How is the moon phase computed?

From a periodic approximation of the moon's synodic period (29.5306 days) anchored to a known new-moon epoch. Accuracy is within a few hours, sufficient for calendars and planning but not astronomical observation.

Are the phases location-dependent?

Slightly. The moon's appearance flips between northern and southern hemispheres, and rise/set times depend on latitude, but the phase itself (illuminated fraction) is global.

What's a synodic vs sidereal month?

Synodic (29.53 days) is the time between successive new moons — the basis of lunar phases. Sidereal (27.32 days) is the time to return to the same position against the stars. Phases use synodic.

Does the calculation use the internet?

No. Phases are computed locally from astronomical formulas.

synodic month · illumination · 30-day calendar · live

Current moon phase & monthly calendar

Pick a date. We compute the lunar age relative to a known new moon (2000-01-06), then label the phase and show the next 30 days.

Quick Start

How the lunar phase is determined

Moon age = (date - reference new moon) mod 29.5306 days. We label the phase by age bucket and compute illumination as (1 - cos(2π·age / synodic)) / 2.

01
Pick a date
Defaults to today.
Parsing accepts ISO 8601, RFC 3339, RFC 2822, and Unix epoch seconds/milliseconds. The Date constructor handles the rest with browser-engine consistency.
dateISO 8601RFC 3339epochlive
in · timestampdate · ISO
```
2026-05-21
```
loaded · ISO 8601zone · ready
02
Diff from new moon
Reference: 2000-01-06 18:14 UTC.
All math runs in UTC internally to avoid DST landmines; presentation converts back to the user’s zone via Intl.DateTimeFormat.
refUTC mathIntlDST-safelive
Δ · optionsdays · live
```
9618.2 days
```
options applied · UTCDST-safe
03
Modulo synodic month
Age = mod 29.53.
Output respects the chosen timezone offset (Z vs +05:30) and pads digits per ISO conventions so sorts stay chronological.
modoffsetzone-awaresortablepadded
mod · computed29.53 · ready
```
age = 4.6 d
```
computed · instantsortable
04
Render
Emoji + label + illumination percentage. Next 30 days shown.
Copy writes via the Async Clipboard API; nothing pings a time server — the browser’s clock is the only oracle.
renderclipboardno oracledeterministicclient-side
✓ · copiedready · done
```
Waxing Crescent
```
shipped · ready to shipsuccess

Pro tips

Time math without DST landmines

Always store UTC

Convert to local for display only. UTC storage avoids DST jumps and timezone drift in databases.

Watch leap seconds

POSIX time doesn’t count leap seconds; some systems do. Don’t use timestamps for sub-second precision across boundaries.

Prefer Temporal eventually

The Date API has many quirks. Temporal is shipping in modern engines and handles zones correctly.

Cron is timezone-tricky

Cron expressions run in the daemon’s local zone unless told otherwise. Pin a TZ explicitly for predictable runs.

ISO 8601 sorts naturally

YYYY-MM-DDTHH:mm:ss sorts correctly as a string. Don’t use US-style MM/DD/YYYY for anything machine-readable.

Beware Y2K38

Signed 32-bit Unix timestamps overflow in 2038. Use 64-bit storage end-to-end for long-lived systems.

Use Cases

When to look up the moon

Astronomy, faith, art.

Stargazing

Dark-sky nights need a new moon. Math runs in UTC under the hood to dodge DST jumps — local time is only for display. That means scheduling logic and date diffs survive March / November fall-back without the classic off-by-one-hour bug.

DST-safe

Religious calendars

Many follow lunar months. Browser clock is the only source — no external time server, no privacy leak, no rate limit. Pair with NTP-synced server time for high-accuracy use cases, but for human-scale work the browser clock is plenty.

UTC-correct

Photography

Capture a full moon over a skyline. ISO 8601 output sorts naturally as strings, perfect for filenames (report-2026-01-15.pdf) and log lines that need chronological ordering without parsing. Drop into S3 keys for free time-bucketing.

ISO-sortable

Art / writing

Set a scene under the right phase. Pairs cleanly with cron, scheduled tasks, and timestamped log analysis. The tool's output drops directly into APScheduler, Linux cron, or GitHub Actions schedule expressions without re-formatting.

spec-grade

Tides

New / full moon → spring tides. Output respects the chosen timezone offset (Z vs +05:30) so REST APIs and database TIMESTAMP WITH TIME ZONE columns round-trip cleanly. No ambiguity between "noon UTC" and "noon local".

zone-aware

Garden

Some growers plant by lunar phase. Confirms the difference between epoch seconds (10 digits) and milliseconds (13 digits) before you commit code that mis-multiplies by 1000 — a classic bug in cross-language time handling.

epoch-accurate

FAQ

Frequently asked questions

How accurate is the phase label?

Bucketed by age in days. Within hours of a phase boundary the next label may be more apt.

Time of day matters?

Only minor — sub-day phase differences are smaller than the bucket width.

Why 29.5306?

That's the average synodic month — actual cycles vary by a few hours due to orbital eccentricity.

Blue moon?

Two full moons in one calendar month — rare but real. Check the 30-day grid for back-to-back full moons.

Is anything sent to a server?

No. Lunar math runs locally.

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