Moon Phase Today

The 29.53 day cycle this page counts

Behind the graphic, the page works out a single number: how far through its cycle of phases the Moon has travelled since the last new moon.

It converts the date you choose into a Julian Day at noon, subtracts a reference new moon (Julian Day 2451550.1, which falls on 6 January 2000), and divides what is left by 29.53058867 days. The whole part of that division is discarded. The fractional remainder, a value between 0 and 1, is the Moon's age.

That divisor is the synodic month: the average gap from one new moon to the next. It is noticeably longer than the sidereal month of 27.32 days, the time the Moon takes to complete one orbit measured against the fixed background stars.

The gap exists because Earth does not stand still. While the Moon goes once around us, we travel about a thirteenth of the way around the Sun. By the time the Moon has finished a lap relative to the stars, the Sun sits in a different direction, so the Moon must keep going for roughly another two and a bit days before it lines up the same way again. Phases are defined by the Sun, so the phase cycle is the longer figure.

Waxing, waning, and the eight names

An age of 0 is new moon, 0.25 is first quarter, 0.5 is full moon and 0.75 is last quarter. The tool rounds the age into eight equal buckets centred on those points, which is where the names come from.

Waxing covers the first half of the cycle, when the lit area is growing. Waning covers the second half, when it is shrinking. Crescent means less than half lit; gibbous means more than half lit but not yet full.

First quarter is the phase name most people misread. It does not mean a quarter of the disc is lit. It means a quarter of the cycle has gone by. At that moment you see a half disc.

What the illumination figure is measuring

The percentage is the fraction of the Moon's visible face that the Sun is lighting, calculated here as a smooth cosine curve running from 0 per cent at new moon to 100 per cent at full.

That curve is not evenly paced, which explains something people notice from the backyard. Around the two quarter phases, the lit fraction shifts by roughly ten percentage points a day, so the Moon visibly changes overnight. Within a day either side of full, it barely moves at all, which is why a full moon looks identical on three consecutive nights even though the age counter on this page keeps ticking over.

Reading the disc from the southern hemisphere

This is worth knowing before you compare the graphic with the sky. The shadow drawn here follows the northern hemisphere convention: waxing moons are lit on the right, waning moons on the left. The phase emoji in the calendar strip are drawn the same way.

Seen from Australia, New Zealand or anywhere else south of the equator, you are effectively standing the other way up relative to a northern observer, so the Moon appears rotated by half a turn. A waxing crescent that a diagram shows lit on the right will look lit on the left from Brisbane or Perth.

Nothing about the underlying numbers changes. The phase name, the illumination percentage and the date of the next full moon are the same everywhere on Earth. Only the orientation of the lit edge flips.

Spring tides, supermoons and blue moons

Tides respond to the geometry the phase describes. At new and full moon the Sun and Moon pull along the same line, the two effects add, and you get spring tides with the largest range between high and low water. At the quarters the two pulls act at right angles and partly cancel, giving neap tides with the smallest range. Each type comes round about twice a month. The word spring here has nothing to do with the season; it carries the older sense of springing up.

A supermoon is a full moon falling near perigee, the closest point of the Moon's slightly elliptical orbit. The Earth to Moon distance swings between roughly 356,500 and 406,700 kilometres, so a perigee full moon appears about 14 per cent wider than an apogee one. Obvious in side by side photographs, hard to pick in the sky alone. This page models phase only, not distance, so it does not flag supermoons.

A blue moon involves no colour at all. The older seasonal definition is the third full moon in a season that contains four. The now more common monthly definition, a second full moon inside one calendar month, began as a misreading in a 1946 magazine article and stuck. Both are possible only because 29.53 days is shorter than most calendar months.

Where this model stops being exact

The 29.53 day figure is an average. Real intervals between new moons run from about 29.18 to 29.93 days, because the orbit is elliptical and the Sun tugs on it unevenly through the year. A mean model like this one can therefore place the exact instant of new or full moon out by several hours, and occasionally by close to a day.

In practice the illumination percentage is reliable, the phase name is reliable except on the day either side of a boundary, and the counters are rounded to whole days. For tide planning, eclipse timing or astrophotography, check a published ephemeris instead.

The date you pick is handled entirely inside your browser, so nothing you look up leaves your device. If you are counting days to or from a lunar event, the Date Difference Calculator and the Age Calculator do that arithmetic directly, and the Chinese Zodiac Calculator covers the lunisolar calendar that these same cycles underpin.

Questions people ask

Why is the moon phase the same date everywhere in the world?

Phase depends on the angle between the Sun, Earth and Moon, which is a single geometry shared by the whole planet. Everyone sees the same phase at the same moment. What differs is the local clock time, and the orientation of the lit edge depending on your latitude.

Does the picture on this page match what I see from Australia?

Not in orientation. The graphic uses the northern hemisphere convention, with waxing moons lit on the right. From southern latitudes the lit side appears on the opposite edge, so mentally rotate the image half a turn. The percentage and the phase name still apply.

How far ahead or back can I check?

Any date the picker accepts. The arithmetic simply counts synodic months forward or back from the year 2000 reference, so it works for past decades and future ones, with the accuracy drifting a little further out the more centuries you travel.

Why does the calendar strip sometimes show the same emoji for several days running?

There are only eight phase symbols for a cycle of about 29.5 days, so each one covers roughly three to four days. The strip shows which of the eight buckets each date falls into, not a unique image per night.