Il Mio Amore, la Luna

Il Mio Amore, la Luna

I love the moon.  It’s not some cockeyed romantic fascination fueled by mythological attributes assigned our cosmological traveling companion: But rather, appreciation of her exemplary illustration of key laws we believe govern the physical universe.

In the Beginning

While no one was around, at least no one who left any record of observation at the time of her formation, the most popular widely-accepted theory asserts she was born in a violent collision between Earth and another planet. Scientists are so sure about this they have assigned a name to the errant wanderer, Theia (named for some obscure Greek Mythological goddess with an affinity for shiny things… so your typical woman).  Now this seems to me a bit presumptive as there was no formal introduction, no trace is to be found in these parts and she apparently left no forwarding address.

The story goes; four and one-half billion years ago, give or take forty-five million years, a planet about the size of Mars (that’s the red one out there next in line) was wandering about our corner of the Milky Way and, influenced by that most universal of phenomena - gravity, attempted to occupy the exact same space-time as Earth did.  It is very important to state this in terms of space-time because both locality and timing are crucial to this event.  Had space been shaped other than it is or time passed at some different rate, no collision would have occurred and you would be working a crossword puzzle rather than reading this, maybe; but we’ll discuss that later.

In the aftermath of the mother of all fender benders, debris flung into space formed into an accretion disk, much the way one was formed with the birth of the Sun thus resulting in the hard, rocky planets; always with the accretion disks, these scientists.  I suppose the reason for such general acceptance of this theory is that modern-day geo-astronomers have built computer models that, applying the made-up data they have created, result in the outcome they were expecting.  This technique also serves global-warming advocates well, but I’ll reserve that rant for another post.

Thus, gravity caused the debris orbiting Earth to accrete, voila, the Moon.  

Ah, the Eternal Dance

What fascinates me about the Moon is her seductive motion across the heavens (or her path through the night sky, for you soulless cretins).  Stanley Kubrick notwithstanding, romantics can hear the faint strains of On the Beautiful Blue Danube as the Moon waltzes across the starry night. But I’m no romantic (as you well know by now) so we’re going to dabble a bit in the rudimentary mechanics that so often inspire poetic effusion.

I will try to minimize the astrophysical esoterica but you will have to bear just a bit.  The planets (especially since that mathematical trouble maker Pluto has been shed from the list) all orbit the sun on roughly the same plane.  That plane (a geometric plane, not an aircraft) is defined by the equator of the Sun. Even our beloved Moon orbits the Earth on this same plane.  Generally, the planets rotate on their axes (the plural of axis, pronounced aks-ees… unfortunately this is the same spelling for the plural of axe, the woodsman’s tool, which is why I have wasted so many words here) which are ninety degrees offset (perpendicular) to this plane.  This plane is called the ecliptic. Now if you are any kind of science geek, you know that there are two exceptions to this; Uranus (stop snickering!) and Earth.  Uranus’ peculiarities have no bearing on our experience with the Moon so we will refrain from discussing them here.

The Earth, however, is absolutely germane to this story, so we will discuss it.  Our axis is tilted 23.4 degrees from the perpendicular with the ecliptic.  This is believed to be the result of some large astronomical collision in the distant past by a wandering planet, say one that might result in the formation of a moon.  Well considering what we’ve learned about the creation of the Moon, that would be pretty convenient, wouldn’t it?  This tilt is responsible for such niggling phenomenon as the seasons, the variance in the length of days and, the north-to-south meandering our moon tracks through the sky.  So, if you find yourself humming some waltz in three-quarters time while gazing at the harvest moon, now you know why. 

I’m Sure It’s Just a Phase

Another unique feature of the Earth, Moon relationship is the spectacle of how she presents her image to those of us who watch such things.  The Moon orbits earth approximately once every 29.5 days.  The Earth, of course, rotates on its axis once each day (give or take one-quarter day per year... well actually, it’s always give).  Thus, each day (or night, which is the best viewing time) we see just a little bit different view of the moon.  I’m sure you know it is all due to the relative positions of the Moon, Sun and Earth. 

A Full Moon occurs when the Moon is directly opposite the Sun from the Earths rotational perspective, this always happens at sunset.  Then each earth day, the Moon’s own shadow grows on itself reducing the observable area by approximately 3.4% until fourteen and three-quarter days later, it is invisible to us as we experience a New Moon, this always happens at sunrise. The period between Full and New, as the moon appears to grow smaller, in known as, “on the wane” or “waning”; the opposite, when the moon appears to grow larger is known as “on the wax” or “waxing”.  Halfway between the full and new phases, occur the first and last quarter, which is confusing because what we see is one-half of the disk we see at full.  When more than one-half of the moon is illuminated, we call it “a gibbous moon” and when less, crescent (hopefully, for obvious reasons). So, to recap, we have: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent, new moon.

If this has confused you, just go outside on a clear night for several days in a row and see how the moon’s shape changes from night to night. If you are still confused I can come to your house and point out which of those heavenly body is the Moon.

A Familiar Face

You may have noticed, that each time you look at the Moon, notwithstanding the variation it its appearance due to phasing, that it always presents the same face to us here on Earth.  This phenomenon is the result of tidal lock.  Simply put (because we don’t have enough time, or let’s be honest, brain power to tackle the math) our moon completes one full rotation on its axis (one lunar day) in the same time it completes one full orbit of the Earth, or 27.5 days.

“But wait!” you cry.  “You said the Moon’s orbit is 29.5 days.”  Well thank you for paying attention and catching that… fasten your seat belts, here goes.  You see, there are two ways of measuring an orbit.  One is the sidereal, in which the point of orbital focus is the Sun (27.5 days) and the other is synodic, in which the orbital focus is the Earth.  This is somewhat advanced stuff so if you are interested you can look it up yourself.

This tidal locking (also known as synchronous rotation) is actually tugging at the Earth so as to cause a slowing of the Earth’s rotation resulting a lengthening of day of fifteen micro-seconds per year; and you thought the days were getting shorter!

What of the Future?

It seems the Moon, like all children, is trying to escape the influence of its parents. Scientists proffer that at formation, the Moon was between 12,000 to 18,000 miles from Earth (center mass to center mass, not surface to surface).  This seems to leave quite the margin for error and may be due to the dearth of reliable measuring tapes available at the time.  Today, the average distance (remember, all celestial orbits are elliptical) is 239,000 miles.  While this may seem a considerable distance, the Moon has had around four and one-half billion years to make the move out of its mother’s basement; it has set a blistering pace of about three inches per year.  The current, laser measured rate of departure is about one and one-half inches per year.

“What” you may ask, “will the night sky be like when the Moon has traveled far enough to break the chains of Earth’s gravity and go a-wandering aimlessly through the Universe.  Well, let me put your mind at ease.  At the present rate, long before we witness her final departure, our sun will have exhausted its fuel supply and grown in size as a Red Giant (look it up, this story is getting too long as it is) engulfing all of the inner planets, ours as well, and we will all have been burned to a cosmic crisp.

There is no need to thank me.  I consider it both honor and duty to put your minds at ease concerning the future destiny of humanity.

Ciao!