Lunar Phase Calculator
Calculate the Moon's current phase and illumination percentage for any calendar date. Useful for photographers, gardeners, and astronomers planning around the lunar cycle.
About this calculator
The Moon completes one synodic cycle — new moon to new moon — in approximately 29.53 days. By measuring elapsed days since a known new moon reference point, we can determine where in its cycle the Moon currently sits. The formula used here converts a calendar date to a day count relative to January 1, 2000, using approximate day counts per year (365.25) and per month (30.44). The phase percentage is: Phase% = round(((elapsed_days mod 29.53) / 29.53) × 100). A result of 0% or 100% indicates a new moon, 50% indicates a full moon, and values between represent waxing (0–50%) or waning (50–100%) phases. Illumination roughly follows a sinusoidal curve peaking at 100% at full moon.
How to use
Let's calculate the lunar phase for March 15, 2024 (year=2024, month=3, day=15). Step 1 — compute elapsed days: (2024 − 2000) × 365.25 + (3 − 1) × 30.44 + 15 = 8766 + 60.88 + 15 = 8841.88 days. Step 2 — find position in lunar cycle: 8841.88 mod 29.53 = 8841.88 − (299 × 29.53) = 8841.88 − 8829.47 = 12.41 days. Step 3 — convert to percentage: (12.41 / 29.53) × 100 ≈ 42%. This places the Moon in its waxing gibbous phase, roughly 42% through its cycle, with the full moon approaching in about 3 days.
Frequently asked questions
How accurate is this lunar phase calculator for planning observations?
This calculator uses a simplified arithmetic approximation rather than a full ephemeris, making it accurate to within about one day for most dates near the year 2000 and somewhat less precise for dates far from that epoch. For casual planning — choosing a dark-sky weekend for stargazing or scheduling a photography session — it is perfectly adequate. For precise lunar rise/set times or exact illumination percentages needed for scientific work, a dedicated ephemeris tool such as JPL Horizons is recommended.
What is the difference between waxing and waning moon phases?
The lunar cycle begins at new moon (0%), when the Moon is between Earth and Sun and its lit face points away from us. During the waxing phase (0–50%), the illuminated portion grows each night, progressing through crescent, first quarter, and gibbous stages until full moon at 50%. During the waning phase (50–100%), illumination decreases through gibbous, last quarter, and crescent back to new moon. Waxing moons are visible in the evening sky, while waning moons rise progressively later and are better seen in the pre-dawn hours.
Why does the Moon's phase cycle last 29.53 days instead of exactly 28?
The 29.53-day synodic month is the time for the Moon to return to the same phase as seen from Earth — say, full moon to full moon. This differs from the Moon's sidereal orbital period of 27.32 days because Earth is also orbiting the Sun. By the time the Moon completes one orbit relative to the stars, Earth has moved roughly 1/13 of the way around the Sun, so the Moon must travel an extra ~2.2 days to 'catch up' and realign with the Sun as seen from Earth. This extra time accounts for the difference between the sidereal and synodic months.