Moon Phase Calculator
Calculate the Moon's phase for any date and time using a real astronomical algorithm based on the Julian Date and the 29.53-day synodic month.
Reviewed October 7, 2026 Runs in your browser — inputs are never stored
Universal Time — adjust for your timezone if needed.
Formula
phase = ((JD − 2451550.1) mod 29.530588853) ÷ 29.530588853 · illumination = (1 − cos 2π·phase) ÷ 2
Uses the synodic month of 29.530588853 days and the reference new moon of JD 2451550.1 (2000-01-06 18:14 UTC). The illuminated fraction is an approximation; true phase varies slightly with lunar orbital eccentricity and the Moon's orbital inclination.
This tool provides educational conversions of astronomical distances only. It is not a navigation, mission-planning, or ephemeris service. For professional or spacecraft work, use official IAU, NASA, or observatory data products.
About the Moon Phase Calculator
The moon phase calculator shows the phase of the Moon and the percentage of its disc that is lit for any date and time. It measures the time since a known new moon and divides by the average synodic month of about 29.53 days — the time from one new moon to the next — to find the Moon’s age in its cycle. From the age it names the phase (new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter or waning crescent). It is handy for planning night photography, stargazing, fishing trips and outdoor events.
How to use it
- 1Enter the date you want to check.
- 2Set the time in UTC (adjust for your local timezone if needed).
- 3Press Calculate to see the phase name, illumination, and Moon age.
- 4Read the days until the next new and full moon for planning.
Worked example
on 14 January 2026 at 12:00 UTC the Moon is a Waning Crescent about 17% illuminated, roughly 25.6 days after the last new moon, with the next new moon in about 4 days — a dark-sky window ideal for deep-sky astrophotography.
How it works
How the Moon phase is calculated
The calculator converts your date and time to a Julian Date (JD), then measures how many days have passed since a known reference new moon (JD 2451550.1, the new moon of 6 January 2000 at 18:14 UTC). Dividing by the mean synodic month of 29.530588853 days gives the phase fraction from 0 (new) to 1 (next new).
Illuminated fraction
The illuminated fraction visible from Earth is approximated as (1 − cos(2π × fraction)) ÷ 2, giving 0 at new moon, 0.5 at the quarters, and 1 at full moon.
Phase names
- 0.00 — New Moon
- 0.25 — First Quarter
- 0.50 — Full Moon
- 0.75 — Last Quarter
Crescent and gibbous phases fill the intervals between these principal phases.
Who it is for
Amateur astronomers, photographers, gardeners, and anyone planning around moonlight or lunar cycles.
Why it helps
Knowing the exact phase and illumination percentage helps you plan moonlit photography, stargazing, fishing, and cultural or religious observances tied to the lunar calendar.
Common use cases
- Planning astrophotography sessions around new or full moons
- Scheduling stargazing trips to avoid bright moonlight
- Tracking lunar cycles for gardening, fishing, or cultural events
- Teaching the phases of the Moon in astronomy classes
Tips for better results
- Use UTC midnight for a clean whole-day phase, or your local time converted to UTC for precision.
- The illuminated fraction is approximate; real illumination shifts slightly with the Moon's elliptical orbit.
- New moon nights are best for faint deep-sky objects; full moon washes out the Milky Way.
- The Moon age (days since new moon) tells you how far through the cycle you are.
Common mistakes to avoid
- Entering local time without converting to UTC when precision matters.
- Expecting the phase to be identical worldwide at the same clock time — it is keyed to UTC.
- Confusing waxing (growing, right-lit in the northern hemisphere) with waning (shrinking, left-lit).
- Assuming full moon always means 100% illumination — it peaks very close but the exact instant is brief.
Frequently asked questions
A:It uses the standard mean-synodic-month algorithm referenced to a known new moon, accurate to within a few hours for the principal phases. Real phases vary slightly because the Moon's orbit is elliptical and inclined.
A:The Julian Date is a continuous count of days since 1 January 4713 BC. Astronomers use it because it avoids calendar and timezone complications.
A:The Moon moves about 13° per day, so the phase shifts noticeably over hours near the quarter boundaries. Entering the exact UTC time gives the most precise result.
A:The synodic month (29.530588853 days) is the average time between two identical phases, such as new moon to new moon. It is longer than the orbital period because Earth also moves around the Sun.
A:Yes. It runs entirely in your browser on CalcWisee with no sign-up required.
Reviewed for accuracy · Last updated October 7, 2026
The astronomical unit is taken as exactly 149,597,870,700 metres per the IAU 2012 definition. Related light-time and light-year factors use the defined speed of light and the IAU Julian year where applicable.
CalcWisee Editorial Team
Calculator development and formula review
CalcWisee is an independent site founded by Ayman. Every calculator is built from a published formula or official definition, tested against hand-worked examples before it goes live, and corrected promptly when a reader reports an issue to ayman@calcwisee.com.
Expertise: Financial mathematics, Health and fitness formulas, Applied mathematics, Unit conversion standards
References & sources
