THE USM SOUTHWORTH PLANETARIUM
207-780-4249     www.usm.maine.edu/planet
70 Falmouth Street     Portland, Maine  04103
43.6667° N                   70.2667° W
Altitude:   10 feet below sea level
Founded January 1970
Julian date:  2458645.5
             "If knees bent the other way, what would a chair look like?"
                                      -Gallagher


THE DAILY ASTRONOMER
Monday, June 17, 2019
Moon Demystified

[image: full-moon3.jpg]
The* full moon* rising above trees branches silhouetted by, well, the full
moon.

Every so often, generally around the time of full moon, we'll revisit some
of the most misleading moon myths and surprising facts.   Of all the
wondrous celestial objects adoring our sky, the moon is the closet, most
changeable, and perplexing.  Even the great Isaac Newton conceded that
contemplation of lunar motion often induced a splitting headache.

*THE MOON DOESN'T ROTATE?*
Status:  MYTH
The moon rotates on its axis just as Earth, the planets and almost every
other celestial object does.   We see only one side of the moon because it
rotates once in the same amount of time it requires to revolve around Earth
once. We refer to this equality as synchronous rotation.      The moon's
orbital and rotational periods are approximately 27.3 Earth days

[image: 2009-02-science-break-02.jpg]
*Synchronous rotation: *  The moon's orbital period is equal to its
rotational period.  For this reason, the same side of the moon faces Earth.

*THE MOON CAUSES TIDES?*
Status:  Not exactly
The moon is principally responsible for tides, but the Sun also exerts a
significant influence.   The tides result from differential gravitational
force: that the gravitational force exerted by one body on another varies
along the body.   This magnitude of this force is quite distance sensitive:
the closer the massive object, the more powerful the tidal forces.    The
Sun is about 400 times more distant from Earth than the moon.
Consequently, the Sun's tidal influence is 44% that of the moon, even
though the Sun is far more massive.

*THE MOON IS NOT VISIBLE DURING THE DAY?*
Status:  MYTH
The moon is as visible during the day as it is at night.    Of course,
during the day it has to compete with the Sun for our attention and so is
often missed.   At night, the Sun is not visible and the moon, if visible,
will be the sky's brightest object.

*THERE IS NO GRAVITY ON THE MOON?*
Status:  MYTH
The moon certainly has gravity,  However, being less massive than Earth,
the moon's surface gravity is about 1/6th as strong as  that of our
planet.   The Apollo astronauts carried so much equipment that on Earth
they weighted about 500 pounds!   On the moon, however, they weighed about
80 pounds.  For this reason they were able to make the type of high jumps
they couldn't have made on Earth.   However, if the moon had no gravity at
all, they would have floated away from the moon entirely.

*THE FULL MOON IS TWICE AS BRIGHT AS THE QUARTER MOON?*
Status:   MYTH
This one is certainly surprising!  The quarter moon's illuminated visible
area is one half that of the full moon.   Geometrically, the full moon
should only be twice as bright as the quarter moon.   However, the full
moon is nine times brighter than the quarter moon!    We see the moon
because it reflects sunlight back to our eyes.    The moon reflects most of
the light along its incidental path, that path along which it receives the
light.    When the moon is full, it is on the far side of Earth relative to
the Sun.    At that time, Earth is right along that light path.

*THE MOON IS CLOSEST WHEN FULL?*
STATUS: MYTH  most of the time
At times, the moon will be at or close to perigee (its point of least
distance from Earth) around the time it is full.   However, most of the
time the moon will not be at its closest when full.  Sometimes, it will be
at apogee (its point of greatest distance from Earth) when full.

The period of time separating successive perigees -called an anomalistic
month- is 27.55 days.    The time period separating successive full moons
-called a synodic month- is 29.53 days.       Perigees and full moons (or
oppositions) will occasionally coincide and in so doing will produce a
"super moon."

*THE MOON IS LARGEST WHEN ON THE HORIZON*
Status:  MYTH
Try this:  when you see the moon along the horizon, extend your thumb out
to arm's length.    You will find that your thumb can completely cover the
moon just as it would if the moon were higher in the sky.     The apparent
enlargement of the horizon moon is a simple eye trick, often known as the
"Ponzo" illusion.  We perceive the enlargement of objects when they are
oriented in a certain way relative to our sight line.   The graphic below
shows two green lines, the upper of which appears to be the larger one.
 However, they are actual of equal lengths.   Our minds perceive the moon
to be larger when it is rising or just above the horizon.     However, as
you can demonstrate with the thumb trick, the moon's apparent size (about
half a degree in angular diameter) is the same at the horizon as it is
anywhere else in the sky.

[image: 4262582.gif]
*Ponzo illusion: *   two lines of equal length appear
unequal when positioned at different altitudes.
The moon appears larger on the horizon than it
does higher in the sky.  However, the enlargement
is illusory.