Venus, if you Will!

It’s mid February, Whabbloggers, and very soon, the evening sky is going to be graced with a rather stunning addition. Yes, our closest planetary neighbor Venus, known since antiquity as the Goddess of Love, is about to assume her position as the dominant “Evening Star”, brightest of all celestial objects save the Moon.

Where has Venus been lately? And why isn’t she the “Evening Star” all the time? To begin to answer these questions, suppose, about a month ago, you headed out to your back yard (or equivalent) at our preferred visualization time, right around sunset. You had taken pains to pick a clear day, of course, so the Sun was clearly visible along the western horizon.  The rest of the sky was still bright blue, with no stars visible just yet. Now, where was Venus? Even though invisible, you could have easily visualized her location by imagining a straight line joining you to the Sun and extending out behind it. Venus was almost directly on that line, some 67 million miles past (beyond) the Sun!

Of course, we know from the earlier Sun blog that the Sun is a stationary object, so there’s nothing special about having gone out near sunset as far as imagining the location of Venus was concerned; you could have just as easily imagined the line connecting the Earth, the Sun, and Venus when the Sun was halfway across the sky at noon, or just clearing the eastern horizon at sunrise, or even when the Sun was below your feet at night. To make this point more concretely, take a look at Figure 1 below, which depicts the green/blue Earth, the yellow Sun, and four half-black circles. These circles represent the position of Venus at four points in her orbit around the Sun. Last month, at the time you made your hypothetical trip to your back yard, Venus was in the position corresponding to the circle on the far right of the figure, on the other side of the Sun from the Earth.

In the Figure, you’re looking straight down on the Solar System from directly above the Sun. Another very useful perspective from which to visualize the relative positions of the Sun, Venus, and the Earth is from a vantage point just behind and slightly above Earth, as in Figure 2 below. A month ago, Venus was in the position labeled “Superior Conjunction”. The Sun occupies the middle position, so the side of Venus that was turned toward the Earth was fully illuminated. We couldn’t see it, though, due to a combination of two factors. First, Venus was fully 160 million miles away, so its disk was quite small. Second, and more important, since Venus rose and set at almost exactly the same time as the Sun, it was never in the sky at night, and during the day, it was completely washed out by the Sun’s glare.

It is interesting to compare the source of Venus’ invisibility last month to the invisibility of the New Moon. As we found out in the lunar blogs, the Moon is located between the Sun and the Earth when it is New, so the side facing us is completely dark. Venus is invisible for entirely different reasons.

Like every other body in the solar system, Venus is never stationary. Day by day, our sister planet travels in the standard leftward (counterclockwise) direction all inner solar system objects travel as they orbit the Sun. The closer the object to the Sun, the faster it moves, and Venus actually whips along at over 75,000 miles per hour. You read that right: Venus covers 21 miles every second! You can see from Figure 2 that over the past month, this movement has been taking Venus gradually away from, and to the left of, the sun.

As Venus moves leftward, the distance she is putting between herself and the Sun (as viewed from our perspective here on Earth) is about to pay dividends for sky watchers. Fast forward in your visualization exercise to about three weeks from now, towards the middle of March. Take some time to settle down in your back yard just after sunset, maybe with a cocktail to keep you company. Sipping slowly, you enjoy the gathering dusk while waiting for the first star to appear. Presto: I guarantee that a bright object will pop into view over near the western horizon, and you will be the first on your block to welcome the Evening Star!

Every day thereafter, Venus will slip a little further to the left along its orbital path, taking her ever further away from the Sun. As she climbs higher in the sky, she will hang around for a longer period after sunset before descending down to the western horizon and disappearing from view (this rapid movement of Venus towards the western horizon is entirely due to the Earth’s leftward rotation; it has nothing to do with Venus’s own movement at all).

