solar-spa/.wiki/API-Reference.md

API Reference

spa(date, latitude, longitude, options?)

Returns a Promise<SpaResult> with raw numeric values.

Parameters

Name	Type	Description
date	Date	Date and time for the calculation
latitude	number	Observer latitude, -90 to 90 (negative = south)
longitude	number	Observer longitude, -180 to 180 (negative = west)
options	object	Optional. See below

Options

Option	Type	Default	Description
timezone	number	auto	Hours from UTC. Auto-detected from the Date object if omitted
elevation	number	0	Meters above sea level
pressure	number	1013.25	Atmospheric pressure in millibars
temperature	number	15	Temperature in Celsius
delta_ut1	number	0	UT1-UTC correction in seconds
delta_t	number	67	TT-UTC difference in seconds
slope	number	0	Surface slope in degrees from horizontal
azm_rotation	number	0	Surface azimuth rotation in degrees from south
atmos_refract	number	0.5667	Atmospheric refraction in degrees
function	number	3	SPA function code (see below)

Result Fields

Field	Type	Unit	Description
zenith	number	degrees	Topocentric zenith angle (0 = directly overhead)
azimuth	number	degrees	Topocentric azimuth, eastward from north (navigational convention)
azimuth_astro	number	degrees	Topocentric azimuth, westward from south (astronomical convention)
incidence	number	degrees	Surface incidence angle
sunrise	number	fractional hours	Local sunrise time
sunset	number	fractional hours	Local sunset time
suntransit	number	fractional hours	Solar noon (sun transit)
sun_transit_alt	number	degrees	Sun altitude at transit
eot	number	minutes	Equation of time
error_code	number	integer	0 on success

Timezone Auto-detection

When timezone is omitted, the value is derived from the Date object's local timezone offset:

timezone = -(date.getTimezoneOffset() / 60);

This works correctly in most cases. Provide an explicit value when computing for a location whose timezone differs from the machine's local timezone.

Error Handling

The SPA validates all inputs against physical bounds (latitude -90 to 90, timezone -18 to 18, etc.). If validation fails, spa() throws an Error with the SPA error code in the message:

try {
  await spa(new Date(), 40, -74, { timezone: 100 });
} catch (e) {
  // "SPA: calculation failed (error code 8)"
}

A null result pointer (WASM memory allocation failure) also throws.

spaFormatted(date, latitude, longitude, options?)

Same parameters and behavior as spa(). Returns a result object with the same fields, but sunrise, sunset, and suntransit are HH:MM:SS strings instead of fractional hours. During polar day or polar night, these strings are "N/A":

const result = await spaFormatted(new Date(2025, 5, 21, 12, 0, 0), 40.7128, -74.006, {
  timezone: -4,
});

console.log(result.sunrise); // "05:25:12"
console.log(result.sunset); // "20:30:42"
console.log(result.suntransit); // "12:57:54"
console.log(result.zenith); // 27.08 (still a number)

formatTime(hours)

Converts fractional hours to an HH:MM:SS string. Returns "N/A" for non-finite or negative values, which occur during polar day or polar night when sunrise or sunset does not happen. Values at or above 24 hours wrap to the next day (e.g., 24.5 becomes "00:30:00").

formatTime(6.5); // "06:30:00"
formatTime(12); // "12:00:00"
formatTime(Infinity); // "N/A"

init()

Pre-initializes the WASM module. Returns a Promise<void> that resolves when the module is ready.

This is optional. The module initializes automatically on the first spa() call. Use init() if you want to pay the initialization cost at application startup rather than on the first calculation.

import { init, spa } from 'solar-spa';

// Somewhere during app bootstrap
await init();

// Later, this call has no initialization overhead
const result = await spa(new Date(), 40, -74);

Calling init() multiple times is safe. The second and subsequent calls return immediately.

Function Codes

The function option controls which outputs the SPA computes. Lower codes skip the rise/transit/set and incidence calculations, which are the most expensive part.

Constant	Value	Computes
SPA_ZA	0	Zenith and azimuth only
SPA_ZA_INC	1	Zenith, azimuth, and incidence angle
SPA_ZA_RTS	2	Zenith, azimuth, and rise/transit/set times
SPA_ALL	3	All output values (default)

import { spa, SPA_ZA } from 'solar-spa';

// When you only need zenith and azimuth (fastest)
const result = await spa(new Date(), 40, -74, { function: SPA_ZA });
console.log(result.zenith, result.azimuth);

When using SPA_ZA or SPA_ZA_INC, the rise/transit/set fields will contain zeros.

TypeScript

Full type definitions are included. Import types directly:

import { spa, SPA_ALL } from 'solar-spa';
import type { SpaResult, SpaOptions } from 'solar-spa';

const options: SpaOptions = {
  timezone: -4,
  elevation: 10,
  function: SPA_ALL,
};

const result: SpaResult = await spa(new Date(), 40.7128, -74.006, options);

---

Home · Architecture · NREL SPA Algorithm · Changelog