manual.js

function getCurrentDate() {
  const now = new Date();
  const year = now.getFullYear();
  const month = now.getMonth(); // Month is zero-indexed, so no adjustment is needed
  const day = now.getDate();

  return new Date(year, month, day);
}

function calculateSunset(latitude, longitude, date) {
    // Convert latitude and longitude to radians
    const phi = latitude * Math.PI / 180;
    const lambda_ = longitude * Math.PI / 180;

    // Calculate the Julian Day (JD)
    const startOfYear = new Date(date.getFullYear(), 0, 1);
    const n = (date - startOfYear) / (1000 * 60 * 60 * 24);
    const jd = n + 1.0008;

    // Calculate the solar noon (in days since 2000-01-01 12:00 UTC)
    const J = jd - longitude / 360;

    // Calculate the solar mean anomaly (M)
    let M = (357.5291 + 0.98560028 * J) % 360;
    const M_rad = M * Math.PI / 180;

    // Calculate the equation of center (C)
    const C = 1.9148 * Math.sin(M_rad) + 0.0200 * Math.sin(2 * M_rad) + 0.0003 * Math.sin(3 * M_rad);

    // Calculate the ecliptic longitude (lambda_sun)
    let lambda_sun = (M + C + 180 + 102.9372) % 360;
    const lambda_sun_rad = lambda_sun * Math.PI / 180;

    // Calculate the Sun's declination (delta)
    const delta = Math.asin(Math.sin(lambda_sun_rad) * Math.sin(23.44 * Math.PI / 180));

    // Calculate the hour angle (H)
    const zenith = 90.83 * Math.PI / 180;  // Official sunset zenith
    const cos_H = (Math.cos(zenith) - Math.sin(phi) * Math.sin(delta)) / (Math.cos(phi) * Math.cos(delta));
    const H = Math.acos(Math.min(Math.max(cos_H, -1), 1));  // Ensure cos_H is within valid range

    // Convert hour angle to hours
    const H_deg = H * 180 / Math.PI;
    const H_hours = H_deg / 15;

    // Calculate solar noon and sunset time
    const solar_noon = 2451545.0009 + J + 0.0053 * Math.sin(M_rad) - 0.0069 * Math.sin(2 * lambda_sun_rad);
    const sunset_jd = solar_noon + H_hours / 24;

    // Convert Julian Day to datetime
    const sunset_utc = new Date(Date.UTC(2000, 0, 1, 12) + (sunset_jd - 2451545.0) * 86400000);
    
    return sunset_utc;
}

// Function to calculate and display sunset time
function calculateAndDisplaySunset() {
    // Get user-provided date
    const day = parseInt(document.getElementById("day").value);
    const month = parseInt(document.getElementById("month").value) - 1; // Month is zero-indexed
    const year = parseInt(document.getElementById("year").value);
    const date = new Date(year, month, day);

    // Get user-provided latitude and longitude
    const latitude = parseFloat(document.getElementById("latitude").value);
    const longitude = parseFloat(document.getElementById("longitude").value);

    // Calculate sunset in UTC
    const sunset_utc = calculateSunset(latitude, longitude, date);

    // Adjust for local time (Bangladesh Standard Time, UTC+6)
    const timezoneOffset = 6;
    const sunset_local = new Date(sunset_utc.getTime() + timezoneOffset * 60 * 60 * 1000);

    // Format time with seconds
    function formatTime(date) {
        return date.toLocaleTimeString('en-US', { hour: '2-digit', minute: '2-digit', second: '2-digit', hour12: true });
    }

    // Display results
    document.getElementById("date").textContent = date.toDateString();
    document.getElementById("sunsetUTC").textContent = formatTime(sunset_utc);
    document.getElementById("sunsetLocal").textContent = formatTime(sunset_local);
}

// Attach event listener to the button
document.getElementById("calculateButton").addEventListener("click", calculateAndDisplaySunset);

// Calculate sunset for the current date on page load
calculateAndDisplaySunset();