Phase 3 of 8 Next: Sunday 18 October 2026

The First Quarter

Exactly half lit, and growing. Called a quarter because it is a quarter of the way through the cycle, which is the single most confusing piece of naming in astronomy.

Where to look: Rises around noon, is highest at sunset, and sets around midnight. The best-placed phase for anybody who wants to look at the Moon in the early evening without staying up.

The terminator — the line between lit and unlit — runs straight down the middle of the disc, and it is where every crater is at its most dramatic, because the Sun is rising there and the shadows are as long as they ever get. A first quarter moon through cheap binoculars beats a full moon through a good telescope.

Commonly muddled: Half the disc is lit, so people call it a half moon and then wonder why the calendar says quarter. Both are right about different things.

#009ObservingPoint binoculars at the line between light and dark. That's where the Moon stops being a disc and becomes a landscape.Along the terminator the Sun sits near the lunar horizon, so crater rims cast long shadows and mountain peaks catch…NASA Moon observing

#071Phases & orbitNobody in an observatory calls it a half moon. "First quarter" counts the orbit, not the shape.By the time half the disc is lit, the Moon has covered 90° of its circuit, one quarter of the way from new moon to…U.S. Naval Observatory

#072Strange but trueAround 270 BC, a Greek worked out how far away the Sun is using nothing but a half moon and an angle.When the Moon is exactly half lit, the Sun, Moon and Earth form a right angle at the Moon, so measuring the…Aristarchus, On the Sizes and Distances (c…

#073ObservingFor about four hours before every first quarter, sunlight catches three crater rims at the right angle and spells a letter X on the Moon.The Lunar X sits on the terminator near the craters Blanchinus, La Caille and Purbach: the rims are already in…NASA Moon observing

#074GeologyThere is a 110-kilometre cliff on the Moon that looks drawn with a ruler. It's a black line after first quarter and a white one after last.Rupes Recta, the Straight Wall, is a fault where the crust dropped a couple of hundred metres along an almost…Lunar Reconnaissance Orbiter

#075ObservingThe first quarter moon rises around noon. Roughly half of all the Moon's time above the horizon is spent in a blue sky, and almost nobody looks up.Only the full moon keeps strictly to the night, rising at sunset and setting at sunrise. Every other phase shares…U.S. Naval Observatory

#076ObservingThe line between light and dark on a waxing moon is a sunrise. On a waning moon it's a sunset. You are watching dawn cross another world at cycling pace.The terminator sweeps the lunar equator at about 15 km/h. On the waxing half it is the morning line, uncovering…NASA Moon observing

#087GeologyNear first quarter, the Moon seems to grow mountains. Low sunlight casts long shadows across the lunar Apennines, making the range stand out in binoculars. Apollo 15 landed at the range’s edge and explored its foothills.Montes Apenninus runs roughly 600 km along the rim of Mare Imbrium, with peaks rising more than 5 km. Near first…Lunar Reconnaissance Orbiter

#151ObservingThe line between lunar day and night creeps along the Moon's equator at about 15 kilometres an hour, and much slower nearer the poles.It follows from a 10,900-kilometre circumference and a 29.5-day day. At the equator you would need a bicycle to…NASA Moon facts

#155ObservingObservers who care about lunar shadows do not talk about phases. They use colongitude — the longitude of the sunrise line on the Moon — because that is what decides how a crater will look tonight.It runs from 0 degrees at first quarter through 90 at full and round again, advancing about half a degree an hour…Rukl, Atlas of the Moon; BAA Lunar Section

#156ObservingThe length of a shadow, the height of the Sun above the lunar horizon and a little trigonometry give the height of the peak throwing it, and that is how the first lunar mountains were measured in 1610.Galileo had trained as an artist and understood shadows; he worked out that some peaks were several kilometres…Galileo, Sidereus Nuncius (1610)

#181Strange but trueWith no air to scatter light, the Moon has no twilight. The Sun goes below the horizon and the landscape goes out.On Earth the sky stays lit for the better part of an hour after sunset because the atmosphere above you is still…Apollo Lunar Surface Journal; NASA Moon facts

The First Quarter

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The First Quarter

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