Moon Phase Calculator

A moon phase is the shape of the sunlit portion of the Moon as seen from Earth. As the Moon orbits Earth over approximately 29.5 days (one synodic month), it cycles through 8 phases: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent. Each phase reflects a different angle between the Sun, Earth, and Moon.

This calculator uses the astronomy-engine library, which implements Jean Meeus's high-precision astronomical algorithms — the same math used by planetariums, NASA mission planning tools, and professional astronomy software. It calculates the Moon's ecliptic elongation, illumination fraction, and exact phase timing with arc-second precision. Results are accurate to within minutes for any date.

Illumination percentage indicates how much of the Moon's visible face is lit by the Sun. At New Moon it's 0% (completely dark), at Full Moon it's 100% (fully lit). A First Quarter moon is roughly 50% illuminated on the right side, while a Last Quarter is 50% illuminated on the left side. The percentage changes continuously as the Moon orbits.

Moon age is the number of days elapsed since the most recent New Moon. A moon age of 0 means it's a New Moon, around 7.4 days is First Quarter, 14.8 days is Full Moon, 22.1 days is Last Quarter, and 29.5 days completes the cycle back to New Moon. This calculator computes the exact moon age by finding the previous New Moon using precise astronomical search algorithms.

Waxing means the illuminated portion is growing — the Moon is moving from New Moon toward Full Moon (phases 0° to 180°). Waning means the illuminated portion is shrinking — the Moon is moving from Full Moon back toward New Moon (180° to 360°). In the Northern Hemisphere, the right side lights up first during waxing, and the left side remains lit last during waning.

Yes. Enter your birth date to discover which lunar phase you were born under. The tool shows the exact phase, illumination, and a personality profile associated with your birth moon phase. Many astrologers believe your birth moon phase influences your emotional temperament and life approach.

The calculator uses astronomy-engine's SearchMoonPhase function, which performs a precise astronomical search for the next occurrence of a specific lunar elongation angle (180° for Full Moon, 0° for New Moon) within 30 days of the selected date. This is not an approximation — it finds the exact moment the Moon reaches that phase.

In the Southern Hemisphere, the Moon appears flipped left-to-right compared to the Northern Hemisphere. A waxing moon that lights up on the right in the North lights up on the left in the South. This calculator shows the Northern Hemisphere perspective, which is the standard convention used by most astronomy tools worldwide.

No. This moon phase calculator runs 100% in your browser using JavaScript and the astronomy-engine library. No date you enter is ever transmitted to any server, stored in any database, or shared with any third party. All astronomical calculations happen locally on your device.

The progress bar shows where the selected date falls within the 29.5-day lunar cycle as a percentage from 0% (New Moon) through 50% (Full Moon) and back to 100% (next New Moon). Labels mark the New Moon, First Quarter, Full Moon, and Last Quarter positions so you can visually gauge where in the cycle any date falls.

Unlike most online moon calculators that use simple modular arithmetic with a single reference date (drifting by 1-2 days over decades), this tool uses the astronomy-engine library — a JavaScript implementation of Jean Meeus's Astronomical Algorithms. It calculates the Moon's ecliptic elongation, illumination fraction, and phase timing with arc-second precision, the same accuracy used by planetariums and NASA mission planning.

Type your date naturally in MM/DD/YYYY format — slashes insert automatically, single digits are padded, and invalid dates are rejected as you type. Tab advances through month, day, and year fields. A calendar picker is available for visual selection, a Today button resets to the current date, and a clear button wipes everything. Month and day validation prevents impossible dates like February 30.

A mathematically accurate SVG renders the Moon's appearance for the selected date — showing the exact proportion of lit versus shadow surface based on the ecliptic elongation angle. The terminator curve (boundary between light and dark) is calculated using the cosine of the phase angle, producing a realistic crescent, quarter, gibbous, or full disc. All moon visuals use consistent off-white and dark blue tones.

Shows the exact percentage of the Moon's visible face illuminated by the Sun, calculated from the astronomy-engine's Illumination function. This is not an estimate — it's derived from the actual geometric relationship between Sun, Earth, and Moon positions. Values range from 0% at New Moon to 100% at Full Moon.

Displays the number of days elapsed since the most recent New Moon, calculated by searching backward from the selected date for the last New Moon using astronomy-engine's SearchMoonPhase function. This gives the true moon age, not an approximation based on a fixed 29.53-day cycle.

Clearly labels whether the Moon is waxing (growing, 0°-180°) or waning (shrinking, 180°-360°) based on the ecliptic elongation. This tells you whether the Moon is gaining or losing light — useful for gardeners, fishers, and anyone tracking lunar rhythms.

A visual horizontal bar shows where the selected date falls in the full 29.5-day lunar cycle. Labeled markers at New Moon, First Quarter, Full Moon, and Last Quarter provide instant context. The percentage updates in real-time as you change dates.

When the selected date falls within 5° of a New Moon, Full Moon, First Quarter, or Last Quarter, a highlighted amber alert appears identifying the special lunar event. This makes it easy to find exact Full Moon or New Moon dates by browsing dates.

Two cards show the next Full Moon and next New Moon from the selected date, with the exact date and a countdown in days. Both are calculated using astronomy-engine's SearchMoonPhase function, which finds the precise moment the Moon reaches 180° (Full) or 0° (New) elongation within the next 30 days.

Each of the 8 lunar phases has an associated personality profile for people born under that phase. Descriptions cover temperament, strengths, and life approach. Whether you believe in lunar influence or enjoy it as a fun exercise, the profiles offer a unique perspective beyond traditional zodiac signs.

When you enter a date other than today, a side-by-side comparison appears showing both today's moon and your selected date's moon with SVG visuals, phase names, and illumination percentages. This makes it easy to see how the Moon has changed or will change relative to right now.

Click the Today button to instantly fill in today's date and see the current moon phase. After exploring other dates, click it again to return to today. The date is always fetched fresh from your system clock, so it stays accurate even if you leave the page open overnight.

A reference section displays all 8 phases with accurate SVG icons (not emoji), illumination ranges, and plain-language descriptions. The current phase is highlighted with a green ring and '(Current)' label. This serves as both an educational guide and a quick-reference for understanding the lunar cycle.

Click Copy to save your complete moon phase result as formatted text — phase name, illumination, moon age, direction, description, personality profile, and next Full and New Moon dates. Paste into messages, journals, social media, or planners.