In the diagram, above, the Sun is assumed to be far off to the right, the inner circle shows the positions of the Moon, seen from above Earth’s North Pole, corresponding to the phases of the seen from Earth as shown on the outer circle. Image: NASA
Here is the sequence of the eight main Moon phases: New Moon: During this phase, the Moon can’t be seen because it is between the Sun and Earth. Waxing crescent: Following the New Moon, a small sliver of the Moon becomes visible at dusk. This sliver is shaped like a crescent because earthlings only see the edge of the part being illuminated by the Sun, and the edge of a round object appears curved, matching the shape of a curved right hand. First quarter: When half of the Moon’s surface is illuminated, one-quarter of the total Moon’s surface is visible from the Earth, hence the counterintuitive name. Waxing gibbous: When more than half of the Moon’s surface visible from Earth is illuminated. Full Moon: When the Moon is on the opposite side of the Earth from the Sun, we see the entirety of the illuminated side of the Moon. Waning gibbous: After the full Moon, the illuminated portion of the Moon visible from Earth begins decreasing in size, but more than half of the Moon’s surface remains visible from Earth. Third quarter: When a curved left hand matches the shape of the Moon, and the Moon displays the side that was not illuminated in the first quarter. Waning crescent: Appears near dawn, when increasingly smaller slivers of the Moon still visibly fit into a curved left hand.