Moon Phases Explained: Why the Moon Changes Shape
The Moon's phase is the part of its sunlit half we can see from Earth. The eight phase names, the 29.5-day cycle, why it isn't Earth's shadow, and 2027 dates.
The Moon’s phases happen because the Sun always lights exactly half of the Moon, and as the Moon orbits Earth we see a changing fraction of that lit half. The full cycle, from new moon back to new moon, takes 29.53 days on average. Earth’s shadow plays no part, except during a lunar eclipse.
The eight phases
Astronomers define four instants precisely and call the stretches between them by descriptive names. The four principal phases are set by the angle between the Moon and the Sun as seen from Earth (strictly, the difference in their ecliptic longitudes): 0° for new moon, 90° for first quarter, 180° for full moon, 270° for last quarter.
| Phase | What you see (Northern Hemisphere) | Angle from the Sun |
|---|---|---|
| New moon | Nothing; the lit side faces away from us | 0° (an instant) |
| Waxing crescent | Thin sliver, lit on the right | between 0° and 90° |
| First quarter | Right half lit | 90° (an instant) |
| Waxing gibbous | More than half lit, growing | between 90° and 180° |
| Full moon | Whole disc lit | 180° (an instant) |
| Waning gibbous | More than half lit, shrinking | between 180° and 270° |
| Last (third) quarter | Left half lit | 270° (an instant) |
| Waning crescent | Thin sliver, lit on the left | between 270° and 360° |
Waxing means growing, waning means shrinking, and gibbous means more than half lit. The “quarter” in first and last quarter refers to a quarter of the way through the cycle, not to the amount of disc you see, which is half.
Because the principal phases are instants, a calendar listing “full moon, 22 March” really means the moment of full moon falls on that date in the stated time zone. To the eye the Moon looks full for a day or two either side.
Two kinds of month
The Moon takes 27.32 days to complete one orbit relative to the distant stars. This is the sidereal month. But in that time Earth has also moved about a thirteenth of the way around the Sun, so the Moon needs roughly two more days to get back to the same position relative to the Sun. That longer period, 29.53 days, is the synodic month, and it is the one that governs phases.
| Length | Measured relative to | |
|---|---|---|
| Sidereal month | 27.32 days | The stars |
| Synodic month | 29.53 days | The Sun (phases) |
The synodic month is an average. Because the Moon’s orbit is elliptical and perturbed by the Sun, individual lunations vary by several hours either side of it. This is why the 2027 dates below are not evenly spaced.
Twelve synodic months add up to about 354 days, eleven days short of a solar year. That mismatch is why lunar calendars drift through the seasons, and why lunisolar reckonings such as the one behind the date of Easter need occasional corrections.
It is not Earth’s shadow
A common explanation of phases is that Earth’s shadow covers part of the Moon. It does not. At a crescent or quarter phase the Moon is well off to one side of the Earth–Sun line, nowhere near our shadow. You can check this yourself: a crescent moon is always close to the Sun in the sky, low in the west after sunset or low in the east before sunrise. Earth’s shadow points directly away from the Sun.
Earth’s shadow does reach the Moon during a lunar eclipse, and that can only happen at full moon, when Sun, Earth and Moon line up. The edge of the shadow on an eclipsed Moon is a curved, fuzzy arc, quite unlike the sharp terminator of a normal phase, and a totally eclipsed Moon often turns a dull red rather than disappearing.
Eclipses do not happen every month because the Moon’s orbit is tilted about 5° to Earth’s orbit around the Sun. Most full moons pass above or below the shadow; most new moons pass above or below the Sun. A solar eclipse is the new-moon counterpart: the new moon of 2 August 2027 produces a total solar eclipse across parts of southern Europe, North Africa and the Middle East.
