Predictable Phases of the Moon:
The Eight Phases of the Moon
In the course of 29.5
days, the moon goes through eight different phases – from New
Moon to Waning Crescent. What causes the moon to look so
different during the month? The answer is “angles” – how the
moon, sun, and Earth are lined up. What does this alignment mean
to how we perceive the moon’s illumination?
The sun is always illuminating the moon; however, depending on
where the moon and Earth are in their respective orbits
(relative to the sun), the sunlit and shadowed portions of the
moon will look different. And, since the Earth and moon orbit at
known rates, the phases of the moon can be predicted, charted,
The moon begins each new cycle as a New Moon. This is when the
moon is between the Earth and sun (approximately aligned), with
the sun illuminating the back of the moon. From Earth’s
perspective, the moon looks to be in shadow as the illuminated
part is hidden from view.
The other major phase of course is the Full Moon. This is when
the moon falls in approximate line with the Earth and sun again,
but this time, the moon is on the other side of the Earth. Thus,
the illuminated part of the moon is facing us and appears to be
completely round. Full.
Most of us are familiar with the crescent shape, which is curved
and appears to be less than half of a moon. This is a good way
to remember the crescent phases – there are two. Paired with
either “waxing” (which means “expanding”) or “waning” (which
means “shrinking”), the crescent is the associated shape for the
2nd and 8th phases.
The other term to remember is “gibbous” which is the opposite of
“crescent” for the moon cycle. Gibbous simply describes the moon
when more than half of it is illuminated. There are two phases
with this name, again paired with “waxing” and “waning” to
properly describe where in the cycle the phase is.
Thus, in order, the eight phases of the moon are:
New Moon | 2. Waxing Crescent | 3. First Quarter
Full Moon | 6. Waning Gibbous | 7. Last Quarter | 8. Waning
Did you know:
“synodic period” takes approximately 29.5 days. This means that,
for an observer on Earth, it takes the moon this long to return
to the same position (relative to the sun).
you know – it actually only takes the moon 27 days to
complete its orbit. Only, because the Earth is also moving along
its own orbit, two days are added on to account for Earth’s