Solar Position Algorithm (SPA)

The Solar Position Algorithm module (SPA) for Rust calculates the sunrise-sunset and azimuth and zenith-angle for specific geo-position and time (UTC); for example for solar-panel-alignment or automotive.

Calculating the Sunrise and Sunset

The following function is calculating the sunrise-sunset at geo-pos lat/lon at time t (UTC).

The algorithm has been ported to Rust from http://lexikon.astronomie.info/zeitgleichung/neu.html Its accuracy is in the range of a few minutes.

Arguments

• utc - UTC time-point (DateTime)
• lat - latitude in WGS84 system, ranging from -90.0 to 90.0.
• lon - longitude in WGS84 system, ranging from -180.0 to 180.0

The function returns a result of type SunriseAndSet.

pub enum SunriseAndSet {
    PolarNight,
    PolarDay,
    Daylight(DateTime<Utc>, DateTime<Utc>),
}

The polar night occurs in the northernmost and southernmost regions of the Earth when the night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnight sun, occurs when the Sun stays above the horizon for more than 24 hours. "Night" is understood as the center of the Sun being below a free horizon, represented by the variants SunriseAndSet::PolarNight or SunriseAndSet::PolarDay.

Since the atmosphere bends the rays of the Sun, the polar day is longer than the polar night, and the area that is affected by polar night is somewhat smaller than the area of midnight sun. The polar circle is located at a latitude between these two areas, at the latitude of approximately 66.5 degrees. The function is approximating the atmospheric bend using a height of 0,833333 degrees.

The variant SunriseAndSet::Daylight(DateTime<Utc>, DateTime<Utc>) represents time of sunrise and sunset.

In case latitude or longitude are not in valid ranges, the function will return Result::Err(BadParam).

pub fn calc_sunrise_and_set(utc: DateTime<Utc>, lat: f64, lon: f64) -> Result<SunriseAndSet, SpaError> {..}

Calculating the Solar Position

The following functions is calculating the solar position (azimuth and zenith-angle) at time t and geo-pos lat/lon

The algorithm has been ported to Rust from http://www.psa.es/sdg/sunpos.htm The algorithm is accurate to within 0.5 minutes of arc for the year 1999 and following.

Arguments

• utc - UTC time-point (DateTime)
• lat - latitude in WGS84 system, ranging from -90.0 to 90.0.
• lon - longitude in WGS84 system, ranging from -180.0 to 180.0

The function returns a result of type SolarPos.

pub struct SolarPos {
    // horizontal angle measured clockwise from a north base line or meridian
    azimuth: f64,
    // the angle between the zenith and the centre of the sun's disc
    zenith_angle: f64,
}

In case latitude or longitude are not in valid ranges, the function will return Result::Err(BadParam).

pub fn calc_solar_position(utc: DateTime<Utc>, lat: f64, lon: f64) -> Result<SolarPos, SpaError> {..}

Performance

Benchmarks have been created using the Bench-Framework of Rust. The Bench-Framework is repeating the test N times and is calculating average time and variance. The following results illustrate the performance compared to a reference-function reading the system time only. The benchmarks have been performed using a Lenovo X230 with Intel Core i5 3320M - 4 x 2,6Ghz and Rust compiler version rustc 1.22.0-nightly (7778906be 2017-10-14) and Ubuntu 17.04 with kernel-4.10.0-38-generic.

Command:

cargo bench --features "benchmark"

Output:

test benchmark::bench_calc_solar_position    ... bench:         557 ns/iter (+/- 10)
test benchmark::bench_calc_sunrise_and_set   ... bench:         490 ns/iter (+/- 6)
test benchmark::bench_reference_read_systime ... bench:          56 ns/iter (+/- 1)