A first quarter Moon may sound like something of a misnomer to beginner lunar observers who note that, rather than being a quarter-lit as the name might suggest, the Moon appears half-lit.
Similarly, towards the end of the lunar month, the ‘last quarter Moon’ phase may cause similar consternations. Last quarter? what happened to the 2nd and 3rd quarters?
During the phases of the Moon, the first quarter marks the point a quarter of the way through the entire lunar cycle.
And, when you think about it, the side of the Moon we’re seeing is a quarter lit, because we are seeing one half of the Moon, half-illuminated.
In contrast, the last quarter Moon occurs towards the end of the lunar cycle and marks the final stage of the waning phase, taking us from full Moon back to new Moon.
The terms first quarter and last quarter probably make more sense if we go through the phases of the Moon, bit by bit.
The whole lunar cycle lasts around 29 days, and during that time it passes through several stages.
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 Moon is illuminated by the Sun beyond the halfway point on the lunar disc, this is known as the waxing gibbous phase, which progresses 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.
First quarter takes place around day 7 of the lunar month, and last quarter around day 22.
Observing a quarter Moon
There’s lots to see on the Moon during the first quarter. Unlike during a full Moon, the terminator (the line marking the division between the lit and unlit sides of the Earth-facing lunar disc) is visible, splitting the side of the Moon we see right down the middle.
Shadows thrown onto the lunar surface at and around the terminator make some of the Moon’s craters and other surface features like lunar Maria really stand out, making this a great time to get out a pair of binoculars or a telescope and observe the lunar surface.
Let’s take a look at some of the best features to see during first quarter and last quarter.
There’s a range of features to see on a first quarter Moon.
Near the middle of the Moon is Hipparchus, a large, low-walled crater. The 150km- (94 mile) wide Hipparchus has another crater, Horrocks, within it near its northeastern rim.
South of Hipparchus is the 136km- (85 mile) wide crater Albategnius, with another crater (Klein) on its southwestern flank.
In the north, the vast Mare Imbrium – a plain over 1,120km wide – is beginning to show. In Latin, Mare Imbrium means ‘Sea of Showers’.
On its eastern edge you’ll find the 55km (34 mile) crater Aristillus, with the smaller (39km) crater Autolycus below it and the larger (57km) Cassini, above. They’re surrounded by mountain ranges.
To the south is Montes Apenninus, named after the Apennines in Italy. Due east is Montes Caucasus, and to the north is Montes Alpes.
Splitting the Montes Alpes is a 190km- (119 mile) long valley, Vallis Alpes.
It’s easy to spot with a small scope, but you’ll need a large (20cm) scope to see the narrow channel that winds along the valley floor.
The last quarter phase of the Moon is wonderful, because there’s great contrast between the bright, rugged craters near the terminator and the dark, flat mare to the west.
Lunar sunset approaches the craters Ptolemaeus, Alphonsus, and Arzachel.
Look to the north of the Moon at the vast Mare Imbrium. The craters and mountain peaks on its eastern side are stunning, especially the 101km (63 mile) crater Plato on the mare’s northernmost fringe.
Its dark, lava-flooded floor stands out against the surrounding bright lunar highlands. Use high magnification to look for shadows cast by Plato Crater’s walls onto its floor.
Can you spot the clair-obscur effect known as Plato’s Hook? This trick of the light is best seen one day after first quarter.
Due south of Plato, standing alone in Imbrium, is Mons Pico, a 2,400m (8,000ft) mountain. It’s only about the same height as the lower peaks of the Italian Dolomites.
But when the angle of light is low, Mons Pico stands out superbly, casting a long shadow onto the Mare Imbrium.
South of Mons Pico is the largest crater in Mare Imbrium – the dark-floored 83km (52 mile) Archimedes. Look out for the triangular promontory that extends from the southeast edge of its walls.
To the northeast of Mare Imbrium is Montes Alpes. Watch the terminator as it moves across this mountain range. As the valleys plunge into darkness, the peaks remain lit.
Pictures of the quarter Moon
Below is a selection of images of the Moon at and around the quarter phases, captured by astrophotographers and BBC Sky at Night Magazine readers.
Or if sketching is more your thing, read our guide on how to draw the Moon.
The Moon by Steve Judge, Manchester, UK. Equipment: Sky-Watcher 150P Newtonian Reflector, Canon EOS 500D, EQ3 mount.