CHAPTER 3 | CYCLES OF THE MOON 35
EclipticEclipticEclipticThe sun and moon are near
each other at new moon.Sagittarius
ScorpiusSagittarius
ScorpiusSagittarius ScorpiusSunSunSunOne sidereal period after new moon,
the moon has returned to the same
place among the stars, but the sun
has moved on along the ecliptic.One synodic period after new moon,
the moon has caught up with the
sun and is again at new moon.New
moonNew
moonMoonThe moon orbits eastward
around Earth in 27.32 days,
itssidereal period. This is how
long the moon takes to circle the
sky once and return to the same
position among the stars.A complete cycle of lunar phases
takes 29.53 days, the moon’s
synodic period. (Synodic comes
from the Greek words for “together”
and “path.”)To see why the synodic period is
longer than the sidereal period,
study the star charts at the right.Although you think of the lunar
cycle as being about 4 weeks long,
it is actually 1.53 days longer than
4 weeks. The calendar divides the
year into 30-day periods called
months (literally “moonths”) in
recognition of the 29.53 day
synodic cycle of the moon.You can use the diagram on the opposite page to
determine when the moon rises and sets at different phases.Full moon
sets at
sunriseNew moon is
invisible
near the
sun
New moon is
invisible
near the
sun
East South
THE SKY AT SUNRISETHE SKY AT SUNRISEWest2726252423 22(^212019)
18
17
16
15
14
Wan
ingc
resce
nt
Wan
ingc
resce
nt Waning
gibbo
us
New moon
One sidereal period
after new moon
One synodic period
after new moon
Phase Moonrise Moonset
New
First quarter
Full
Third quarter
Dawn
Noon
Sunset
Midnight
Sunset
Midnight
Dawn
Noon
TIMES OF MOONRISE AND MOONSET
3
The last two weeks of the cycle of
the moon are shown below by its
position at sunrise on 14 successive
mornings. As the moon shrinks from full
to new, it is said to wane.
2b
The third quarter moon
is 3 weeks through its
4-week cycle.