Why Does the Moon Have Phases?

** My apologies on the lateness - it's been a busy month. **

 

Full moon. Crescent moon. New moon. There's no doubt you've previously heard these or even talked about them. Perhaps you even know the less common or more specific phases of the moon. Yet, there still seem to be misconceptions about the moon's appearance. I've already covered why the full moon can seem so large and why the moon can appear orange when it's on the horizon. Here's some more information about the moon's appearance:

The Moon's Orbit & Apparent Size

Let's start with some important background knowledge. The moon orbits the Earth on a nearly, but not perfectly, circular path. As a consequence, there are two points where the moon will be at its closest (perigee) and farthest (apogee) distance from the Earth with every other point somewhere in between. The difference in distance between the two extremes is enough to be noticeable, at least with pictures. It might be a bit difficult to see the difference by just going outside and taking a look. Another neat thing about the moon is that it's tidally locked in its orbit. For every one orbit around the Earth, it will rotate exactly one time. The result is that someone on the Earth can only ever see one side of the moon. If you compare the two pictures above, you'll notice that the dark lunar mare near the bottom right corner of the crescent moon is the same as that on the right side of the gibbous moon. On the flip side, there is one side of the moon that we can never see from the Earth. This is where the "dark side" of the moon comes from. The dark side of the moon isn't dark in the true sense of the word. There are times when the sun shines on it. However, it's considered "dark" because people were never able to see it until lunar orbiter spacecraft were built and sent into space.

Moon Phases

Unlike the sun, the moon does not emit light on its own - we only see it because of the sunlight that it reflects. Like the Earth, the moon will only have sunlight illuminating half of it at a time. The half that is illuminated is not always that half of the moon that faces the Earth, which leads to the phases. I'll be a bit more specific with a few examples to help clear up any confusion. When the moon is full, the half that is illuminated is also the half that faces the Earth. The moon is on the opposite side of the Earth than the sun, so the sunlight illuminates the half that faces us. The opposite of this is the new moon. In this case, the moon and sun are on the same side of the Earth, so the half that is illuminated is the "dark" side of the moon. 

This case also brings up another interesting point: the new moon should be located close to the sun in the sky. In other words, we see the new moon during the day rather than at night. It doesn't disappear from the sky or become dark. Instead, it is just visible at a different time of day. 

Let's leave this little detour and get back to the phases. The last example that I'll give is the half moon. To see a half moon in the sky, the moon would be 90 degrees from the sun. If you have trouble visualizing that, picture it this way: if the Earth is a clock, the sun will appear near the 12 and the moon will appear near the 3. For this phase, half of the "dark" side is illuminated and half of the side that we see is illuminated. To get the rest of the phases, we can just rotate the moon around the Earth. The farther it gets from the sun, the closer it will get to a full moon. As it moves from full to new, a smaller and smaller section (visible to us) will be illuminated. If you need some extra visualization, you check out this chart or try the activity listed below.

Eclipses

I'm sure most people are well aware of solar eclipses, but did you know that the moon can be eclipsed, too? A lunar eclipse happens when all of the moon, Earth, and sun are aligned with the Earth in between the other two. As the moon passes behind the Earth, it moves into the Earth's shadow. Since the moon appears lit due to reflected sunlight, being in a shadow will make the moon appear darker. At the start of the lunar eclipse, a growing fraction of the moon will appear blacked out. As more and more of the moon moves into the Earth's shadow, it starts to turn a dark orange color. The next total lunar eclipse visible for the Americas will be on October 8, 2014, so get your binoculars ready.

Try It at Home

You can see for yourself how this process works with just a (solid) ball and a flashlight. Begin by placing the ball on a table with the flashlight pointed at it from a couple or so feet away. Walk around the table with the surface at eye level and watch the appearance of the ball. When you're opposite the flashlight (sun), the ball (moon) appears as a dark, "new moon" phase. Go around to the same side as the flashlight, and you'll see a full moon (or rather full ball?). As you walk around, you'll see the phases of the ball changing just like the moon. However, the moon orbits the Earth, not the other way around. For a more realistic demo, place a larger ball (or globe if you have one on hand) a few feet from the flashlight. You can simulate the moon and its phases by moving the small ball around the large one. Place a "person" on the large ball. How does the small ball's phase change from your person's point of view as it moves around in its orbit?

Keep in mind that the moon's orbit is slightly tilted. This means that the moon and Earth rarely appear directly between the sun and the other. By aligning all three, you are creating a solar (moon in middle) or lunar (Earth in middle) eclipse.

