The phases of the Moon
The Moon is always half lit by the Sun. How much of that lit half you see depends on where the Moon is in its orbit, and that is what the phases describe. The U.S. Naval Observatory defines the four principal phases by the difference between the Moon's and the Sun's position along the ecliptic: new moon at 0 degrees, first quarter at 90, full moon at 180 and last quarter at 270 degrees. Each of them is a single instant, the same for everyone on Earth.
Between them lie the intermediate phases, each lasting about a week. While the lit part grows, the Moon is waxing: a waxing crescent when less than half of the disk is lit, a waxing gibbous Moon when more than half is lit. After full moon it is waning, first gibbous, then a crescent again.
| Phase | Disk illuminated |
|---|---|
| New moon | 0% |
| Waxing crescent | between 0% and 50%, growing |
| First quarter | 50% |
| Waxing gibbous | between 50% and 100%, growing |
| Full moon | 100% |
| Waning gibbous | between 100% and 50%, shrinking |
| Last quarter | 50% |
| Waning crescent | between 50% and 0%, shrinking |
Near full moon the illuminated share changes slowly, so the Moon can look full for several nights in a row. USNO gives about 97 to 99 percent illuminated on the nights just before and after the exact full moon, and 93 to 97 percent two days away from it. The calendar names a principal phase only on the date it happens in the time zone you picked.
Moon age and the lunar cycle
The age of the Moon is the time since the last new moon, in days. One full lunar cycle, from new moon to new moon, takes about 29.5 days on average, according to the U.S. Naval Observatory. NASA notes that the Moon needs about 27.3 days for one trip around the Earth, but because the Earth moves along its orbit at the same time, it takes about 29.5 days to get back to the same phase. That is why most months have one full moon and some have two.
The Moon from the Southern Hemisphere
The icons show the Moon as it looks from the Northern Hemisphere, where a waxing Moon is lit on the right. The Royal Astronomical Society of New Zealand explains that observers in the Southern Hemisphere see the Moon the other way up, so before full moon the left side is lit. Switch the view to see the phases that way.
How the calculator works
The times of new moon, first quarter, full moon and last quarter come from the method in chapter 49 of Jean Meeus, Astronomical Algorithms, with all of its periodic and planetary correction terms. The illuminated share of the disk uses the phase angle from chapter 48 of the same book. Meeus works in Dynamical Time, so the calculator subtracts the difference to ordinary clock time with the formulas Espenak and Meeus published on NASA's eclipse website. We checked 24 principal phases in 2000, 2026 and 2030 against the U.S. Naval Observatory's tables: all agreed within one minute. Everything is calculated in your browser for any date from 1900 to 2100.
How to use the moon phase calendar
- 1Open the page to see the moon phase right now, or pick any date to see the Moon at noon that day.
- 2Choose your time zone or UTC, and the Northern or Southern Hemisphere view.
- 3Read the next phases, and page through the month calendar with the arrows. Tap a day to see its phase, illumination and age.
Frequently asked questions
What is the moon phase today?
The calculator at the top of this page shows it for the current moment in your time zone: the phase name, how much of the disk is lit and the age of the Moon in days since the last new moon. On the day of a new moon, first quarter, full moon or last quarter, that phase is named; on all other days it is a waxing or waning crescent or gibbous Moon.
When is the next full moon?
The next full moon is shown right under the result, with its date and time in your time zone. The list of upcoming phases and the month calendar show the following new moon, first quarter, full moon and last quarter as well.
Why does the full moon date differ between countries?
A full moon happens at one instant for the whole Earth. When that instant is late in the evening in one time zone, it is already the next day in a time zone further east. Switch the calculator between your time zone and UTC to see this.
What is the age of the Moon?
The age is the time since the last new moon, in days. The U.S. Naval Observatory gives the average length of a full cycle of phases as about 29.5 days, so the age runs from 0 at new moon to about 29.5 just before the next one.
How accurate are the phase times?
The times use the method in chapter 49 of Jean Meeus, Astronomical Algorithms. We compared 24 new moons, quarters and full moons in 2000, 2026 and 2030 with the tables of the U.S. Naval Observatory, and all agreed within one minute.