Phase-locked events advance around calendar-month spokesLive current inputs
Date & Time
Moon Phase Date Recurring Date Calculator
Generate a defensible sequence of recurring lunar-phase dates from a start date, selected phase, cycle interval, observation lead time, and timing tolerance instead of treating a lunar month as a fixed calendar month.
First selected phase—
Final recurrence—
Mean event spacing—
Drift from equal calendar months—
First preparation start—
Observation window width—
Total schedule span—
Cadence interpretation—
Lunar recurrence orbit
Phase-locked events advance around calendar-month spokes
Each node is tied to a synodic-cycle position; a secondary calendar ring exposes the accumulated drift that fixed monthly dates would hide.
Recurrence ephemeris
Event-by-event phase dates, preparation starts, and timing windows
Dates are modeled from a mean synodic month and should be checked against an authoritative ephemeris for operational use.
| Event | Modeled phase instant | Preparation starts | Tolerance window | Days since prior | Calendar drift |
|---|
Recurrence setup
Build the cadence around the astronomical event
- Choose the first date from which planning may begin.
- Select the required major phase explicitly.
- Use multiple cycles only for less frequent events.
- Set the number of occurrences needed for the planning horizon.
- Add operational lead time and a realistic timing tolerance.
Phase-locked scheduling
A lunar recurrence is not the same as a monthly calendar recurrence
A mean synodic month is about 29.53 days, so equal calendar-month dates gradually separate from the intended phase. The calculator preserves phase cadence first and reports calendar drift rather than concealing it.
The mean-cycle model is suitable for planning and comparison, not precision astronomy. Topocentric visibility, atmospheric conditions, time zones, and orbital perturbations require ephemeris data.
Calculation method
Project a selected lunar phase by synodic-cycle recurrence
The selected major phase is located after the planning start. Later events add whole multiples of the mean synodic month; calendar drift compares that physical cadence with equal calendar-month anniversaries.
Detailed calculation process and general formulas
A_target = phaseIndex * S / 4delta_1 = mod(A_target - A_start, S)T_i = T_1 + (i - 1) * k * SP_i = T_i - L / 24Drift_i = T_i - addCalendarMonths(T_1, (i - 1) * k)Symbols, meanings, and units
- S
- mean synodic month29.530588853 days
- A_start, A_target
- lunar age at start and selected major-phase agedays
- T_i
- modeled instant of recurrence iUTC date-time
- k, i
- cycle interval and recurrence numberdimensionless
- L, P_i, Drift_i
- lead hours, preparation start, and calendar drifthours; date-time; days
Cadence diagnostics
Four checks before publishing a lunar schedule
The schedule separates astronomical recurrence, calendar labeling, preparation workload, and acceptable timing error.
Astronomical anchor
—First modeled instant after the entered start.
Schedule horizon
—Final event in the requested series.
Calendar divergence
—Accumulated difference from equal calendar months.
Operational window
—Total width around each modeled event.
Decision takeaway: Publish the phase-locked dates, then verify each important event against a current ephemeris.
Schedule review
When to regenerate the series
- Observation site or time zone changes
- A different lunar phase becomes relevant
- The recurrence interval changes
- Weather or access constraints alter lead time
- An authoritative ephemeris revises the event instant
Astronomical evidence
Inputs and records worth preserving
- Selected phase definition
- Planning start and time-zone convention
- Authoritative ephemeris source
- Observation-site access hours
- Tolerance rationale and operating notes
Practical applications
Decisions this calculator is designed to support
Monthly full-moon field survey
A team needs eight full-moon visits and must begin equipment preparation 18 hours before each event.
What the result clarifies: The recurrence table replaces equal monthly dates with a phase-linked cadence.
Quarterly new-moon dark-sky session
An observatory schedules every third new moon with a narrow acceptable window.
What the result clarifies: Cycle interval and tolerance describe the actual observing rule without implying calendar-quarter alignment.
Worked example
Current-input substitution and reconciliation
Model limitations
This calculator uses a mean synodic month for planning. It does not predict local moonrise, altitude, weather, visibility, eclipses, or high-precision phase instants. Confirm consequential dates with an authoritative astronomical ephemeris.
Moon Phase Date Recurring Date Calculator | Lunar Recurrence Planner FAQ
Why do the dates drift from the same day each month?
The mean lunar phase cycle is about 29.53 days, while calendar months have different lengths.
Does the tolerance change the phase date?
No. It defines an operating window around the modeled instant.
Can this predict moonrise?
No. Moonrise depends on location and requires positional astronomy.
Are these dates suitable for legal or safety deadlines?
Not without authoritative verification and the correct local time-zone rules.