Let’s continue to follow Venus around in her orbit. As she travels ever further “up, up and away” from the Sun, eventually she reaches the half-way point, equivalent to the Moon’s “half full” position, the point of maximum displacement from the Sun (again, that statement pertains only to our perspective; from her own perspective, Venus stays remarkably close to 67 million miles from the Sun throughout her orbit). The period of maximum displacement will happen over this summer. The black and white ball at the top of Figure 1, and the extreme left of Figure 2, illustrates what position she will take.   By then, Venus has swung far enough from the Sun that she is quite a ways from the Western horizon at sunset, and remains visible for several hours after sunset.

Let’s pause for just a moment, consider Venus at this point in her orbit, and visualize her behavior in our sky over a 24-hour period beginning at sunset (i.e., through a full rotation of the Earth). Once it has disappeared, the Sun commences following the standard path below your feet (the path we visualized in the Sun blog) due to the Earth’s eastward rotation. Meanwhile, that same earthly rotation is driving Venus ever rightward, down to and eventually below the western horizon. By the time the Sun pops back into view along the eastern horizon in the morning, where is Venus? Still below your feet, of course, tracing out pretty much the same path as the Sun, just several hours behind. Since Venus doesn’t rise for several hours after the Sun, the event goes completely unnoticed because the planet rises into full daylight.

When the Moon swings into view at the half full point in it’s cycle, our satellite is clearly visible in the daytime sky.  Believe it or not, Venus is, too, if you know exactly where to look. The best opportunity comes when the half moon and Venus rise at almost exactly the same time, very close together, and then move in unison as the Earth’s rotation carries them up and across the daytime sky. I’ll be sure to point this opportunity out when it happens this summer. If you can spot Venus, you’ll be one of the few human beings on the planet who’ve ever seen it in broad daylight!

Now, let’s return to the main event, the changes in Venus’ appearance that accompany its orbit around the Sun. You can see from the highly informative Figure 2 that, at the time of greatest elongation, Venus is actually “rounding the curve” and about to start swinging back to the right along our line of sight: A path that will bring it back into alignment with the Sun. Venus is still drawing closer to Earth during this time, so it continues to grow larger. However, as late summer gives way to autumn, each night the distance between our brilliant Evening Star and the Sun will shrink steadily, and the interval between when the Sun sets and Venus sets will shrink as well.

By Halloween, Venus’s orbital motion will have brought it right back into alignment with the Earth and the Sun, but between the two, rather than on the far side of the Sun as it is now.   Once again, rising and setting in tandem with the sun, Venus will fade into invisibility in our night sky. This time, her invisibility is caused both by the fact that Venus and the Sun will rise and set together, and also because our point of view is now aligned with the dark side.   In that sense, the Halloween “New Venus” and the New Moon of November 6th will have a lot in common!

And what then? Once again, Figure 2 tells the story: Venus’s incessant movement will cause it to move past the sun and off to the Sun’s right. Remember from our Moon blogs what happens to celestial objects that are located to the right of the Sun? When the Sun sets, objects to the right are already below the western horizon, and have a head start when it comes to being whisked around by the Earth’s rotation toward their appointment with the Eastern horizon. So it is that Venus will start to rise before the Sun, and take her appointed place as the blazing Morning Star. Over the course of next winter, the planet will swing further and further to the right, giving her more and more of a head start on the Sun. Consequently, Venus will rise earlier and earlier than the Sun, and stay visible for a longer period of time before dawn arrives and she fades into the morning twilight.

To really imagine this Morning Star behavior, let’s go back to Figure 1 and its “God’s Eye perspective” above the Sun. Pretend you are a specific location along the Earth’s equator (along the very edge of the disk), and the entire green ball representing the Earth is spinning in a counterclockwise direction. Venus is at the bottom of the figure. Now imagine spinning throughout the night. As you pass midnight, which is when you are exactly opposite the Sun on the Earth’s far side, you can see from the geometry in the Figure that Venus is soon going to swing into your view along the Eastern horizon. The Earth will have to complete almost another quarter turn before the Sun does its own appearance act, also to the East.

