What Are We? ScorekeepersThe moon is a companion in our daily lives,
in our history, and in our mythology. It
makes a dramatic sight as it moves through
the sky, cycling through a sequence of
phases that has repeated for billions of
years. The moon has been humanity’s
timekeeper. Moses, Jesus, and Muhammad
saw the same moon that you see. The moon
is part of our human heritage, and famous
paintings, poems, plays, and music celebrate
the beauty of the moon.Eclipses of the sun and moon have
frightened and fascinated for millennia,
and some of humanity’s earliest efforts to
understand nature were focused on counting
the phases of the moon and predicting
eclipses. Some astronomers have found
evidence that Stonehenge could have been
used for eclipse prediction, and the ancient
Maya in Central America left behind
elaborate tables that allowed them to
predict eclipses.Our lives are ruled by the moon as it
divides our year into months, and its cycle
from new to fi rst quarter to full to third
quarter and back to full divides the month
into four weeks. In a Native American story,
Coyote gambles with the sun to see if the
sun will return after the winter solstice to
warm Earth. The moon keeps score. The
moon is a symbol of regularity, reliability,
and dependability. It is the scorekeeper
counting out our weeks and months.Summary
▶ The moon orbits eastward around Earth once a month and rotates on its
axis so as to keep the same side facing Earth throughout the month.
▶ Because you see the moon by refl ected sunlight, its shape appears to
change as it orbits Earth and sunlight illuminates different amounts of
the side facing Earth.
▶ The lunar phases wax from new moon to fi rst quarter to full moon and
wane from full moon to third quarter to new moon.
▶ A complete cycle of lunar phases takes 29.53 days, which is known as
the moon’s synodic period (p. 35). The sidereal period (p. 35) of
the moon—its orbital period with respect to the stars—is a bit over
two days shorter.
▶ If a full moon passes through Earth’s shadow, sunlight is cut off, and
the moon darkens in a lunar eclipse (p. 33). If the moon fully enters
the dark umbra (p. 33) of Earth’s shadow, the eclipse is a total lunar
eclipse (p. 36); but if it only grazes the umbra, the eclipse is a
partial lunar eclipse (p. 36). If the moon enters the partial shadow of
the penumbra (p. 33) but not the umbra, the eclipse is a penumbral
lunar eclipse (p. 37).
▶ (^) During totality (p. 36), the eclipsed moon looks copper-red because of
sunlight refracted through Earth’s atmosphere.
▶ (^) The small-angle formula (p. 39) allows you to calculate an object’s
angular diameter from its linear diameter and distance. The angular
diameter of the sun and moon is about 0.5 degrees.
▶ (^) A solar eclipse (p. 38) occurs if a new moon passes between the sun
and Earth and the moon’s shadow sweeps over Earth’s surface along the
path of totality (p. 39). Observers inside the path of totality see a
total solar eclipse (p. 38), and those just outside the path of totality
see a partial solar eclipse (p. 38).
▶ (^) When the moon is near perigee (p. 39), the closest point in its orbit,
its angular diameter is large enough to cover the sun’s photosphere and
produce a total eclipse. But if the moon is near apogee (p. 39), the
farthest point in its orbit, it looks too small and can’t entirely cover
the photosphere. A solar eclipse occurring then would be an annular
eclipse (p. 39).
▶ During a total eclipse of the sun, the bright photosphere (p. 41) of
the sun is covered, and the fainter corona (p. 41), chromosphere
(p. 41), and prominences (p. 41) become visible.
▶ (^) Sometimes at the beginning or end of the total phase of a total solar
eclipse, a small piece of the sun’s photosphere can peek out through
a valley at the edge of the moon and produce a diamond ring effect
(p. 41).
▶ (^) Looking at the sun is dangerous and can burn the retinas of your eyes.
The safest way to observe the partial phases of a solar eclipse is by
pinhole projection. Only during totality, when the photosphere is com-
pletely hidden, is it safe to look at the sun directly.
▶ (^) Solar eclipses must occur at new moon, and lunar eclipses must occur
at full moon. Because the moon’s orbit is tipped a few degrees from the
plane of Earth’s orbit, most new moons cross north or south of the sun,
and there are no solar eclipses in those months. Similarly, most full
moons cross north or south of Earth’s shadow, and there are no lunar
eclipses in those months.
▶ (^) The moon’s orbit crosses the ecliptic at two locations called nodes
(p. 43), and eclipses can occur only when the sun is crossing a node.
During these periods, called eclipse seasons (p. 43), a new moon
will cause a solar eclipse, and a full moon can cause a lunar eclipse.
An eclipse season occurs each time the line of nodes (p. 44) points
toward the sun. Knowing when the eclipse seasons occur would allow
you to guess which new moons and full moons could cause eclipses.
▶ (^) Because the orbit of the moon precesses, the nodes slip westward
along the ecliptic, and it takes the sun only about 347 days to go from
a node around the ecliptic and back to the same node. This is called an
eclipse year (p. 44).
▶ (^) Because the nodes of the moon’s orbit move westward, eclipse seasons
begin about 19 days earlier each year.
▶ (^) Eclipses follow a pattern called the saros cycle (p. 44). After one
saros of 18 years 11^13 days, the pattern of eclipses repeats. Some
ancient astronomers knew of the saros cycle and used it to predict
eclipses.