Moon Phase Calculator

How to use this tool

Pick a date. The page opens on today at noon in your own time zone and recalculates as you type. Add a time if a particular hour matters, and switch to the southern hemisphere if that is where you stand; the drawing flips left to right. The cards list each surrounding principal phase in your zone with the UTC time beside it.

The input people most often get wrong is the offset. It is applied exactly as entered, with no daylight saving logic, so Toronto is UTC−4 from March to November and UTC−5 in winter. The wrong choice shifts every time by an hour, and a phase near midnight can land on the wrong calendar day. The instant of a phase, the age and the lit fraction are the same everywhere; only the clock reading changes.

What the calculation is doing

Phase instants come from chapter 49 of Jean Meeus, Astronomical Algorithms. With k the lunation index, zero at the new moon of January 6, 2000, and T = k ÷ 1236.85, the mean phase in Julian Ephemeris Days is JDE = 2451550.09766 + 29.530588861 k + 0.00015437 T² − 0.000000150 T³ + 0.00000000073 T⁴, with k raised by 0.25, 0.5 or 0.75 for first quarter, full moon and last quarter. Because the orbit is elliptical and perturbed by the sun, the true instant can sit up to about 19 hours from the mean, so 25 periodic and 14 planetary terms are added. Subtracting ΔT, the gap between this dynamical time and clock time (69.1 seconds in 2026, from the US Naval Observatory), gives UTC.

The lit fraction comes from positions, not phase times. The moon (chapter 47) and sun (chapter 25) give their separation ψ from cos ψ = cos β cos(λ − λ⊙), with β the latitude of the moon and λ, λ⊙ the longitudes; with R and Δ the distances of sun and moon, tan i = R sin ψ ÷ (Δ − R cos ψ) gives the phase angle, and the fraction is (1 + cos i) ÷ 2. A calendar day in your zone holding a new moon, quarter or full moon takes that name; other days are crescent or gibbous. The 29.53 day count from January 6, 2000 appears for comparison only: it is out by 0.29 days on average and 0.75 at worst.

Why no two lunations are the same length

The 29.53 days everyone quotes is an average. The moon moves faster near perigee and slower near apogee, and the sun stretches the orbit through the year, so in 2026 the gap from one new moon to the next runs from 29.28 to 29.81 days, and over the long run from about 29.27 to 29.83. A fixed count therefore drifts up to three quarters of a day from the true phase: harmless for planning a night walk, decisive when the question is which date holds the full moon or which month gets a second one.

Full moon is rarely 100.0% lit for a related reason. Phases are defined by longitude, but the orbit is tilted 5.1° to the ecliptic, so the moon usually passes a few degrees above or below the point opposite the sun. At the instant of full moon in 2026 the lit fraction ranges from 99.81% to 99.97%, except on March 3 and August 28, the only two to reach 100.00%; both are lunar eclipses, a reminder that the fraction is pure geometry and ignores the shadow of the Earth. The new moons nearest a node, February 17 and August 12, are the solar eclipses of 2026, and the calculator puts both within 11 minutes of the NASA time of greatest eclipse.

The twelve lunations that begin in 2026

Every principal phase in UTC for the lunations starting with the new moons of 2026. The January 3 full moon belongs to the lunation that began in December 2025, so only the year table in the tool shows it.

A worked example you can check on paper

Enter September 22, 2026 at 12:00 with the offset at UTC. Noon UTC is Julian Day 2,461,306.0000; adding ΔT = 69.1 seconds (0.0008 day) gives JDE 2,461,306.0008. The lunation index is k = 330 (Brown lunation 1283), so T = 330 ÷ 1236.85 = 0.26681 and the mean new moon is 2451550.09766 + 29.530588861 × 330 + 0.00015437 × 0.26681² = 2,461,295.1920. The periodic and planetary terms add −0.5475 day, giving JDE 2,461,294.6445, which is Julian Day 2,461,294.6437 in UTC once ΔT comes back off: 03:27 UTC on September 11, 2026, the same minute as the USNO table.

Age = 2,461,306.0000 − 2,461,294.6437 = 11.36 days, and with k = 330.5 the full moon falls at 16:49 UTC on September 26, 4.2 days ahead, so the day is a waxing gibbous. The moon is 130.00° east of the sun and 1.80° south of the ecliptic, so ψ = 129.98°. With the moon 397,423 km away and the sun 150.2 million km, tan i = R sin ψ ÷ (Δ − R cos ψ) gives i = 49.90°, and (1 + cos 49.90°) ÷ 2 = (1 + 0.6441) ÷ 2 = 0.8220: the disc is 82.2% lit, against 82% in the USNO daily table. The quick count gives an age of 10.65 days, 0.7 day short, yet by luck of the cosine a fraction of 82.0%.