References & Further Reading

Lunar Eclipse Page - by NASA

Volcanism on the Moon - by Robert Wickman

The Lunar Orbiter Program - by Lunar and Planetary Institute (NASA)

Earth's Moon Phases, Monthly Lunar Cycles (Infographic) by Karl Tate

Water Waves

Chances are you've been to a beach before, and it's quite likely that there were waves whether you were at the ocean, a lake, or some other sizeable body of water. If you've ever watched the waves for a while, you might have noticed that they tend to arrive parallel, or nearly so, to the shoreline. Even if they are at angle when they are far away, they still reach the shoreline nearly parallel. Why? Another thing noticeable to anyone who has watches waves, and is visible in the above photo, is the crashing and "breaking" of waves as them come towards the shoreline. How does that happen?

The first question can be answered by fluid dynamics. In shallow water, that is at a depth which is smaller than the wavelength (measure of distance between wave crests), the speed of the waves depends on the water depth and gravity only. Specifically, the speed is equal to the square root of gravity times depth. This tells us that waves will move slowest in the shallowest water. Let's assume that there is a nice downward slope as you move away from the shoreline. Start by imagining a wave that begins parallel to the shore. As it moves closer, the shallower water will cause it to slow down. Now, what happens if the wave is rotated so that it starts at an angle to the shoreline?

If you think of the wave crest as a straight line, one end will be closer to shore than the other if it's at an angle. The far end is in deeper water and will travel faster than the shallow end. This causes the wave crest to turn until both ends are at the same depth or it hits the shore (whichever comes first). If you're having trouble imagining this scenario, try thinking about an axle with a wheel on each end. If you move both wheels at the same speed, the axle will move in a straight line. What happens if you hold one wheel stationary (more or less) and move the other? The moving wheel will rotate around the other one. Now, allow both wheels to turn but move one a little faster than the other. The axle will move forward while also gradually turning. This is analogous to our wave scenario.

That's one question answered, so on to the next one: what causes waves topple over? It may seem strange but this answer relies on the same principle as the first one. In this case, we can no longer treat the wave crest as just a line; it does have a finite width, after all. Since the back of the wave is in deeper water than the front, the back will travel faster and eventually catch the front. The water particles will accumulate behind the crest until it reaches a point when the added water forces the wave to topple over. If the wave is large enough, you may notice how the top rolls over the bottom. As water piles up behind the wave, the water becomes even deeper than the water in front. Consequently, the water in back moves even faster, particularly that on the surface. This helps the top of the wave crest to roll as the wave topples over. Keep in mind, however, that this is only valid for waves in shallow water. It's not often that you see breaking waves in the middle of a lake, partly for this reason. When you do see them, the wind is generally very strong and helps force them to topple over.

Fortunately, these are easy things to go see for yourself, so get out and enjoy the coming summer. Maybe the next time you find yourself at a beach, you'll pay a little more attention to the waves.


References
Waves in Fluids (video) by National Committee for Fluid Mechanics Films (1960s; more here)

Sonoluminescence

I believe the credit for this one goes to my lab partner.

At first sight, those three little pin-pricks of light may not seem very spectacular. However, the phenomenon that creates them (sonoluminescence) is really quite interesting. Some scientists once believed that the light was created by the ever elusive nuclear fusion, although this idea has since been almost completely rejected. We can get to the basic idea by breaking apart the name, sonoluminescence. The "sono" refers to sound, while "luminescence" is the glow. In essence, the lights are created by sound.

In the experiment above, we start with a rectangle container filled with purified water. The metal cylinder at the top that sticks into the water is a transducer; in other words, it will create the sound for us. Sound is a pressure wave; when it travels through the water (in this case), it causes the water molecules to oscillate back-and-forth causing regions of compression, where molecules are pushed together, and rarefaction, where particles are pulled away. The sound that we used was a higher pitch than humans can hear, known as ultrasound. If the frequency of the sound (that is, how many times it oscillates per second) is chosen correctly, a standing wave will form in the water. If you and a friend each hold one side of a rope and shake it in sync with one another, a standing wave will form. You'll know when one forms because there will be at least one point on the rope that doesn't seem to move. The stationary point is called a node. Between each node is an anti-node, the point that has the most motion. The anti-node is what we're interested in.

