAz; // Where the sun will rise in degrees from north.
float sr.setAz; // Where the sun will set in degrees from north.
time_t sr.queryTime; // The *time* passed as the third argument.
time_t sr.riseTime; // The sun rise event, in UTC seconds from the Unix epoch.
time_t sr.setTime; // The sun set event.