ForHosting KIT · Science

Moon phase calculator

Enter a date and see the Moon's phase: on 1 January 2026 the Moon is 12.421 days old, 93.9% lit and waxing gibbous. The calculation uses the mean lunar month of 29.530588853 days. Free in your browser.

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Worked example

If you enter

Date: 2026-01-01

You get

Lunar age (days): 12.421
Illumination (%): 93.9
Phase: Waxing gibbous
Lunar cycle (days): 29.530588853

Given an ISO date, returns the Moon's age in days since a known new moon, its illuminated fraction as a percentage, and the named phase, using the mean synodic month of 29.530588853 days.

How the Moon's phase is worked out

The Moon goes from new to full and back in a mean synodic month of 29.530588853 days. The calculator counts the days between a known new moon and your date, evaluated at 12:00 UTC, and keeps only the remainder within the current cycle. That remainder is the Moon's age. The share of the disc that is lit follows a smooth curve of that age, close to 0% at new moon and peaking at 100% near full moon. On 1 January 2026 the age is 12.421 days and the illumination is 93.9%.

What the result is good for, and its limits

The named phase comes from splitting the cycle into eight bands of roughly equal length, from new moon through the crescent, quarter and gibbous stages and back again. That makes it a dependable source for calendars, almanac pages, dashboards and lunar icons for each day of a month. It is a mean-phase approximation that ignores the slightly elliptical orbit of the Moon, so the phase and illumination can be off by about a day. For precise astronomical positions, use a proper ephemeris instead.

What you can do with it

Calendars and almanac widgets

Show the phase and illumination for each day of a month, driving lunar icons from the deterministic named phase.

FAQ

What synodic period does it use?

The mean synodic month of 29.530588853 days, anchored to a reference new moon at 2000-01-06 18:14 UTC.

How accurate are the phase and illumination?

It is a mean-phase model that ignores orbital eccentricity, so results are accurate to roughly a day, ideal for calendars and dashboards.

What are the eight phases of the Moon in order?

New moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent. Each covers roughly one eighth of the lunar month, and after the last one the cycle starts again at new moon.

What does the Moon's age mean?

It is the number of days since the last new moon, running from 0 up to about 29.53 before the cycle restarts. On 1 January 2026 the age is 12.421 days, which places the Moon in its waxing gibbous phase.

At what time of day is the phase calculated?

Every date is evaluated at 12:00 UTC, the middle of the day in universal time. That keeps one answer per date, so a calendar shows the same phase and illumination wherever it is opened.

Related tools

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/dev/moon-phase

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/dev/moon-phase \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"date":"2026-01-01"}'

const res = await fetch("https://api.kit.forhosting.com/dev/moon-phase", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "date": "2026-01-01"
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/dev/moon-phase",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "date": "2026-01-01"
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/dev/moon-phase", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"date":"2026-01-01"}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"date":"2026-01-01"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/dev/moon-phase", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "date": "2026-01-01"
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "dev.moon_phase",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Technical notes

The synodic-month age model

Illumination is derived from that age as (1 - cos(2*pi*age/SYNODIC_DAYS)) / 2 * 100, a smooth curve peaking at 100% near full moon and near 0% at new moon. It evaluates each date at 12:00 UTC, takes the elapsed milliseconds since the epoch, divides by 86400000 to get days, and reduces that modulo the synodic length; a negative remainder is wrapped forward by adding one full period, so age_days is always in [0, 29.53).

How the phase name is chosen

The phase string is picked by comparing age_days against eight fixed thresholds, each roughly one eighth of the synodic month. Ages below 1.84566 are 'new', then 'waxing_crescent' below 5.53699, 'first_quarter' below 9.22831, 'waxing_gibbous' below 12.91963, 'full' below 16.61096, 'waning_gibbous' below 20.30228, 'last_quarter' below 23.99361, and 'waning_crescent' below 27.68493; anything at or above that final boundary wraps back to 'new'. Because the age for 2026-01-01 is 12.421, it falls in the waxing_gibbous band. This is a mean-phase approximation: it ignores the Moon's orbital eccentricity, so the named phase and illumination are accurate to about a day, which is more than enough for calendars, dashboards and content but not for precise ephemeris work.

Date validation and rounding

The date must be a string in strict YYYY-MM-DD form, checked by regex; month must be 1 to 12 and day 1 to 31, and the real calendar is verified with a UTC probe before any arithmetic so that an impossible date like 2026-02-30, which JavaScript's Date would silently roll over into March, is rejected as invalid_input instead of quietly accepted. The optional decimals parameter defaults to 3 and must be an integer from 0 to 6. age_days is rounded half-up to that precision, while illumination_percent is always rounded to a single decimal regardless of decimals. The response also echoes the input date and the synodic_days constant, and any non-finite intermediate raises invalid_input.

Integration use cases

Photography and outdoor planning

Query illumination_percent across a date range to find the darkest or brightest nights for astrophotography or fieldwork.

Content and scheduling automation

Batch dates through the API to tag posts, events or fishing/gardening guides with the correct moon phase.

Technical questions

Which date formats are accepted?

Only strict YYYY-MM-DD strings. Impossible dates such as 2026-02-30 are rejected as invalid_input rather than rolled over.

What are the possible phase values?

new, waxing_crescent, first_quarter, waxing_gibbous, full, waning_gibbous, last_quarter and waning_crescent, chosen by age thresholds.

Is it free and what does the API cost?

The on-page widget is free (tier A) and runs the same deterministic core; API calls are billed per successful request from $0.002.

Read the full KIT documentation →

Run — free