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.