The next step is to make a very small bubble very near the anti-node. It is a difficult task, but if done correctly, the bubble move into the anti-node and get trapped there. Because the anti-node is where the wave motion varies the most, this point will rapidly oscillate between a high pressure state (compression) and a low pressure state (rarefaction). During the high pressure state, the air in the bubble will be squeezed into a smaller volume. The opposite happens during the low pressure state; that is, bubble's volume grows and the air inside can spread out more. This should help to explain why it is so difficult to trap a bubble. If you squeeze or stretch it just a little too much, the bubble will pop forcing you to start over.

So, we have a bubble in water that is changing volume due to being trapped in an anti-node of a sound wave. Now that the experiment is set-up, we can move to our initial question: why does the bubble light up? This is a difficult question to answer and, frankly, the exact answer remains a mystery of physics. However, we can make measurements that give us some insights into the process and allow us to speculate on what it could be. The light is emitted when the bubble is squeezed. At this point, the volume that air molecules have is very small. When the bubble is squeezed, the air molecules heat up. In essence, this heat causes the molecules to emit light. What isn't known is the process through which the light is emitted. The most accepted idea is that the heat energy pulls the molecules apart. When the molecules re-form, they emit light. However, this does not explain the spectrum (or variety) of light emitted. As the experiments continue, scientists hope to one day find the answer.

While this is a very neat experiment, it might not have much practical use. However, sonoluminescence does occur in nature. Have you ever heard of the beautiful mantis shrimp? Perhaps you've seen the comic about it that's been getting passed around lately. The mantis shrimp can close its claw so quickly that it creates a high enough pressure wave to form a cavitation bubble that can glow by sonoluminescence. Just be careful if you go looking for one. Not only do they create a large enough pressure wave to kill their prey, but it is even strong enough to break through the glass of an aquarium.


References and Further Reading
The Sonoluminescence Process by the American Institute of Physics
Sound is a Pressure Wave by The Physics Classroom
Sonoluminescence: Sound into Light (Scientific American article) by Seth Putterman
Mantis Shrimp by Chesapeake Bay Program
Why the Mantis Shrimp is my new favorite animal by The Oatmeal

Schrodinger's Cat

Up to this point, I've covered mostly topics involving geology and light. To shake things up a bit, this post will cover something a bit different - quantum mechanics. Generally speaking, quantum things are too small to photograph without a powerful microscope. However, there is a good macroscopic analogy for one of the most important quantum processes. The Schrodinger's cat paradox was originally used by Erwin Schrodinger to show how silly quantum mechanics seemed when applied to the macroscopic world in which we live. In contrast, his cat paradox has turned into one of the most widely used analogies for explaining the quantum phenomenon of superposition.

If you're not familiar with the Schrodinger's cat paradox, here's how it goes:
(Note: Do NOT try this at home. This is a thought experiment only, not something actually carried out. Please do not needlessly harm animals.)

Start with a solid box. Inside the box is a vial of poison attached to a special device that contains an unstable atom. This unstable atom has a 50% chance of decaying within an hour. If the atom decays, the device will trigger a hammer to smash the vial of poison. Let's now place a cat inside the box and then close the box. If we wait an hour, there is a 50% chance that the device was trigger and the vial smashed. If that happened, we would expect to see a dead cat upon opening the box. However, there is also a 50% chance that nothing happened and the cat is still alive. So, without opening the box, is the cat dead or alive after an hour of waiting?

As long as we don't observe the cat by opening the bow and looking at it, we can say that it is in a state of being both dead and alive. Now, this doesn't make much sense. Obviously, an animal can only be dead or alive, not both. This is why it is a paradox and why Mr. Schrodinger used this as an example of how ridiculous the quantum theory of superposition seemed. Now, what exactly is superposition and why is it important?

From the cat paradox, you may have figured out what superposition is. In simple terms, superposition means that a quantum particle, such as an electron, can be in multiple states at the same time. There is one catch though - if you try to measure or observe the particle, it will assume only one of the states. Going back to the cat paradox, once you open the box and check to see what happened, the cat will be either alive or dead. You can no longer say that it exists in both states.

One consequence of this is that the act of measuring a system can affect how the system is behaving. This is something that rarely happens in the macroscopic world. If you use a ruler to measure the length of a pencil, the act of measuring will not change the length of the pencil. Its length will be exactly the same before and after you measure it. In the quantum world, however, things are different. Because particles are so small, even light can easily affect the properties of a particle. For example, when light collides with an electron, it can pass momentum to the electron and change its speed and/or path of motion. This consequence becomes significant when trying to do things on a quantum level, such as building a quantum computer. These computers use the fact that an atom can simultaneously be in multiple states to speed computations by doing more than one at the same time. However, to get the computation results you would need to interact with the atoms. This causes them to lose all but one of their current states and breaks the multiple-state basis of the computer.

