Moon Phase Calculator — FAQ

It uses the Julian Day Number (JDN) method combined with the synodic month — the 29.53-day cycle between new moons. Starting from a known reference new moon (2000-01-06), it calculates how many days have elapsed to the date you enter, then finds where in the cycle that date falls. The result gives the moon age, phase name, and illumination fraction.

The calculator is accurate enough for everyday use — correctly identifying the phase name and approximate illumination. It may be off by about one day for the exact timing of phase peaks (e.g., the precise moment of full moon), because it uses the synodic-period method rather than a full gravitational ephemeris. For precise astronomical timing, consult the US Naval Observatory or a dedicated ephemeris tool.

The eight phases are:

• 🌑 New Moon — 0% illuminated; the Moon is between Earth and the Sun
• 🌒 Waxing Crescent — a thin lit sliver growing on the right (Northern Hemisphere)
• 🌓 First Quarter — right half lit (~50%), growing toward full
• 🌔 Waxing Gibbous — more than half lit and still growing
• 🌕 Full Moon — fully illuminated; Earth is between Moon and Sun
• 🌖 Waning Gibbous — more than half lit but shrinking
• 🌗 Last Quarter — left half lit (~50%), shrinking toward new
• 🌘 Waning Crescent — a thin lit sliver shrinking on the left (Northern Hemisphere)

Waxing means the Moon is getting brighter — the illuminated portion is growing from new moon toward full moon. Waning means the Moon is getting darker — the illuminated portion is shrinking from full moon back toward new moon. The calculator shows this label alongside the phase name.

Because the Southern Hemisphere is oriented upside-down relative to the Northern Hemisphere when looking at the Moon. The same phase appears horizontally flipped — a waxing crescent that shows its lit side on the right in the Northern Hemisphere appears lit on the left in the Southern Hemisphere. The N/S hemisphere toggle on this tool flips the visual to match your perspective.

Yes. The calculator works for dates from 1700 to 2300. The Julian Day Number algorithm works equally well for past and future dates, so you can look up the Moon phase on any birthday, historic event, or future date you're curious about.

Moon age is the number of days since the most recent new moon. It runs from 0 at new moon to about 14.77 days at full moon, then back toward 29.53 days at the next new moon. It tells you exactly where in the lunar cycle the date falls and helps you gauge how close to a major phase the date is.

Illumination is derived from the moon age using a cosine function: (1 − cos(2π × age / synodic)) / 2. This gives 0% at new moon, approximately 50% at the quarter phases, and 100% at full moon. It models the smooth transition of sunlit area visible from Earth across the lunar disc throughout the cycle.

Completely free, with no sign-up, no downloads, and no limits. The entire calculation runs in your browser — no data leaves your device.

No. Nothing is stored, logged, or sent to a server. The date you enter is used only within the page to compute the result and is never recorded anywhere. There is no history, no tracking, and no account required.