Swiss Ephemeris in Node.js
swisseph npm · Full Setup Guide

# Install swisseph (requires build tools: node-gyp, python)
npm install swisseph

# On macOS (if you see gyp errors):
xcode-select --install
npm install swisseph

# On Ubuntu/Debian:
sudo apt-get install -y python3 build-essential
npm install swisseph

# Verify installation
node -e "const swe = require('swisseph'); console.log(swe.SE_SUN);"
# Should print: 0

const swisseph = require('swisseph')
const path = require('path')

// Point to ephemeris data files
swisseph.swe_set_ephe_path(
  path.join(process.cwd(), 'node_modules/swisseph/ephe')
)

// Convert birth date/time to Julian Day Number
function toJulianDay(year, month, day, hourDecimal) {
  return swisseph.swe_julday(year, month, day, hourDecimal, swisseph.SE_GREG_CAL)
}

// Get sidereal (Vedic) planet position with Lahiri ayanamsa
function getPlanetSidereal(jd, planetId) {
  // Set Lahiri ayanamsa (most common in Vedic astrology)
  swisseph.swe_set_sid_mode(swisseph.SE_SIDM_LAHIRI, 0, 0)

  const flags = swisseph.SEFLG_SWIEPH | swisseph.SEFLG_SIDEREAL
  const result = swisseph.swe_calc_ut(jd, planetId, flags)

  if (result.flag === swisseph.ERR) {
    throw new Error(`Ephemeris error for planet ${planetId}`)
  }

  return {
    longitude: result.longitude,  // 0-360 degrees
    latitude:  result.latitude,
    speed:     result.longitudeSpeed,
    retrograde: result.longitudeSpeed < 0,
  }
}

// Example: Get Sun and Moon for Mumbai birth chart
const jd = toJulianDay(1990, 6, 15, 8.5)  // 15 Jun 1990, 08:30 IST

const sun  = getPlanetSidereal(jd, swisseph.SE_SUN)
const moon = getPlanetSidereal(jd, swisseph.SE_MOON)
const mars = getPlanetSidereal(jd, swisseph.SE_MARS)

console.log('Sun longitude:', sun.longitude.toFixed(4))   // e.g. 60.3421 (Taurus)
console.log('Moon retrograde:', moon.retrograde)           // false
console.log('Mars retrograde:', mars.retrograde)           // true/false

// Convert longitude to nakshatra, pada, and sign
const NAKSHATRAS = [
  'Ashwini','Bharani','Krittika','Rohini','Mrigashira','Ardra',
  'Punarvasu','Pushya','Ashlesha','Magha','Purva Phalguni','Uttara Phalguni',
  'Hasta','Chitra','Swati','Vishakha','Anuradha','Jyeshtha',
  'Mula','Purva Ashadha','Uttara Ashadha','Shravana','Dhanishtha','Shatabhisha',
  'Purva Bhadrapada','Uttara Bhadrapada','Revati'
]

const SIGNS = [
  'Aries','Taurus','Gemini','Cancer','Leo','Virgo',
  'Libra','Scorpio','Sagittarius','Capricorn','Aquarius','Pisces'
]

function parsePosition(longitude) {
  const nakshatraIndex = Math.floor(longitude / (360 / 27))
  const pada           = Math.floor((longitude % (360 / 27)) / (360 / 108)) + 1
  const signIndex      = Math.floor(longitude / 30)
  const degreeInSign   = longitude % 30

  return {
    nakshatra:      NAKSHATRAS[nakshatraIndex],
    pada,
    sign:           SIGNS[signIndex],
    degree:         degreeInSign.toFixed(2),
    fullDegree:     longitude.toFixed(4),
  }
}

// Example usage
const sunPos = parsePosition(sun.longitude)
console.log(sunPos)
// { nakshatra: 'Mrigashira', pada: 4, sign: 'Taurus', degree: '0.34' }

// Compute ascendant (lagna) — requires birth location
function getAscendant(jd, latitude, longitude, ayanamsa = swisseph.SE_SIDM_LAHIRI) {
  swisseph.swe_set_sid_mode(ayanamsa, 0, 0)

  // swe_houses returns house cusps and ASC/MC
  const houses = swisseph.swe_houses(jd, latitude, longitude, 'P') // 'P' = Placidus

  // Subtract ayanamsa for sidereal ascendant
  const ayanamsaValue = swisseph.swe_get_ayanamsa_ut(jd)
  let siderealAsc = houses.ascendant - ayanamsaValue
  if (siderealAsc < 0) siderealAsc += 360
  if (siderealAsc >= 360) siderealAsc -= 360

  return parsePosition(siderealAsc)
}

// Mumbai: lat 19.0760, lon 72.8777
const ascendant = getAscendant(jd, 19.0760, 72.8777)
console.log('Ascendant:', ascendant.sign, ascendant.degree + '°')