Mistakes that catch people out

Using the winter offset in summer. UTC−5 for Toronto in July puts every time an hour early, and a full moon at 00:30 local slides to the previous day. Pick the offset the clocks show on that date.

Reading the age as the days to full moon. The age counts from the last new moon, and full moon comes at about 14.8 days, so an age of 20 days is a waning gibbous five days past full. Read the card for the next full moon instead.

Adding 29.53 days to a known full moon. Counting eight mean lunations on from the January 3, 2026 full moon lands on August 27 at 15:55 UTC; the real full moon is August 28 at 04:19, 12.4 hours later and on the next date.

Forgetting which way up you are. A waxing moon is lit on the right from Canada and on the left from Australia, and near the equator an evening crescent is lit from below. Set the hemisphere and the drawing follows.

Popular calculations

Each address opens the calculator on that date, with the time and offset filled in where they matter.

Full moons of 2026

New moons of 2026

Eclipses of 2026, at greatest eclipse

Other dates

How this was checked

The Meeus transcription was checked against the worked examples in the book: the new moon of February 1977 (example 49.a, JDE 2443192.65118) and the last quarter of January 2044 (49.b, JDE 2467636.49186) to five decimals, the position of the moon in 47.a to six decimals and the lit fraction of 0.6786 in 48.a to within 0.0001. All 198 phases in the US Naval Observatory tables for 2000, 2024, 2025 and 2026 were then compared: the largest difference is 0.84 minutes and the mean 0.29, with the calculator 8.5 seconds later on average. Both round to the minute, so 165 displayed times match and 33 differ by one. On nine dates the lit fraction rounds to the USNO daily percentage every time, the largest gap before rounding being 0.46 points.

As an independent route, the instant when the chapter 47 moon longitude minus the chapter 25 sun longitude reaches 0°, 90°, 180° or 270°, found by bisection, agrees with the chapter 49 series within 1.14 minutes (mean 0.34) over 341 phases from 1900 to 2100, about the accuracy of the low precision sun. Three thousand random instants from 1700 to 2100 kept the age inside its lunation and the fraction between 0 and 1. Eclipse times are from the NASA catalogues. Every address above was loaded to confirm a distinct title and canonical link, and the page was checked at eight widths down to 320 pixels.

Common questions

What phase is the moon in tonight?

The calculator opens on today at noon in your time zone; set an evening time for the figure while you are outside, since near a quarter the lit fraction moves up to 6 percentage points between noon and midnight. The name belongs to the calendar day, so a day holding a full moon is a full moon day even if the instant falls at breakfast.

When is the next full moon?

Enter today and read the line under the phase name. The remaining full moons of 2026 are October 26 at 04:12 UTC, November 24 at 14:54 UTC and December 24 at 01:28 UTC. The table inside the tool lists every phase of any year you enter.

Why does the calculator show 99.9% rather than 100% at full moon?

Full moon is defined by longitude alone, and the moon usually passes a few degrees north or south of the point opposite the sun, so a sliver of the far edge stays dark. Only a full moon near a node reaches 100%, which makes a lunar eclipse: March 3 and August 28 in 2026.

Why does my calendar give a different date for the full moon?

Calendars use the local time of the publisher. The full moon at 01:28 UTC on December 24 is December 23 at 20:28 in Toronto and 17:28 in Vancouver. Pick your own offset and the dates come out for your zone.

How accurate are the phase times?

Within a minute of the US Naval Observatory, the largest gap being 0.84 minutes over 198 phases; because both round, a displayed time can differ from the USNO table by one minute. Outside 1973 to 2033 ΔT is fitted or extrapolated, so the error grows with distance from the present.

What is a blue moon, and is there one in 2026?

A blue moon is usually the second full moon in a calendar month, which comes about every two and a half years because 12 lunations fall 11 days short of a year. May 2026 has two, May 1 at 17:23 UTC and May 31 at 08:45 UTC, so May 31 is a blue moon in every time zone.

Related Calculators and Reference

• Sunrise and Sunset Calculator: when it gets dark where you are.
• Date Duration Calculator: count the days between two phases.
• Days From Today Calculator: turn a phase 18 days away into a calendar date.
• Time Zone Converter: find which offset a city uses on a given date.
• Week Number Calculator: the ISO week a full or new moon falls in.
• US Naval Observatory, Dates of Primary Phases of the Moon: the official tables this calculator was checked against.