Moon Phase Calculator

In short The moon is always exactly half lit by the sun. What we call the phase is simply how much of that lit half happens to be turned toward us, and it follows directly from one angle: the elongation, the angle between the moon and the sun as seen from earth. At 0 degrees the lit side faces entirely away and we see a new moon; at 180 degrees it faces us and the moon is full; the quarters sit at 90 and 270 with exactly half the visible disc lit. The illuminated fraction is not linear in time, which is why a moon three days past new looks like a thin sliver rather than a fifth. One cycle from new moon to new moon is the synodic month, and its long-run mean is 29.530589 days. Individual months are not that: they range from about 29.28 to 29.82 days, because both the earth orbit and the moon orbit are ellipses, so the geometry that defines a phase does not come round at a constant rate. That variation is real and this tool shows the actual length of the cycle you are in. Two things worth clearing up. The far side of the moon is not the dark side — it receives exactly as much sunlight as the near side over the same month, and it is the far side only because the moon is tidally locked and always shows us the same face. And the lit edge points in different directions depending on where you are: on the right in the northern hemisphere while waxing, on the left in the southern, and lying along the bottom near the equator.