Moon Phase Calculator
Calculate date and time values locally with visible assumptions and copy-ready results.
Use calendar math with the assumption visible
Use it for gardening notes, night photography planning, tide-adjacent context, classroom examples, and curiosity about a date. Moon phase is estimated from lunar age within the synodic month, which is close enough for planning but not astronomy-grade.
The phase is estimated from days since a known new moon and the 29.530588-day synodic month. The age of the moon maps to labels like first quarter or waning gibbous. The phase label and illumination estimate make the output more useful than a single vague moon icon.
Limits and local processing
This is an algorithmic approximation, not an observatory-grade lunar ephemeris. The selected date is evaluated locally against the known new-moon reference. For navigation, tides, or observatory work, use a full lunar ephemeris. The illumination estimate gives a rough brightness clue, which is often more useful for planning than the phase label alone. Expect a planning-grade estimate.
Frequently Asked Questions
How does the moon phase calculator work?
The phase is estimated from days since a known new moon and the 29.530588-day synodic month. The age of the moon maps to labels like first quarter or waning gibbous.
When should I use this tool?
Use it for gardening notes, night photography planning, tide-adjacent context, classroom examples, and curiosity about a date.
What should I watch out for?
This is an algorithmic approximation, not an observatory-grade lunar ephemeris.
Is this date tool private?
Yes. It runs entirely in your browser, so dates, schedules, and notes are not uploaded or stored.
More free browser utilities for planning, formatting, and technical cleanup.