And that’s pretty much it! When Venus is located to the right of the Sun (from our perspective) she forms the Morning Star. When she is located to the left of the Sun, she forms the Evening Star. Over the course of this year, I encourage you to follow the progress of Venus, starting about three weeks from now along the western horizon. Check out how high she gets in the sky over the summer. In the autumn, track her movement back in the direction of the Sun, before she disappears completely. And then, next winter, if you find yourself with insomnia some night, or happen to get up unusually early, check out the Morning Star!

Figure 2 illustrates one more notable change in Venus brought on by her orbital motion, although you need a telescope or good pair of binoculars to see it. Right now, in the middle of February, the side of Venus that faces Earth is almost fully illuminated. As the planet swings in our direction, the proportion of the lit side that is visible from our vantage point shrinks steadily. When Venus is at her maximum elongation from the Sun, she is exactly “half full”, just like the Moon. Then, as Venus swings back into alignment with the Sun, late next summer and into fall, the proportion of her disk that is illuminated from our vantage point shrinks to a thin crescent (again, these phases are just like the Moon). You will see these phases of Venus clearly if you ever examine the planet in a good pair of binoculars or a small telescope.

Before leaving Venus completely, I’d like to take this opportunity to talk about the place for just a little bit. Virtually a twin of the Earth in terms of size, Venus has always been my favorite planet. I don’t really know why. She certainly isn’t more likely to harbor life than Mars; the surface temperature is about 800 degrees Fahrenheit, 300 degrees hotter than the highest setting on your oven. That temperature is pretty constant across the entire surface, too, despite Earth-like variations in latitude and altitude, and despite the fact that nights on Venus are longer than 100 Earth nights. The long nights are because Venus spins on its axis very slowly, a lot more slowly than the Moon even (you do remember from earlier blogs how slowly the Moon spins, right)? As with many other features, the slow rotation rate of Venus is still a mystery. As for how Venus maintains such a constant temperature, the answer is that Venus’s atmosphere is 90 times as dense as ours. The air pressure at the surface is equivalent to the pressure 3000 feet down in our oceans! Just as there isn’t a lot of variation in water temperatures at that depth, so too is there little temperature variation on Venus.

There is a great deal more that is fascinating about our sister planet, such as the current thinking about why she has such a thick atmosphere, but I will save that discussion for another blog.

Until next time, then, Whabbloggers… when I think I’ll tackle a little astrology!

From the Earth to the Moon

Several blogs ago, we engaged in a visualization exercise to help understand the apparent movement of the Sun in our sky.  In today’s blog, I want to continue the fun by visualizing the Sun’s behavior again, but this time from the point of view of a tourist (you! you!) on the surface of the Moon.  The purpose?  To illuminate one of the Moon’s best-kept secrets!

Before boarding your spacecraft, I’ll briefly review how the Moon behaves from our vantage point here on Earth.  As you know, the Moon is constantly moving in an easterly (leftward) direction, circling the Earth every 27 days or so.  The cycle starts with a small crescent that grows a little fatter every night until, about two weeks later, the Moon has the appearance of a big round ball.  Then, the pattern reverses itself; each night, darkness steals more and more of the ball from our sight until it disappears completely, and we say that the Moon is “New”.

Through the last two blogs, we’ve discovered that these changes in appearance are completely explained by the change in the relative positions of the Sun, Moon, and Earth due to the movement of the Moon around the Earth.  When the Moon is new, it is between us and the Sun; when the Moon is full, we are between it and the Sun.  But although the Moon’s constantly shifting position fully explains its phases, a mystery remains. Imagine the Moon in its “New” position, between the Sun and the Earth.  The “far side” is fully illuminated by the Sun, and the “near side” is in darkness.  If there was a small city on the near side, we’d be able the lights of the city twinkling in the lunar night.

Now, using your powers of visualization, imagine just picking the Moon up and moving it to the position it occupies when full.  Where is our hypothetical city now?  By rights, it should now be on the “far side”, the side invisible to us.  The side that is completely illuminated, and the side we actually see, should be what was the Moon’s far side (the side facing the Sun) when the Moon was new.  But that’s not the case.  What you see, instead, is the same side that faced you when the Moon was new: The so-called near side, the side containing our hypothetical city.  Assuming the city were big enough, you could see the buildings with a telescope.

