The eight moon phases are a labeling convention, not eight events
Half the Moon is lit at all times. What changes is the angle we see it from — and how eight names get attached to one continuous 29.53-day cycle.
A moon phase is an angle, not a shape. The Moon is a sphere lit from one direction, and at every moment half of it is in sunlight. What changes across a month is where we stand relative to the lit half.
That angle — the Moon’s position east of the Sun along the ecliptic — is called the elongation. It runs from 0° to 360° over 29.53 days, and the eight familiar names are eight labels placed on that number line.
| Elongation | Name | Lit | What is happening |
|---|---|---|---|
| 0° | New moon | 0% | Moon between Earth and Sun; lit side faces away |
| 45° | Waxing crescent | 15% | Lit limb growing on the west side |
| 90° | First quarter | 50% | Half the disc lit, growing |
| 135° | Waxing gibbous | 85% | More than half lit, still growing |
| 180° | Full moon | 100% | Moon opposite the Sun; whole disc lit |
| 225° | Waning gibbous | 85% | More than half lit, now shrinking |
| 270° | Last quarter | 50% | Half the disc lit, shrinking |
| 315° | Waning crescent | 15% | Thin lit limb on the east side |
Two things in that table are load-bearing.
The lit percentage is not linear. It is (1 − cos θ) / 2, so it changes slowly near new and full and
fastest at the quarters. At 135° the disc is already 85% lit; at 170° it is 99.2%. That is why the Moon
looks full for two or three nights: the last 1% of the disc takes about 19 hours to fill.
Four names are instants and four are ranges. New, first quarter, full and last quarter happen at a computable moment. Crescent and gibbous describe where you are between them, and last for days. A calendar can only put a time on the first four — see the phase calendar for those instants across the coming year.
The month that is two months
The cycle a phase calendar counts is the synodic month: 29.530589 days from new moon to new moon. It is longer than the sidereal month of 27.321662 days — the time to return to the same place against the stars — because Earth is moving too. By the time the Moon has completed one orbit, Earth has travelled about 27° around the Sun, so the Moon needs roughly two more days to catch up to the Sun–Earth line.
That difference is the source of a persistent confusion: a calendar says a lunation is about 29.5 days, and a sky atlas says the Moon returns to the same stars in 27.3. Both are right; they measure different things.
Why the same face always looks at us
The Moon rotates once per orbit — synchronous rotation, a consequence of billions of years of tidal braking by Earth. The result is that the far side is never visible from Earth, but it is not dark: it receives exactly as much sunlight as the near side, on the same schedule. “Dark side” is a nineteenth-century phrase that survived because the far side was genuinely unknown until the Soviet Luna 3 probe photographed it in October 1959.
What the phases are not
They are not caused by Earth’s shadow. That misconception is worth naming because it predicts the wrong thing: if Earth’s shadow caused phases, the phase would depend on Earth’s position, and a lunar eclipse — the only time the shadow is actually involved — would happen every month. It does not; the Moon’s orbit is tilted 5.1° to the ecliptic, so the shadow usually passes above or below it. Phases are geometry of illumination; eclipses are geometry of shadow.
They are also not a schedule for anything on Earth. No phase has been shown to govern sleep, mood, plants, markets or weather, and this site does not publish claims of that kind — the editorial policy says why in general terms.
Checking a phase yourself
The fastest test needs no equipment. On a waxing Moon the lit limb is on the west — the side the Sun set on — and the terminator, the line between light and dark, is a curve whose ends point east. If you are in the northern hemisphere and the bright side is on the right, the Moon is waxing; if it is on the left, waning. South of the equator the whole picture is mirrored, which is why the mnemonic fails for half the planet.
If you want to know the phase for a specific date rather than understand the cycle, the lookup tool gives the elongation, the lit fraction, the age and the distance, computed from the same lunar theory this page uses.
Astronomical facts only. No phase is claimed to influence events on Earth.
Frequently asked questions
- Is there a dark side of the Moon?
- No. Half the Moon is lit at every moment except during a lunar eclipse. What people mean by "dark side" is the far side, which is lit just as often as the near side but faces away from Earth — it was first photographed in 1959 by the Soviet Luna 3 probe. The Moon rotates once per orbit, which is why the same hemisphere always faces us.
- Why are there eight phases and not four?
- Four are instants and four are ranges. New moon, first quarter, full moon and last quarter are moments when the elongation is exactly 0°, 90°, 180° and 270°. The four crescent and gibbous names cover the roughly seven days between them. A calendar gives times only to the first kind; the second kind has no single moment to name.
- Does the Moon's phase look the same from the southern hemisphere?
- No, it is inverted. Seen from south of the equator the waxing crescent is lit on the left rather than the right, because you are standing on the other side of the Earth looking at the same illuminated hemisphere from a different angle. The phases themselves happen at the same instants everywhere.
- How long is a lunar month, exactly?
- Two different answers, and they differ by about two days. The synodic month — new moon to new moon, which is what a phase calendar counts — is 29.530589 days. The sidereal month, one orbit against the stars, is 27.321662 days. The difference exists because Earth moves around the Sun while the Moon circles Earth, so the Moon has to travel a little further to line up with the Sun again.
Sources
Each source is a primary or official reference, checked before publication. Links open the original.
- NASA Science — Moon: the synchronous rotation, the 5.1° orbital inclination and the far side photographed by Luna 3 in 1959 (science.nasa.gov/moon/)
- Meeus, Astronomical Algorithms, 2nd edition, chapters 47 and 49 — the lunar theory used for the elongation, and the 29.530589-day mean synodic month
- U.S. Naval Observatory — published new, first quarter, full and last quarter instants, used to check this site's solver to within 3 minutes (aa.usno.navy.mil)
- JPL Horizons (DE441) — the lunar ephemeris against which this site's right ascension, declination and distance are measured (ssd.jpl.nasa.gov/horizons/)