There are experiments that modern day scientists can use to test the theory of superposition. Even though it was questioned back in the 1930s when the cat paradox was first introduced, it is now a well accepted theory. That said, from Mr. Schrodinger's days until now, one thing remains the same - the quantum world remains a very strange one that few people can even begin to fully understand.


Further Reading
Schrodinger's Cat Comes into View - By physicsworld.com
No. 347: Schrodinger's Cat - By John Lienhard at the University of Houston
Another step toward quantum computers: Using photons for memory - By Eric Gershon, phys.org
What Is Quantum Superposition? - By the Science Channel

The Life of a Star

(Apologies for the blurriness - long exposure photos require a tri-pod, which I didn't have on hand that night.)

Orion, the warrior. This is probably one of the most well known constellations, particularly Orion's belt. If you're not familiar with constellations, here's how to pick out Orion. The three stars that make a vertical line at center are Orion's belt. Just to the right of Orion's belt, there is a near horizontal chain of stars. This is Orion's sheathed sword hanging from his belt. The four brightest stars that form a box around the belt and sword are Orion's feet and shoulders. You may also notice a sub-horizontal arc of bright stars between the top of the frame and Orion's (highest) shoulder. This is the bow that he is using to hunt Taurus the bull, or so the legend goes.

If you look closely, you might notice that the lower of Orion's shoulders appears to have a reddish color as opposed to the more usual blueish white color of stars. This star is called Betelgeuse (generally pronounced "Beetle-juice") and is a type of star known as a red supergiant. Similarly to people, stars are born, change as they age, and eventually die. The very largest of the stars die young, sometimes after only a few tens of thousand years. The smaller stars, however, can look forward to a long life, often reaching the ripe old age of billions (our sun) or possibly even trillions of years. Astronomers use the Hertzsprung-Russell (or H-R) diagram to help find out what stage of life a star is in.

Stars spend most of their lifetimes as part of the main sequence. Our sun is roughly an average main sequence star - not a big, bright blue giant nor a small, dim red dwarf. The hotter the star is, the more blue it appears. Therefore, the red Betelgeuse is now cooler than it was as a youngster. As stars age, most move off of the main sequence and into the realms of the red giants and supergiants. (The smallest stars may never quite reach the "giant" phase.) When stars become red giants or supergiants, it is a sign that they are nearing the end of their lifetime. The smaller red giant stars, such as what our sun will become, release their outer atmospheres when they die. The "stardust" expands into space over time, forming a planetary nebula and leaving just the star's core, now called a white dwarf, at the center to slowly cool and fade away. The biggest giants and supergiants, however, meet a much more exciting end. They die in massive stellar explosions, supernovae, which send much of the star's matter flying through space. The remaining core cannot support its own weight anymore and collapses into a very dense neutron star or, in the case of the biggest of the big stars, a black hole.

For stars to continue living, they require energy just as we do. People and animals get their energy from breaking down the food that they eat. Stars, on the other, are more like plants, in the sense that they both make their own energy. However, stars do not require energy from an outside source like plants do when they take in sunlight to help produce their energy. Stars are actually made of their food - in a sense, they live by eating themselves. The most common element in the universe, by far, is hydrogen followed by helium, lithium, and so on down the periodic table. Stars get energy by fusing these small "light" elements into big "heavy" ones. Inside the core of a star, temperatures and pressures are so extreme that the electrons are ripped off the atoms to form a plasma of electrons and ions, the atomic nuclei that is made of protons and neutrons. This hot, dense plasma is the perfect environment for fusion to occur.

Within the hot, charged plasma soup at the star's center, collisions between nuclei are unavoidable. If the collision has enough energy involved, the two nuclei will fuse together into a single larger one. For example, two hydrogen nuclei can combine to create a helium nuclei. This is the process which builds about half of the elements on the periodic table. However, fusion will only release energy when small nuclei are involved. The larger the nuclei, the more energy it takes to fuse them and the less energy is released in the process. As the star turns more and more of its light nuclei into heavier ones, it becomes harder and harder for the star to continue fusing nuclei to get the energy it needs to survive. This is the stage of life that Betelgeuse and other red giant stars have reached. After reaching the red giant stage, the star will continue fusing nuclei until it reaches the point when fusion requires more energy than it produces. For smaller stars like the sun, this limit is reached when carbon is produced. The giant stars, however, can continue fusing nuclei until the entire core has been changed into iron.