How can this be the case, when the Moon has traveled all the way around the Earth?  You are on your way to the Moon’s surface to answer this question.

Flash forward a couple of days, and you’ve arrived at your destination. Your spacecraft has deliberately landed smack dab on the Moon’s equator (half way between the “top” and the “bottom” of the Moon as seen from Earth), and smack dab along the extreme right edge of the disk, as viewed from here (we want to be able to see you, so you’re just barely inside the edge).  In addition, you’ve timed your arrival so that it coincides exactly with new Moon.

Imagine getting out of your spacecraft and lying flat on the ground with your head closer to the North Pole and your feet closer to the South Pole (so that if you look down at your feet, you’re looking south). From our perspective here on Earth, your feet would be toward the “bottom” of the moon, and your head toward the top.  Now, stretch your arms out so that your left arm is pointing due East, and your right arm is pointing due West.

Having positioned yourself in exactly the right configuration, all you have to do now is wait, and tell me, back here on Earth, what you experience during the next two weeks.   The goal is to connect your experience with the changes I will see in the Moon’s appearance.   For a short time after you lie down, you’d be in pitch darkness; your particular location along the right limb is still in darkness.  But not for long.  Just a short time past new Moon, I see the Moon as the thin crescent, as in the photograph below that you’ve seen before.  From my perspective, the area around the right limb (the area around you) has transitioned from being in darkness to being in sunlight.  The area off to your west (your right) has not yet undergone this transition, and is still shrouded in darkness.

What causes this transition from darkness to light?  Sunrise, of course: The same event that transforms night into day here on Earth.   You’ve been right on the dividing line between night and day, and now the Sun has risen right at the location along the eastern horizon that you are pointing to with your outstretched left arm.

24 hours now pass. Viewing the Moon from my vantage point here on Earth, the crescent has grown a little fatter, which means that a larger area of the Moon’s surface beyond the limb (off to your right) has become illuminated. I’ve captured this situation in the photograph of the Moon above.  From where you are located, the only way that areas to the west of you can have transitioned into daytime is if the Sun has climbed higher in the sky over to your east, exactly the track it takes across the sky after sunrise here on Earth.

Over the two weeks that the Moon waxes, you stay completely still, and we compare notes every night.  At “half full” from my perspective here on Earth, where is the Sun located for you?  Straight overhead.  It’s high noon on the limb of the Moon, and the temperature is approaching the boiling point of water!  Skip ahead anther week, to when I report seeing a full Moon.  For you, the Sun has moved all the way across the sky and down to the western horizon, where it is now poised to set.  The very next night, I report that the Moon is just past full; the right limb of the Moon has fallen into darkness.  That’s entirely consistent with your report, which is that the Sun has now set, and the temperature is plunging.  Your long lunar night has begun. 

Okay.  You’ve been describing how the position of the Sun has changed every day, rising in the East and setting in the West, just the way the Sun behaves for us here on Earth.  We know that the Sun’s apparent movement across our sky is an illusion, brought on by the Earth’s counterclockwise rotation.  Is the similarity between the Sun’s behavior here and on the surface of the Moon just a lucky coincidence? Hardly.  The only way that the Sun can behave in the same way from your vantage point on the Moon, as it does here on Earth, is if the Moon, too, is spinning on its axis in an easterly direction (or to the left, from your perspective on the Moon’s surface).  

How can the Moon’s rotation be reconciled with the fact that the same side of the Moon always faces the Earth?  At first, this seems rather difficult.  If the Moon spins in a counterclockwise direction, like the Earth does, over time, shouldn't new regions of the Moon’s surface become visible to us? Specifically, why doesn't new lunar territory constantly spin into view along the left limb of the disk (as viewed from Earth), and constantly disappear from view along the right limb?  Why, in other words, don’t you disappear behind the right limb?

