beaivi

Beaivi

Beaivi is a tiny BSD-licensed JavaScript library for calculating sun position, sunlight phases (times for sunrise, sunset, dusk, etc.), moon position and lunar phase for the given location and time.

The original code is from SunCalc created by Vladimir Agafonkin (@mourner) as a part of the SunCalc.net project. This repo has updated to code to add types and ES modules.

Most calculations are based on the formulas given in the excellent Astronomy Answers articles about position of the sun and the planets. You can read about different twilight phases calculated by SunCalc in the Twilight article on Wikipedia.

Usage example

import * as Beaivi from 'beaivi';

// Get today's sunlight times for London
const times = Beaivi.getTimes(new Date(), 51.5, -0.1);

// Format sunrise time from the Date object
const sunriseStr = times.sunrise.getHours() + ':' + times.sunrise.getMinutes();

// Get position of the sun (azimuth and altitude) at today's sunrise
const sunrisePos = Beaivi.getPosition(times.sunrise, 51.5, -0.1);

// Get sunrise azimuth in degrees
const sunriseAzimuth = sunrisePos.azimuth * 180 / Math.PI;

Reference

Sunlight times

Beaivi.getTimes(/*Date*/ date, /*Number*/ latitude, /*Number*/ longitude, /*Number (default=0)*/ height)

Returns an object with the following properties (each is a Date object):

Property	Description
sunrise	sunrise (top edge of the sun appears on the horizon)
sunriseEnd	sunrise ends (bottom edge of the sun touches the horizon)
goldenHourEnd	morning golden hour (soft light, best time for photography) ends
solarNoon	solar noon (sun is in the highest position)
goldenHour	evening golden hour starts
sunsetStart	sunset starts (bottom edge of the sun touches the horizon)
sunset	sunset (sun disappears below the horizon, evening civil twilight starts)
dusk	dusk (evening nautical twilight starts)
nauticalDusk	nautical dusk (evening astronomical twilight starts)
night	night starts (dark enough for astronomical observations)
nadir	nadir (darkest moment of the night, sun is in the lowest position)
nightEnd	night ends (morning astronomical twilight starts)
nauticalDawn	nautical dawn (morning nautical twilight starts)
dawn	dawn (morning nautical twilight ends, morning civil twilight starts)

Beaivi.addTime(/*Number*/ angleInDegrees, /*String*/ morningName, /*String*/ eveningName)

Adds a custom time when the sun reaches the given angle to results returned by SunCalc.getTimes.

SunCalc.times property contains all currently defined times.

Sun position

Beaivi.getPosition(/*Date*/ timeAndDate, /*Number*/ latitude, /*Number*/ longitude)

Returns an object with the following properties:

• altitude: sun altitude above the horizon in radians, e.g. 0 at the horizon and PI/2 at the zenith (straight over your head)
• azimuth: sun azimuth in radians (direction along the horizon, measured from south to west), e.g. 0 is south and Math.PI * 3/4 is northwest

Moon position

Beaivi.getMoonPosition(/*Date*/ timeAndDate, /*Number*/ latitude, /*Number*/ longitude)

Returns an object with the following properties:

• altitude: moon altitude above the horizon in radians
• azimuth: moon azimuth in radians
• distance: distance to moon in kilometers
• parallacticAngle: parallactic angle of the moon in radians

Moon illumination

Beaivi.getMoonIllumination(/*Date*/ timeAndDate)

Returns an object with the following properties:

• fraction: illuminated fraction of the moon; varies from 0.0 (new moon) to 1.0 (full moon)
• phase: moon phase; varies from 0.0 to 1.0, described below
• angle: midpoint angle in radians of the illuminated limb of the moon reckoned eastward from the north point of the disk; the moon is waxing if the angle is negative, and waning if positive

Moon phase value should be interpreted like this:

Phase	Name
0	New Moon
	Waxing Crescent
0.25	First Quarter
	Waxing Gibbous
0.5	Full Moon
	Waning Gibbous
0.75	Last Quarter
	Waning Crescent

By subtracting the parallacticAngle from the angle one can get the zenith angle of the moons bright limb (anticlockwise). The zenith angle can be used do draw the moon shape from the observers perspective (e.g. moon lying on its back).

Moon rise and set times

Beaivi.getMoonTimes(/*Date*/ date, /*Number*/ latitude, /*Number*/ longitude[, inUTC])

Returns an object with the following properties:

• rise: moonrise time as Date
• set: moonset time as Date
• alwaysUp: true if the moon never rises/sets and is always above the horizon during the day
• alwaysDown: true if the moon is always below the horizon

By default, it will search for moon rise and set during local user's day (frou 0 to 24 hours). If inUTC is set to true, it will instead search the specified date from 0 to 24 UTC hours.