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

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

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.  

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.