The reason is simple, but subtle. Let's shift perspective for just a moment and pose a different question.  If the Moon didn't rotate on its axis the way we’ve established it does, what parts of its surface would we see during the two-week period that it moves from new (barely visible as a crescent) to fully illuminated? Well, since the Moon moves continuously in a leftward direction along its orbital track around the Earth, our viewpoint should be constantly shifting "around" the moon in a rightward direction.  We should be seeing new territory appearing constantly on the right side of the Moon's disk (right limb), where you are, while territory constantly disappears from the left limb.  This fact is a little easier to visualize with our old friend, the phases of the Moon figure, so I'm including it again below.

Let’s pause and summarize these points.  Considering the Moon’s movement around the Earth, the sides of the Moon’s disk where territory should be becoming visible and invisible are exactly opposite the sides where territory should be becoming visible and invisible, given the Moon's own rotation.  That’s a big conceptual mouthful to swallow, so it’s worth kind of savoring it, if you have the time and patience.

Are you still with me, patient Whabbloggers? I hope so, because if you are, you have all the conceptual ingredients needed to put the big picture together.  A key to the entire issue is the speed at which the Sun moves across the lunar sky from your perspective on the surface.  Recall that, approximately one week after your sunrise, the Sun was directly overhead (it was locally noon)?  That means that what takes approximately 6 hours to happen on the Earth (the time needed for the Sun to move from it’s position on the eastern horizon at sunrise to fully overhead at noon) has taken a full week on the Moon.  A week after that, when the Moon is full, the Moon's rotation has pushed the Sun all the way to "sunset position” off to your right.  A day on the Moon is two weeks long, and following sunset, two weeks will pass before the sun once again peeks above your eastern horizon.

It takes exactly one month for the moon to rotate once around on its axis. It also takes exactly the same amount of time for the Moon to complete one revolution around the Earth.  Consider: During the time it takes the Moon to revolve from the position it occupies when it is new (right in front of the Sun) to when it is half full (so its position forms a right angle with the Sun and the Earth), it has moved through 90 degrees, exactly one quarter of its orbital circle.  If the Moon wasn’t spinning on its axis, exactly one quarter of the far side would have swung into view along the right side of the disk.   But, in the week it takes to reach that “half moon” position, the Moon has also rotated, in a counterclockwise direction, exactly one quarter of the way around on its axis, effectively blocking any new territory from appearing.  The two motions completely cancel each other out, leaving the same side of the moon permanently turned toward the Earth! 

There’s just one loose end to wrap up.  For the Earth to complete a day in a scant 24 hours, it has to be rotating at an extremely fast clip, reaching a thousand miles an hour at the equator.  What about the moon, where a day lasts a month?  It turns out that even at the Equator, the Moon only rotates at about 10 miles an hour!  As you move away from the Equator, toward one of the lunar poles, the rotation rate slows down all the way to walking speed and below.  Yes: There are places on the moon where you could walk toward the west and keep the Sun permanently fixed at one position in the sky.  Someday, I can imagine moon settlers living in specially designed double-wide boxcars on a railroad track that completely circumnavigates the Moon.  With the boxcar moving along the track at just walking speed, the residents would live in perpetual daylight, with the Sun permanently frozen in a position low enough in the sky (shortly enough after sunrise) that the ambient temperature would always be a balmy 72 degrees!   Yes, you could construct things on the Moon so you lived your life in permanent daylight, and endless summer!

And with that, it’s time for both of us to leave the Moon.  Next blog, I will tackle the behavior of another compelling object in our sky, the planet Venus, the Evening and the Morning Star.  The question is, how can it be both?

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!

