Phase 4 of 8 A stretch, not a moment

The Waxing Gibbous

More than half lit and still filling out, with the shadowed part reduced to a bite out of one edge. Gibbous is from the Latin for hump-backed.

Where to look: Rises in the afternoon and sets in the small hours, so it is the phase most likely to be up when you happen to look. It also does the single most common thing anybody notices about the Moon: it is visible in broad daylight.

The longest of the eight, and the one that convinces people the Moon is out of order, because it is high in the south at dinner time and obviously not full. It is the last good week for craters before the shadows flatten out.

Commonly muddled: Being visible in the afternoon is not unusual or a sign of anything. The Moon is above the horizon for roughly twelve hours out of every twenty-five, and daylight is half of those.

#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

#032ObservingThe Moon looks enormous on the horizon and normal overhead. It is the same size in both. The effect is entirely in your head.Photograph it in both positions with the same lens and the discs measure identically, if anything the horizon Moon…NASA Moon facts

#060ObservingThe brightest spot on the Moon is a crater called Aristarchus. It's bright enough that people have mistaken it for activity on the surface.Aristarchus sits on a plateau of unusually reflective material and is geologically fresh, making it stand out…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

#079ObservingThe most common reason a meteor shower disappoints is not cloud. It's the Moon.A gibbous moon washes out everything but the brightest few meteors. Shower dates are fixed by Earth's orbit while…American Meteor Society

#080Strange but true"Gibbous" means humpbacked. English has no everyday word for the shape, so astronomy borrowed a medical one.From the Latin gibbus, a hump. It described hunched posture in medieval medical writing long before it described…Oxford English Dictionary

#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

#126GeologyHadley Rille is a kilometre and a half wide and three hundred metres deep, and it meanders like a river — but nothing has ever flowed through it except lava.Sinuous rilles are collapsed lava tubes or channels cut by molten rock, and Apollo 15 landed beside this one…Apollo 15 Preliminary Science Report, NASA…

#147ObservingWhen the Moon passes in front of a star the star does not fade, it vanishes — instantly, because there is no air up there to blur the edge.The disappearance takes a fraction of a second and is startling the first time you see it. Timing occultations was…International Occultation Timing Association

#148ObservingStand in exactly the right place — a track a few hundred metres wide — and a star will flick on and off several times as it grazes the mountains and valleys of the Moon's limb.It is the sharpest way to measure the profile of the lunar edge, which matters because the same profile decides…International Occultation Timing Association…

The Waxing Gibbous

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The Waxing Gibbous

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