THE SOUTHWORTH PLANETARIUM
207-780-4249       www.usm.maine.edu/planet
70 Falmouth Street  Portland, Maine 04103
43.6667° N,                    70.2667° W
Founded January 1970
           "A little night music."



THE DAILY ASTRONOMER
Tuesday, February 23, 2016
The Moon



A few billion years ago, when our infant solar system was a violent
maelstrom of aggregating planetesimals, colliding boulders, and
lava-hot proto-worlds, an errant body careened inexorably toward our
largely molten Earth.   At that time, our planet's crust was little
more than a thin membrane glowing dull ember from the churning liquids
beneath.     Had any observer managed to find a secure perch on this
unstable crust, she would have noticed a planet-large projectile in
approach.    From that perspective, it would have appeared to be
moving slowly, for the sky offered no markings by which to measure
velocity.  The only indication of motion was the body's slowly
increasing angular diameter.   At one point, it would have been as
large as the full moon appears to us today. Later, it would have
appeared as encompassing as a cloud form.   Eventually, it would have
covered most of the sky. By that time, even before the contact, Earth
would have lurched forward, for gravity is mutually attractive and the
two bodies would have approached each other.

It was called a 'violent collision,' although the word 'violent'
doesn't begin to describe it.  The impact obliterated the smaller body
and almost destroyed Earth.  Fortunately, Earth was then almost
entirely a liquid and was therefore capable of sustaining such a blow.
  Were it more rigid, as it is now, the collision would have likely
annihilated it, as well.   A large part of our planet is still molten,
of course, but the crust is much thicker.  Though Earth survived the
impact, much of its material was propelled into space, along with the
vaporized remains of the assaulting body.     This mixture remained
trapped by Earth's gravity and, in the cooling environs of space,
rapidly coalesced into an orbiting body: the moon.

This scenario roughly summarizes the "Wham Theory," which developed
after geochemists analyzed the Apollo moon samples and determined that
the lunar surface is in some respects similar and, in others,
dissimilar to Earth.   From these findings, some researchers concluded
that the moon must consist of matter derived from Earth and another
planet: one that struck Earth and was, itself, destroyed in the
process.

It is difficult to know the moon's original distance from Earth.
Astronomers believe that the moon must have been about 4 -5 Earth
radii away from the planet, or about 23,000 - 30,000 kilometers: about
10 - 14 times closer than its current distance. Had it been much
closer, such as 3 Earth radii away, the tidal forces would have
prevented its formation.   From the beginning, the moon has receded
slowly from Earth and, consequently, Earth's rotation rate has slowed.

The moon would have appeared quite large in our sky and, as tidal
forces are very distance sensitive, it induced constant turbulence on
the planet even when the first seas gathered on the planet's surface.
Some biologists believe that the moon's influence induced such high
tides and energy to the oceans that it hastened the propagation of
life through Earth's seas and eventually onto land.      This notion
is a mere hypothesis that can be suggested, but hardly proven.

We know that life abounds on Earth now and the Moon, whose birth just
pre-dates life's inception on Earth, still remains with us.    The
moon has figured quite prominently in humanity's mythological history.
Our own Greco-Roman tradition identifies the moon with Artemis,
goddess of the hunt and wildlife, for her full radiance can illume all
but the darkest forests, to the benefit of the creatures that inhabit
them and the hunters than pursue the creatures.

The moon has also been symbolic of rebirth and rejuvenation, for it
proceeds through an entire lunar cycle every 29.5 days, a span of time
known as the "synodic period."  In fact, our modern calendar consists
of twelve months, which were originally known as "moonths," as the
duration of each approximated this period.        A new lunation cycle
begins at "conjunction," or new moon.    At this phase, the moon is
between the sun and Earth and its illuminated half points away from
us.   After new, the moon passes through the waxing crescent phase:
the moon is a crescent that appears to grow each night until it
reaches first quadrature, or "first quarter."   The first quarter moon
rises around noon and sets around midnight.   Following first quarter
is waxing gibbous, The illumination percentage increases each night
until it becomes full.  When full, the moon is on the opposite side of
Earth relative to the sun, hence the term "opposition."   After full
moon, the moon appears to diminish in size, as is illumination
percentage decreases through the phase called "waning gibbous."   The
next phase is second quadrature, better known as last quarter.  The
moon rises around midnight and sets around noon.  The cycle ends with
waning crescent, when the moon "shrinks" even further each night as it
then proceeds to new moon and the cycle begins anew.

The moon, being spherical, always remains half illuminated by the Sun.
Its constantly changing aspect is a result of its motion around Earth.
  It also rotates on its axis, just as Earth does.   However, its
rotational period equals its revolutionary period so that one side
always faces us.  Over time, the combined gravitational influence of
Earth and the Sun have caused it to attain a type of equilibrium known
as "synchronous rotation."      It is said that over time, as the moon
recedes from Earth and the planet's rotation slows, the two bodies
will attain a mutual synchronous rotation.    Mathematically, we know,
however, that such a state would occur billions of years after the
Sun's life cycle ends.

This knowledge at least lends us an assurance that through the
planet's history, the moon will remain a prominent feature in both the
day and night skies.     Solar eclipses shall continue for hundreds of
millions of years. Lunar eclipses will only end when the Sun does.
 It is comforting to know that in an ever changing world, at least
Artemis shall prove to be a true immortal.