Of Wax and Men

Last time, with a little help from our friends the Beatles, we used our powers of visual imagination to better understand the daily behavior of the Sun in the sky (and below it, at night).  As we discovered, the key is to think of the Sun’s motion as the product of the Earth spinning round and round and round on its axis.   This time out, we’re going to start to tackle the behavior of our nearest neighbor in space, the Moon.   First up will be the remarkable changes that take place in the Moon’s appearance, and position, over the course of each month.  What’s in this for you?  Suppose you went outside last night and casually noticed the Moon.  Could you have answered a companion’s questions about where the Moon would be in the sky tonight?  Where it will be tomorrow night?  And how its shape is going to alter?  Well, if you can get through the next two blogs with me, answering questions like that will become child’s play.  You’ll be able to effortlessly analyze the Moon’s appearance at any time of the month and know just where it is in its monthly cycle, why it appears the way it does, and where it is going.  Fun, right?  But not only that.  In the final installment of the Moon series, we’ll build on your newfound understanding to uncover and discuss a fascinating “secret” about our nearest neighbor and its behavior.

As you know already, the Moon’s monthly cycle starts and ends with the “New Moon” phase, when the Moon is completely invisible.  What hides it from our sight?  The answer rests with two simple facts.  First, the Moon is a big round ball.  Just like the Earth, exactly half of the Moon’s surface (one hemisphere’s worth) is always illuminated by the Sun, and the other half is always in darkness.  The figure below illustrates the second fact:  The Moon continuously circles (orbits) the Earth, completing one revolution every 27 days.   Notice from the figure that when the Moon is new, that corresponds to the point in the Moon’s orbit where the Earth, Moon, and Sun are perfectly aligned, and the Moon is between the Earth and the Sun.  See where the sun’s rays are hitting the Moon’s surface?  All the illumination is confined to the “far side”, the hemisphere that’s opposite the hemisphere that faces us (the near side).  In other words, when the Moon is new, the far side is experiencing daytime, and “our” side is experiencing night.

As I noted, the Moon takes 27 days to complete one orbit, and another two days before it again lines up with the Sun (the reason for the discrepancy is that the Earth is in constant motion around the Sun.  But that’s a topic for a future blog about, of all things,  astrology).  At an average distance from the Earth of about 239,000 miles, the Moon covers over 750,000 miles per orbit.   To go that far in just 27 days, the Moon has to be moving in excess of 1000 miles per hour.  That’s rather fast for an object that looks completely stationary when you view it in the night sky, wouldn’t you agree?  But I’m getting ahead of myself because, since we’re still talking about the new Moon, you can’t see it yet!

I know I said (and the figure gives the impression) that the new Moon is positioned directly between the Sun and us, but that’s not quite true.  Usually, the Moon’s path takes it just slightly above or below the Sun rather than directly in front of it.   On the rare occasions where the Moon does pass right in front, some lucky locations here on Earth experience a solar eclipse.  However, with that speed of 1000 miles an hour, the Moon doesn’t stay aligned with the Sun very long; very soon, and very quickly, it moves off to the left, and keeps on moving.

One of the primary tools in our “Moon visualization” arsenal is the simple fact that the Moon moves in an eastward direction, which means that it is always moving to the left through the sky.  An amateur astronomer took the photograph on the right immediately after sunset, scant hours after the moon was new.  That razor-thin crescent is, of course, the Moon, having traveled just a small way to the left (east) of the Sun and, since the orbit is curved, a small way “back” in our direction. 

The Moon assumes this form of crescent once every month.  You’d have to be really on your toes to see it, though, because it is very close to the Sun, and quickly follows the Sun below the horizon (that is, it quickly sets).  Even though the Moon is moving east, you’re viewing it from a vantage point that is itself spinning around in an easterly direction, and this rotation is “pushing” the Moon (and the Sun) down and to the right much faster than the Moon is moving to the left around its orbit.  Recall in the Beatle’s blog that I discussed how the Earth’s rotation controls the movement of the Sun after sunset, “pulling” it ever further down and to your left (if you’re looking south) until the Sun swings back into view along the Eastern horizon (dawn)?  That reminder should give you a sufficient basis to visualize what happens to the Moon after it sets on the heels of the Sun.  At any rate, I encourage you to try.   The first commenter who correctly identifies when the Moon “rises” in relation to the Sun gets today’s comment section prize!  

The next order of business is to understand why a sliver of the near side is now illuminated.  Looking back at the “phases of the Moon” figure, fast-forward about two weeks, to the point when the Moon has traveled exactly half way around its circular orbit.  You see from the illustration that the Sun, Moon, and Earth are once again lined up, but this time, with the Moon furthest from the Sun and the Earth in the middle.