When each phase is up
The phase tells you roughly where the Moon is relative to the Sun, so it also tells you roughly when the Moon rises and sets. The times below are approximate and shift with latitude and season.
| Phase | Rises around | Highest around | Sets around |
|---|---|---|---|
| New moon | Sunrise | Noon | Sunset |
| First quarter | Noon | Sunset | Midnight |
| Full moon | Sunset | Midnight | Sunrise |
| Last quarter | Midnight | Sunrise | Noon |
On average the Moon rises about 50 minutes later each day, because it moves about 12–13° eastward against the sky daily. This is why a first-quarter moon is easy to see in the afternoon and a last-quarter moon lingers in the morning sky. For exact moonrise and moonset times, see the London moon page or New York moon page.
Same phase everywhere, different orientation
At any given instant the Moon is in the same phase for everyone on Earth. A full moon in Tokyo is a full moon in Buenos Aires. What can differ is the calendar date: a full moon at 23:04 UTC on 15 September 2027 falls on 16 September in East Asia.
What also differs is how the Moon looks. Observers in the Southern Hemisphere see the Moon “upside down” compared with the north. A waxing crescent that is lit on the right in London is lit on the left in Sydney. Near the equator the lit side tends to face up or down, so a young crescent can lie almost flat, like a bowl.
Blue moons
“Blue moon” has nothing to do with colour. It has two definitions in use:
- Seasonal blue moon (the older one): the third full moon in an astronomical season that contains four full moons instead of the usual three. The spring of 2027, from the March equinox to the June solstice, has four full moons (22 March, 20 April, 20 May and 19 June), so the full moon of 20 May 2027 is a seasonal blue moon.
- Monthly blue moon: the second full moon in a single calendar month. This usage spread after a 1946 article in Sky & Telescope misread the seasonal rule. In UTC, May 2026 had full moons on 1 and 31 May; the next is 31 December 2028 (on 1 January 2029 in time zones well east of UTC, so whether it is a “blue moon” depends on where you are).
There is no monthly blue moon in 2027.
New and full moons in 2027
All times are UTC, rounded to the minute, computed with the same algorithm used on the site’s 2027 calendar. Convert to your own zone with the time zone converter.
| Month | New moon | Full moon |
|---|---|---|
| January | 7 Jan, 20:25 | 22 Jan, 12:18 |
| February | 6 Feb, 15:56 | 20 Feb, 23:24 |
| March | 8 Mar, 09:30 | 22 Mar, 10:44 |
| April | 6 Apr, 23:51 | 20 Apr, 22:27 |
| May | 6 May, 10:59 | 20 May, 10:59 |
| June | 4 Jun, 19:41 | 19 Jun, 00:45 |
| July | 4 Jul, 03:02 | 18 Jul, 15:45 |
| August | 2 Aug, 10:05 and 31 Aug, 17:41 | 17 Aug, 07:29 |
| September | 30 Sep, 02:36 | 15 Sep, 23:04 |
| October | 29 Oct, 13:37 | 15 Oct, 13:47 |
| November | 28 Nov, 03:25 | 14 Nov, 03:26 |
| December | 27 Dec, 20:13 | 13 Dec, 16:09 |
August 2027 has two new moons, the new-moon equivalent of a monthly blue moon. There is no common name for it.
Quick reference
- The phase depends only on the Sun–Earth–Moon angle, so it is the same worldwide at any instant.
- A crescent is always near the Sun in the sky; a full moon is always opposite it.
- If the right side is lit (Northern Hemisphere), the Moon is waxing and best seen in the evening. Reverse that in the Southern Hemisphere.
- Phases repeat every 29.53 days on average; the Moon’s orbit around Earth takes 27.32 days.
Sources
- NASA Science: Moon Phases
- U.S. Naval Observatory: Phases of the Moon and Percent of the Moon Illuminated
- U.S. Naval Observatory: Phases of the Moon (data service)
- NASA Science: Lunar Eclipses and Solar Eclipses
Dates, offsets and astronomical times quoted here are computed with the same code that powers the site's tools — see data sources and methods. Found an error? Tell us.