To Everything, Turn, Turn, Turn...

One of my favorite activities while on a tropical vacation is to sip a cocktail at sunset.  Let’s go with that theme for this blog, a continuation of our series to understand the behavior of the Moon.  By great good fortune, pretend you are enjoying a month-long holiday at a fabulous resort somewhere along the Earth’s equator.  By interesting coincidence, the start of your vacation, two weeks ago, coincided exactly with New Moon.   Every day since your arrival, you’ve made it your business to be on your fabulous open patio at sunset, facing due south, enjoying your own favorite cocktail.  The drink goes down easily, the tropical breezes soothe your brow, and you haven’t a care in the world.  Why would you?  You’re only half-way through a long fabulous vacation! 

But, you actually do have one small concern.  Over the last two weeks, you’ve watched the moon’s position and appearance change as it went from New to Full.   Tonight, the Sun is slipping below the western horizon, setting up a gorgeous tropical sunset.  Off to your left, the full Moon is coming into view along the eastern horizon.   Although the scene is dripping with beauty and romance is definitely in the air, your mind is occupied with trying to puzzle out what’s going to happen next to the Moon, now that it’s full.

To understand this, it is helpful to and try and connect your local view to the zoomed-out “birds eye” view that you would get if you were out in space, many thousands of miles above the Earth’s North Pole. That perspective is exactly what’s captured in the figure above (the same one I used in the last blog to trace the Moon’s behavior while it was waxing).  Looking down on the central little ball that is the Earth in the figure, you are in essence seeing the half of the Earth’s surface that corresponds to the Northern Hemisphere.   Half of that area is lit up by the Sun (which is off to the right), and is experiencing daytime.  The other half, turned away from the location of the Sun, is black, signifying night.

Now, comfortably ensconced in your tropical get-away on the equator at sunset, where does that put you on the surface of the little Earth ball?  If you can stop reading and answer that question right now, yourself, then you’re way ahead of this game!  If not, well, let me tell you: You’re right at the top of the ball, exactly where the line is separating white from black (day from night).  The Sun is setting because the Earth is spinning counterclockwise (eastward), so your position on the Earth’s surface is shifting leftward into the dark side. 

As we noted before, the Moon, the Earth, and the Sun are all lined up, with the Earth directly between the Moon and the Sun.  That configuration explains why the Moon rises over your eastern (leftward) horizon at this exact point in time.  It is only now, once you spin into the dark side, that the Moon becomes visible; a little earlier, while you were still on the lit (daylight) side, the Moon was still hidden below the Earth’s Eastern horizon.  In essence, throughout the daylight hours, the Moon was positioned below your feet.  However, the eastward-spinning Earth was constantly “pushing” the Moon toward your left until, just at nightfall, it got “pushed” above the eastern horizon and into view.

As the night of the full moon commences, you keep on spinning to the left, which now ”pulls” the Moon ever westward across the sky.  Just as you’re about to spin into the lighted side (Sunrise), the Moon passes out of view over in the West.

What then?  In the last blog, we discovered that when the Moon was waxing, it was located to the left of the Sun. That meant the Moon was in essence “trailing” the Sun, both setting after the Sun did (always during the night) and rising after the Sun did (always during the day).  But, moving at the standard 25,000 miles per hour, the Moon doesn’t stay aligned with the Sun and the Earth for long.  From the “top-down” perspective of the “Phases Figure”, you can see the impact of this perpetual counterclockwise motion on when the Moon rises during the next two weeks.  Every night, the Moon doesn’t appear until longer and longer after sunset, and doesn’t set in the West until longer and longer after sunrise.  A good benchmark for these changes is the “3^rd quarter”, when the Moon is exactly half-way back to the vicinity of the Sun (and halfway back to another New Moon phase).  At this point, the Moon forms a right angle triangle with the Earth and the Sun.  The Moon doesn’t swing into view in the east until you are halfway through the “dark side of the Earth”, or midnight, and doesn’t again get obscured by the Earth (set in the West) until you have spun around to the point where you are halfway through your “daylight phase”.

Although the moon continues to rise at night throughout the waning period, the gap between moonrise and sunrise shrinks steadily as the Moon churns ever closer to the Sun, and the amount of time that the moon is visible during the day expands steadily.  However, that doesn’t necessarily mean that the Moon is getting easier and easier to see during daylight (though it is actually pretty easy to spot if you know where to look).  As the Moon approaches closer and closer to the vicinity of the Sun, less and less of the surface is illuminated.  You can visualize how this is happening by returning to your vantage point on the patio of your tropical vacation paradise.  What does the Moon look like in the sky as it wanes?

The answer is illustrated in the series of phases in the figure below.  The top row shows how the Moon looks while waxing, which we covered in the last blog.  The bottom row is what you’d see if you kept track of the Moon while waning.  Right after full Moon, darkness start to encroach along the right side limb, similar to how brightness appeared while the Moon was waxing.  Over successive days, less and less of the near sight is lighted, until the Moon reverts to that familiar crescent shape again.  The difference from the waxing phase is that the crescent is on the left side of the Moon’s surface, rather than the right.

This behavior makes perfect sense when you realize that the Moon is moving ever closer to alignment with the Sun from off on the Sun’s right flank.  As the Moon moves back toward full alignment, more and more of the sunlit hemisphere slips around to the far side, leaving less and less of the near side illuminated.   Eventually, the moon is almost completely aligned with the Sun and the Earth, and only a sliver of near side is visible along the left limb.  Of course, the rest of the sunlight region is now behind that limb, heating up the far side of the moon. 

A day or so later, the moon slips into full alignment, directly between you and the Sun.  Just as it was when you started your vacation, the near side is completely shrouded in cold and darkness.  The good news is, we’re back where we started, at the New Moon phase!  The bad news is, your vacation is over.

All, right, it’s time to leave fantasy land and get back to reality.  Wherever you actually live on the Earth’s surface, I encourage you to start looking for the Moon when you find yourself outdoors, day or night.  When you spot it, think about what its appearance tells you about where it is in its cycle and where it will be (and what it will look like) in the coming days.  Then, go out and confirm your predictions with actual sightings.  In no time, you’ll be an expert on the Phases of the Moon!

In the next blog, I’m going to build on this “introduction to lunar behavior” to uncover and describe something else about the Moon that explains a major part of its appearance, at least as seen from Earth.   Hint: Over the course of the Moon’s orbit, regardless of whether the Moon is waxing or waning, only the near side (or a portion thereof) is ever revealed to us. 

If the moon is circling all the way around the Earth during this period, why don’t we ever see the far side?  If you can answer that question in the comment section, you won’t even have to bother reading the next installment!