Sun and Moon Calculator

Moon Phase Calculator

Enter a date to discover the moon's phase, illumination, and age.

function calculateMoonPhase() { var year = parseInt(document.getElementById("moonYear").value); var month = parseInt(document.getElementById("moonMonth").value); var day = parseInt(document.getElementById("moonDay").value); // Input validation if (isNaN(year) || isNaN(month) || isNaN(day) || year 2100 || month 12 || day 31) { document.getElementById("moonPhaseResult").innerHTML = "Please enter valid date values (Year: 1900-2100, Month: 1-12, Day: 1-31)."; return; } // Adjust month for Julian Date calculation (Jan=1, Feb=2, …, Dec=12) // For JD calculation, months 1 and 2 are treated as months 13 and 14 of the previous year. var a = Math.floor((14 – month) / 12); var y = year + 4800 – a; var m = month + (12 * a) – 3; // Calculate Julian Date (JD) for the given date at 00:00 UTC // Formula from Wikipedia / Fliegel and Van Flandern var JD = day + Math.floor(((153 * m) + 2) / 5) + (365 * y) + Math.floor(y / 4) – Math.floor(y / 100) + Math.floor(y / 400) – 32045; // Reference New Moon: January 6, 2000, 18:14 UTC (JD 2451549.34305556) // Synodic Month: 29.530588 days var referenceNewMoonJD = 2451549.34305556; var synodicMonth = 29.530588; // Calculate days since the reference new moon var daysSinceNewMoon = JD – referenceNewMoonJD; // Calculate the moon's age within the current synodic cycle var moonAge = daysSinceNewMoon % synodicMonth; if (moonAge < 0) { moonAge += synodicMonth; // Ensure positive value for dates before reference } // Calculate illumination percentage (simplified approximation) // A more accurate formula involves phase angle, but this is a good approximation for illumination. var illumination = (1 – Math.cos(moonAge / synodicMonth * 2 * Math.PI)) / 2 * 100; var phaseName = ""; if (moonAge = 28.75) { phaseName = "New Moon"; } else if (moonAge >= 0.75 && moonAge = 6.75 && moonAge = 8.25 && moonAge = 13.75 && moonAge = 15.25 && moonAge = 21.25 && moonAge = 22.75 && moonAge < 28.75) { phaseName = "Waning Crescent"; } var resultHTML = "

Moon Phase Details:

"; resultHTML += "Phase: " + phaseName + ""; resultHTML += "Illumination: " + illumination.toFixed(1) + "%"; resultHTML += "Moon Age: " + moonAge.toFixed(2) + " days (since New Moon)"; document.getElementById("moonPhaseResult").innerHTML = resultHTML; } // Set default date to today for convenience and calculate on load window.onload = function() { var today = new Date(); document.getElementById("moonYear").value = today.getFullYear(); document.getElementById("moonMonth").value = today.getMonth() + 1; // getMonth() is 0-indexed document.getElementById("moonDay").value = today.getDate(); calculateMoonPhase(); // Calculate for today's date on load };

Understanding the Moon Phase Calculator

The Moon Phase Calculator is a tool designed to determine the current phase of the moon, its illumination percentage, and its age (days since the last New Moon) for any given date. While the sun's position dictates our seasons and daylight, the moon's phases are a result of its orbit around Earth and the changing angles at which we view its illuminated surface.

Why Track Moon Phases?

Understanding moon phases has been significant across various cultures and practices for millennia:

  • Astronomy & Stargazing: Knowing the moon phase is crucial for astronomers. A bright full moon can wash out fainter celestial objects, making new moon periods ideal for deep-sky observation.
  • Gardening & Agriculture: Some traditional and biodynamic gardening practices suggest planting and harvesting according to moon phases, believing it influences plant growth and vitality.
  • Fishing & Tides: The moon's gravitational pull is the primary driver of Earth's tides. Anglers often consult moon phases, particularly around new and full moons (spring tides), which can influence fish activity.
  • Cultural & Spiritual Significance: Many festivals, rituals, and spiritual practices worldwide are tied to specific moon phases, such as Diwali, Eid al-Fitr, and various pagan celebrations.
  • Personal Planning: Some individuals find that their energy levels or moods are influenced by the moon's cycle, using moon phase information for personal reflection or planning.

How Does the Calculator Work?

This calculator uses a well-established astronomical algorithm to determine the moon's phase. Here's a simplified breakdown:

  1. Julian Date Conversion: First, the entered calendar date (Year, Month, Day) is converted into a Julian Date (JD). The Julian Date system is a continuous count of days and fractions of a day since noon Universal Time on January 1, 4713 BC. This uniform system simplifies astronomical calculations.
  2. Reference Point: The calculator uses a known New Moon event as a reference point (e.g., January 6, 2000, 18:14 UTC).
  3. Days Since Reference: It then calculates the total number of days that have passed between the reference New Moon and your specified date.
  4. Synodic Period: The moon's synodic period (the time it takes for the moon to complete one cycle of phases as observed from Earth) is approximately 29.530588 days. By taking the remainder of the "days since reference" divided by the synodic period, we get the moon's current age within its cycle.
  5. Phase Determination: Based on this "moon age," the calculator assigns a specific phase name (e.g., New Moon, Waxing Crescent, Full Moon) and estimates the illumination percentage.

Moon Phase Definitions:

  • New Moon: The moon is between the Earth and the Sun, so the side facing Earth is not illuminated. It's generally not visible.
  • Waxing Crescent: A sliver of the moon becomes visible, growing larger each night. "Waxing" means growing.
  • First Quarter: Half of the moon's face is illuminated, appearing as a "D" shape in the Northern Hemisphere.
  • Waxing Gibbous: More than half of the moon is illuminated, continuing to grow towards full. "Gibbous" means bulging.
  • Full Moon: The entire face of the moon is illuminated as seen from Earth. It rises around sunset.
  • Waning Gibbous: The illumination starts to decrease after the Full Moon. "Waning" means shrinking.
  • Last Quarter: Half of the moon's face is illuminated again, but it's the opposite half from the First Quarter, appearing as a "C" shape in the Northern Hemisphere.
  • Waning Crescent: Only a sliver remains illuminated, shrinking towards the New Moon.

How to Use This Calculator:

  1. Enter the Year: Input the four-digit year (e.g., 2023).
  2. Enter the Month: Input the month number (1 for January, 12 for December).
  3. Enter the Day: Input the day of the month (1-31).
  4. Click "Calculate Moon Phase": The calculator will instantly display the moon's phase, its illumination percentage, and its age for the specified date.

Limitations:

This calculator provides a good approximation of the moon's phase and illumination. However, it does not account for:

  • Exact Time of Day: The calculation is based on 00:00 UTC for the given date. The moon's phase can shift slightly over a 24-hour period.
  • Geographic Location: While moon phases are largely universal, the exact moment of a phase (e.g., precise time of full moon) can vary slightly depending on your longitude and time zone. This calculator provides a general phase for the date.
  • Lunar Eclipses or Other Phenomena: This tool focuses solely on the phase and illumination, not on specific astronomical events.

For highly precise astronomical data, specialized software or observatories should be consulted. Nevertheless, this calculator offers a reliable and easy way to understand the moon's cycle for general purposes.

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