The Moon does not produce its own light; it shines by reflecting sunlight, and the phase we see depends entirely on the changing angle between the Sun, the Moon and the Earth as the Moon orbits us roughly once a month. As that geometry shifts, the portion of the Moon's sunlit half that faces us grows and shrinks in a continuous, predictable cycle. Astronomers divide this cycle into eight recognisable phases.
The cycle begins at the new moon, when the Moon sits between the Earth and the Sun, so its lit side faces away from us and it is essentially invisible. Over the following days it becomes a waxing crescent, a slim curve of light on the right-hand side (as seen from the northern hemisphere) that thickens each night β "waxing" simply means growing. About a week after new moon comes the first quarter, when exactly half the disc is lit; despite the name "quarter," it looks like a half moon, and the name refers to the Moon being one quarter of the way through its cycle.
The light continues to grow through the waxing gibbous phase, "gibbous" meaning more than half but not yet full, until the Moon reaches the full moon, when it sits opposite the Sun with Earth in between, and its entire face is lit and brilliant. From here the process reverses: the Moon wanes, or shrinks. It passes through the waning gibbous, then the last quarter (again half-lit, but now the opposite half from first quarter), and finally the slender waning crescent that fades in the pre-dawn sky before the cycle returns to new moon and begins again.
The whole cycle, from one new moon to the next, takes about 29.5 days β a period known as a lunation or synodic month. This is slightly longer than the time the Moon takes to physically orbit the Earth, because the Earth is also moving around the Sun, so the Moon must travel a little further to return to the same phase. That familiar 29.5-day rhythm is the origin of the month as a unit of time, and it is the number at the heart of every moon-phase calculation, including the live one on this page.
Beyond its beauty, the Moon's phase has real, practical effects, the most significant of which is on the tides. Tides are driven by the gravitational pull of both the Moon and the Sun on the Earth's oceans. When the Sun, Moon and Earth line up β which happens at both new moon and full moon β their gravitational pulls combine to produce the largest tidal range, the so-called spring tides, with the highest high tides and lowest low tides. When the Moon is at first or last quarter, the Sun and Moon pull at right angles to each other, partly cancelling out, and we get the smaller neap tides. Anyone who fishes, boats, or spends time on the coast learns to read the Moon's phase as a guide to the tides, and this is why tide-dependent activities are so closely tied to the lunar calendar.
The phase also governs how much natural light the night holds, which matters more than people accustomed to electric lighting might think. A full moon can be bright enough to cast shadows and read by, while the nights around new moon are the darkest. This is of great importance to astronomers and stargazers, who plan observations of faint objects for the dark nights around new moon, when the sky is not washed out by moonlight. It matters to wildlife, whose behaviour and breeding cycles are often tuned to the lunar cycle, and it has mattered throughout human history for navigation, agriculture and the timing of countless traditions and festivals, many of which are still set by the phases of the Moon.
Why does the Moon look different from the southern hemisphere? The Moon appears "upside down" relative to the northern hemisphere view, because observers south of the equator are, in effect, standing the other way up. A waxing crescent lit on the right in the north appears lit on the left in the south. The phase is the same; only the orientation differs.
What is a "blue moon"? The popular modern definition is the second full moon in a single calendar month, which happens occasionally because the 29.5-day lunar cycle does not divide evenly into most months. It has nothing to do with the Moon's colour, and it is simply a quirk of fitting the lunar rhythm into our calendar.
What are supermoons and why do they look bigger? The Moon's orbit is slightly elliptical, so its distance from Earth varies. A "supermoon" is a full moon that happens to occur when the Moon is near its closest point to Earth, making it appear a little larger and brighter than an average full moon. The difference is real but modest, and often exaggerated by the well-known "moon illusion" that makes a low moon near the horizon seem enormous.
Does the Moon have a dark side? Not permanently. The Moon keeps the same face turned toward Earth, so there is a "far side" we never see from here, but that far side receives just as much sunlight as the near side over the course of a lunar cycle. The "dark side of the Moon" is really the far side, and it is only dark to us in the sense that we cannot see it, not in the sense that sunlight never reaches it.