Half the disc lit and still filling. A first quarter moon rises around noon and sets around midnight, which makes it the easiest phase to see without staying up or getting up early.
This is the phase most people can see without changing anything about their day. A first quarter moon rises around midday, is highest in the sky at about sunset, and sets around midnight. That means it is already up and obvious during the afternoon, and it dominates the early evening sky.
The afternoon visibility surprises people. A first quarter moon is perfectly easy to see in broad daylight, pale against a blue sky, and if you have never noticed one it is because you have not looked up rather than because it was not there. It is the single best demonstration that the Moon is not a night-time object.
Northern hemisphere observers see the right half lit; southern hemisphere observers see the left, and the moon tool draws the northern view. The terminator, the straight-looking line down the middle, is where the Sun is rising on the lunar surface, and it is the most rewarding part of the Moon to look at through binoculars. Craters near that line are lit from the side and throw long shadows, so the surface looks three-dimensional in a way it never does at full moon, when the Sun is overhead there and the detail flattens out.
The phase is the angle between the Moon and the Sun as seen from Earth. At first quarter that angle is ninety degrees, which puts the Moon a quarter of the way around its circuit from new moon, and that is where the name comes from. It is a quarter of a cycle, not a quarter of a disc.
Half the visible face is lit because you are looking at a sphere illuminated from exactly the side. Illumination is one minus the cosine of the elongation, all over two, which at ninety degrees gives precisely a half. Nothing here involves Earth's shadow. A shadow on the Moon is a lunar eclipse, it happens only at full moon, and it is a different phenomenon entirely.
One point worth being exact about. First quarter is an instant: the moment the angle passes ninety degrees and the disc is exactly half lit. A name has to cover more than an instant to be usable, so the moon tool prints it for six degrees of elongation either side, which is roughly eleven hours before and after. Across that whole window the lit fraction only moves between about forty-five and fifty-five per cent, so the fifty per cent given above is the value at the instant itself and the tool is never far from it.
The quarter phases are read as points of tension in almost every system that uses them. Where the new moon is the intention and the full moon the culmination, first quarter is the point at which something started has met its first genuine resistance and a decision is required to keep going.
The almanac tradition treated the waxing half generally as the time to sow and plant, with the days approaching first quarter favoured for crops with above-ground fruit. As with every almanac rule about the Moon, the practice was consistent, widespread and unsupported by any mechanism, and controlled trials have not found the effect.
The modern reading owes a good deal to twentieth-century astrology, where Dane Rudhyar's work on the lunation cycle in the 1960s set out the eight phases as stages of an unfolding process, with the quarters as crisis points. That framework is now so common in popular writing about moon phases that it reads as traditional. It is about sixty years old, and knowing that does not make it useless, only recent. Rudhyar was explicit that he was building a psychological model rather than recovering an old one, which is more honest than most of what has been made from it since.
Phase and sign measure different things at the same moment. The phase is the angle between the Moon and the Sun; the sign is the Moon's position around the ecliptic, and it changes roughly every two and a half days whatever the phase is doing.
At first quarter the Moon is ninety degrees ahead of the Sun, which is three signs. So a first quarter moon always falls about three signs on from the Sun's current sign: during Aries season it sits in Cancer or thereabouts, and during Libra season it sits near Capricorn. That relationship is fixed by the geometry, and it is why the quarter phases are described as squares in astrological language, a square being a ninety-degree angle.
That relationship is tight enough to rely on. The tool prints first quarter only within six degrees of the exact angle, and six degrees is a fifth of a sign, so the Moon will be almost exactly three signs on from the Sun unless the Sun itself happens to sit within a few degrees of a boundary. If the precise sign matters, take it from the tool for the moment you are asking about.
The evidence on lunar effects does not change from phase to phase, so the honest summary belongs on every page rather than only on the dramatic ones.
The Moon exerts no meaningful force on a human body. Tidal force falls off with the cube of distance and scales with the size of the object being stretched, so it moves ocean basins thousands of kilometres across and does essentially nothing across something under two metres tall. The popular reasoning from the tides to the water in a person fails on the numbers.
Studies looking for lunar effects on behaviour have not found them. Rotton and Kelly's 1985 meta-analysis of thirty-seven studies reported no reliable relationship between phase and psychiatric admissions, crime or crisis calls, and subsequent work on birth rates and emergency attendance has been similarly negative. The better-supported finding is that a bright moon disrupts sleep, most plausibly through light rather than anything more exotic. A first quarter moon sets around midnight, so it lights the first half of the night and leaves the second half dark, which puts it midway between the two extremes of the cycle on that measure. If moonlight is the mechanism, this phase should sit between the new and the full on any effect, and that is a testable claim rather than a symbolic one.
The quarter phases have a long history as calendar anchors, because they divide the month into four roughly seven-day stretches. That division is one of the plausible origins of the seven-day week, though the case is circumstantial and competing explanations exist, most notably the seven classical planets.
The naming is old and has confused English speakers for centuries. Calling a half-lit moon a quarter makes sense only if you are counting position in a cycle rather than describing a shape, and the alternative name, half moon, has been in common use alongside it since at least the medieval period. Astronomical writing kept the quarter, everyday speech kept the half, and both are still current.
Galileo's observations of the Moon in 1609 and 1610 were made largely at phases like this one, and for the reason described above: near the terminator, shadows are long and the surface relief is obvious. His drawings of a mountainous, cratered Moon, published in the Sidereus Nuncius in 1610, depended on catching the light at a low angle rather than on the full face, and they overturned the idea that the Moon was a perfect sphere.
The most common misreading is the name. A first quarter moon is not a quarter lit, it is half lit, and the quarter refers to how far round the cycle it has travelled. Anybody who reads the phase name as a description of the illuminated area will find that the figures on the moon tool contradict them, and the tool is right.
The second is the assumption that half the disc being lit means half the Moon is lit. Exactly half the Moon is lit at all times, as any sphere near a distant light source is. What changes through the month is how much of the lit half is turned toward Earth, which is a fact about viewing angle rather than about illumination.
The third is treating the quarter as a crisis. Popular writing about phases leans heavily on the language of tension and decision points, which comes from a twentieth-century framework rather than from anything ancient, and it can be read as an instruction that something must be confronted now. Nothing in the astronomy supports that, and a person who reads a scheduled crisis into an ordinary Tuesday has been handed a difficulty by the calendar. The useful version is milder: a recurring marker roughly a week after new moon is a convenient point to check whether something started is still moving. Compare last quarter, which sits at the same angle on the other side.
See where the Moon is right now.