Moon Phase Calculator

This tool relies on established astronomical mathematics to determine the moon's position and appearance without needing external APIs or network requests.

1. Date Conversion: The selected calendar date is converted into a Julian Day Number, a continuous count of days since the beginning of the Julian Period, which simplifies astronomical time calculations.
2. Ecliptic Longitude: Using Jean Meeus's astronomical algorithms (from Astronomical Algorithms, 1998), the calculator computes the apparent geocentric ecliptic longitude of both the Moon and the Sun for the given moment.
3. Elongation and Phase: The angular distance (elongation) between the Sun and the Moon is calculated. This angle directly determines the phase fraction (from 0.0 to 1.0) and the illuminated percentage of the lunar disc visible from Earth.
4. Event Extrapolation: To find upcoming or previous phase events, the tool solves for the exact moments when the elongation equals 0° (New Moon), 90° (First Quarter), 180° (Full Moon), and 270° (Third Quarter), applying periodic correction terms for high precision.
5. Lunar Calendar Generation: For the monthly view, the phase is sampled at noon UTC for each day of the selected month, mapping exact phase transitions to their respective calendar days for easy visual reference.