See what happens now?  The Sun’s rays are falling directly on the near side, lighting it up fully, and it’s the far side that’s shivering in darkness (by the way, this illustration helps you see why lunar eclipses, which occur when the Moon passes through the Earth’s shadow, only happen when the Moon is full). 

Now, turn the clock back two weeks, when the Moon was still a thin crescent near the Sun.  Every day, the Moon travels about 25,000 miles along it’s orbit, so every day, it draws a little closer to the position that it’s eventually going to occupy when full.   Moving ever closer to that position causes us to be able to see a little bit more of the sunlit hemisphere each night; a good way to think about this is to imagine the moon moving, not only out and to the left of the Earth, but also a little more “along side” the Earth, thus revealing a bigger and bigger piece or fraction of the sunlit hemisphere that used to be hidden behind the right limb.  After about a week, the Moon is at the top of the figure, and forms a right angle triangle with the Earth and the Sun.  At this point, exactly half of the illuminated hemisphere is visible to us, and we say the Moon is “half full”. Where is the other half? On the far side, of course, meaning exactly half of the far side is illuminated too.

But there’s something else that’s important to glean from the figure.  Take a look at the little circle in the middle representing the Earth, half of which is lit, and half of which is not.  You’re seeing the Earth as it would look if you were directly above the North Pole.   Now imagine the little circle rotating in its counterclockwise (Eastward) direction around the pole.  Now change your perspective, and pretend to move  from your “eagle eye”, way above the North Pole, to your present location on the Earth’s surface, right on the line between the dark side and the light side.  That position corresponds to sunrise, of course; the Sun is coming into view on the eastern horizon.  As the day wears on, you continue to rotate in a counterclockwise direction.  Although the half-full Moon starts out below the horizon, the Earth’s spin pulls it closer and closer to your eastern horizon, until, right at local noon, the Moon swings into view.  Yes, when the Moon is half full, it rises right around local noon (and is clearly visible, even thought it’s broad daylight).  Your day progresses, the Earth continues to spin, and the Sun slowly sinks into the West.  Meanwhile, the Moon continues to climb higher into the sky.  At local sunset, you are now positioned right at the very top of the little Earth ball in the figure, and the Moon is now directly overhead. With nightfall, the Moon blazes brightly in the sky.  Over the evening hours, though, it follows the sun into the west, until finally setting half-way through the night.

As the Moon proceeds towards full, her thousand mile-per-hour orbital speed opens up more and more distance between her and the Sun.  If you visualize the relative locations of the Sun and Moon near sunrise, (i.e., below your feet and off to your left if you are outside facing south), you’ll see that the increasing Sun-Moon distance is creating a longer and longer delay between sunrise and moonrise.  The increase also means that the Moon is visible in the night sky for a longer and longer period after sunset. 

Eventually, the Sun-Moon gap gets big enough that the Moon is both full, and rises just as the Sun is setting.  Again, the figure shows why this is the case:  The Moon is only full when it is on the other side of the Earth from the Sun, as far away from the Sun as it can get.

OK.  We’ve covered things up to the full moon phase.  In the next blog, we’re going to tackle the other half of the cycle, when the Moon starts waning.  If you’re like most people, and like me before I started to pay attention to these things, the waning Moon is much less familiar than the waxing Moon.   This is because most people stay up after sunset, so they gain plenty of experience with seeing the Moon when it is visible in the early nighttime sky.  In the waning phase, though, the constant leftward movement of the Moon in its orbit causes it to move back in the Sun’s direction, on a trajectory that has it approaching the Sun from the right-hand side.  As we’ll see next time, this geometry means that the Moon rises later and later each night, after we’ve typically gone to bed, and remains visible longer and longer in the morning.  However, since we much more naturally associate the Moon with a nighttime object, we almost never look for (or see) it in the morning, and our nearest neighbor completely exits our consciousness.

But more on that next time.