How to observe a gibbous Moon
A gibbous Moon is a spectacular sight. Here are some of the top features to see on a waxing and waning gibbous.
The term 'gibbous Moon' describes the phase when the side of the Moon that faces Earth appears more than half-lit by the Sun, but is less than fully lit.
Visually speaking, a gibbous Moon is the opposite of a crescent Moon. It can either be a waxing gibbous - as it approaches full Moon - or a waning gibbous - as it approaches new Moon, and is punctuated by the first and last of the quarter Moon phases.
This is perhaps better illustrated if we consider the phases of the Moon, stage-by-stage.
During a lunar month, the Moon goes from new Moon, through the waxing crescent phase up to first quarter when the Moon's disc appears half illuminated.
As more of the Earth-facing side of the Moon is illuminated by the Sun each day, this is known as the waxing gibbous phase, which occurs until full Moon, when the lunar disc is fully illuminated.
Then, the cycle reverses and goes from full Moon, through a waning gibbous phase until last quarter when, again, the lunar disc is half-illuminated (but this time on the opposite side).
Slowly, less and less of the Moon's disc is illuminated during the waning crescent phase, until we're back to new Moon and the cycle begins again.
The waxing gibbous phase lasts from about day 8 to day 14 (full Moon) of the lunar cycle, and the waning gibbous phase lasts from about day 15 to day 22 (last quarter).
Observing a gibbous Moon
Unlike a full Moon, when the entirety of the Earth-facing lunar disc is lit by sunlight, during the gibbous phase, the terminator (the line dividing the lit and unlit portions of the disc) moves steadily across the surface.
If you observe the Moon during the gibbous phase, night after night you'll see the terminator creep across the lunar disc, casting shadows on certain features such as craters and lunar maria.
As a result, the gibbous phase makes for a great time to observe the Moon with binoculars or telescope to get a closer look at some of its features.
Let's take a look at some of the key features to spot on the lunar surface during the gibbous phases.
Or if lunar sketching is more your thing, read our guide on how to draw the Moon.
Observing a waxing gibbous
Day 8: Rupes Recta
Just south of the Moon’s equator is the Moon’s sixth biggest mare – the vast Mare Nubium, a basin 750km (466 miles) in diameter.
Look for a pencil-thin dark line running almost parallel to the nearby terminator. That’s the shadow cast by Rupes Recta, a 110km- (68 mile) long fault better known as the Straight Wall.
Roughly 300m (1,000ft) high, it rises above the mare at an angle of about 20°.
East of the mare, the second great trio of craters is now lit: Ptolemaeus, Alphonsus and Arzachel. At 153km (96 miles) wide, Ptolemaeus is the largest.
If the conditions are right, and your telescope is up to the job, you may be able to spot saucer-like depressions on the shallow crater floor.
To the south of Ptolemaeus is the 118km (74 mile) Alphonsus, and below Alphonsus is the 97km- (61 mile) wide crater Arzachel. It boasts high ramparts and a tall central peak.
Meanwhile, just to the northeast of Mare Imbrium, the beautiful dark-floored crater Plato has emerged.
Day 9: Copernicus
The crater Copernicus, just below Mare Imbrium, is a spectacular sight.
Use high magnifications to explore this 93km-(58 mile) wide crater and you’ll see its terraced walls, central peaks and the shadows cast by its eastern ramparts on the crater’s floor.
Its rim is about 900m (3,000ft) above the surrounding plain, and it’s easy to see debris stretching away in all directions.
Despite the crater’s rugged look, the Apollo 17 mission revealed it was quite shallow.
If you now follow the terminator south, you’ll see a large crater standing out from the rest near the bottom of the Moon.
This giant is Clavius – at 225km (141 miles), it’s often incorrectly labelled the Moon’s largest (the 256km Bailly is the record holder).
Clavius crater’s floor and walls have numerous craterlets; the larger your scope, the more you’ll spot. Can you see the clair-oscur effect known as the Eyes of Clavius? This trick of the light is best seen 1 day after first quater.
Above Clavius is the 85km (53 mile) crater Tycho. Explore it tonight when the shadows are long..
Day 11: Mare Humorum
Nicely illuminated in the southwest is Mare Humorum, an oft-overlooked lunar sea.
Search for craterlets and wrinkle ridges on Humorum’s dark floor. Four conspicuous craters flank this mare.
The biggest is the 110km (69 mile) Gassendi to the north, with its shallow floor, channels (‘rilles’), hills and central peak.
In the west is Mersenius, on the southern edge is Doppelmayer and to the east is the 58km (36 mile) Hippalus, a half crater flooded by lava from Humorum.
Now follow the terminator to the northwest portion of the Moon and you’ll see the 40km (25 mile) Aristarchus Crater, the brightest large crater on the Moon.
Northwest of it is the Vallis Schröteri or Schröter’s Valley. This 160km (100 mile) rille was formed by lava flowing from the Cobra’s Head – a lunar volcano.
Observing a waning gibbous
Day 16: Petavius
With Mare Crisium’s eastern edge now in darkness, the mountains that rim this sea show enough shadow to give a three-dimensional aspect to the scene.
At high magnification, the small craters Picard and Peirce in western Crisium pop into view. On the mare’s eastern floor at least one wrinkle ridge is visible.
Far south of Crisium lies a string of four large craters. There’s the terraced 132km (83 mile) Langrenus with a nice peak, the shallow Vendelinus, Petavius with its massive central peak, and Furnerius.
Day 19: Apollo 11 landing site
Tonight, find the site of the famous Apollo 11 landing site, where Neil Armstrong became the first man to take a ‘small step’ onto the lunar surface.
You won’t be able to see the lunar module though, because no telescope is powerful enough.
Look near the terminator in the southeast portion of the Moon and find three craters Theophilus, Cyrillus and Catharina.
Using a high magnification, look north from Theophilus and find the small craters Torricelli and Maskelyne – both are 24km wide.
Create a triangle with these two as the base and the 7km (4 mile) Moltke to their west.
One-third of the way between Moltke and the crater Sabine, and just above a line between the two, is the landing site.
Pictures of a gibbous Moon
Below is a selection of images of gibbous Moons, captured by astrophotographers and BBC Sky at Night Magazine readers.
Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.