Iron is the most stable of all elements. The elements that come before it on the periodic table can produce energy when they are fused together, and those after iron produce energy when they are split (fission). Fission is the process in nuclear bombs that use uranium or plutonium. Because a star cannot create nuclei heavier than iron with fusion, these nuclei can only form when there is enough energy for fusion without needing any energy to be produced - during the star's death. The extreme forces involved in supernovae are capable of creating the heavy nuclei. Since supernovae are basically giant explosions, both the heavy nuclei and the lighter nuclei that were never fused are thrown out into space and end up as part of the next generation of stars and planets. Eventually this stardust, created in extreme environments with fusion and scattered throughout the galaxy by explosions, is brought together in the perfect way to create beings capable of understanding this process - us. As it turns out, we all have an explosive past.


References
Orion - by the Peoria Astronomical Society
The Hertzsprung Russell Diagram - by Richard Powell
Supernovae - by C.R. Nave at Georgia State University
Periodic Table of the Elements - by Mark Winter at The University of Sheffield
Basics: What are Plasmas? - by Dr. Timothy Eastman at Plasmas International
Fission and Fusion - by Yoseph Murtanu on the ChemWiki by University of California, Davis

Geologic Faults

By far the most famous fault, in the US at least, is the San Andreas Fault in California. As a kid on the other side of the country, I always pictured it as one giant chasm that went on forever - similar to what you might see in a movie. Of course, I later learned what the San Andreas Fault really is and also that faults are not really giant gaps in the ground as the movies might have you believe. In fact, most faults, such as the one pictured above, are much smaller, sometimes even microscopic. Nor is California the only state with faults and earthquakes. Over half of the states in the US have at least a moderate earthquake risk, which can be seen on earthquake hazard maps published by the USGS.

Simply put, a fault is a crack in rock on which slip can occur. The rocks on top can either slip downward (normal fault), upward (reverse fault), or move side-to-side (transform fault). Can you figure out what type of fault is pictured? Hint: try to match the strata (or rock layers) on each side to see which way the rocks on top moved. This fault is far from California; it's located at the Seneca Stone quarry in the Finger Lakes region of New York. There is a big difference between this fault and the San Andreas.

Faults form for different reasons. The San Andreas Fault is a major fault because it lies at a plate boundary. The Earth's crust is made of multiple large pieces called tectonic plates. These plates "float" on the mantle and are slowly moving (a few inches per year). At plate boundaries, three main things can happen: one plate can move underneath the other, the plates can move apart, or they can move past each other sideways. Of course, they aren't moving continuously. Friction can cause the plates to get stuck on each other at the boundary. However, stress builds up if the plates are stuck. When the stress becomes large enough to overcome friction, the plates will slip and release the energy that had been stored. This energy is mostly released as waves in the Earth that we call earthquakes.

However, many faults don't occur on plate boundaries. Central New York, for one, is far from a plate boundary. Volcanic processes, such as magma intrusion and gas release, can create faults or enlarge cracks. Colliding plates create stress and strain throughout the plate, not just at the boundary. This can cause intra-plate faulting and create fold mountains. Old, brittle rocks tend to break more easily and will be more likely to have faults and cracks. For our New York fault, we can look at the geologic history of the region. The Finger Lakes were created by glaciers. When glaciers form, their weight causes the crust to sink - similar to what happens when you sit on a mattress. Eventually, the glaciers receded which allowed the crust to rise back up. The stress and strain associated with the crust's bending can create cracks in the rock. This would be a good guess to the origin of the fault in the picture.

Many faults are hidden beneath the surface. We only find them when they become active (i.e. move and create earthquakes) or when we dig into the ground for things like rock quarries, mining, and roads. At this moment, you could be sitting right above a fault and not even know it. Don't worry too much though. Few faults are large enough or have enough energy built up to create a major earthquake. The areas that do have to worry are well known and generally well prepared. Make sure to check out the earthquake hazard map to see how likely an earthquake is to occur near your house.



References and Further Information
San Andreas Fault - by David Lynch
Hazard Mapping Images and Data - by the US Geological Survey (USGS)
United States Geological Survey
Fault Motion - by the Incorporated Research Institutions for Seismology (IRIS)
Plate Boundaries - by Maggi Glasscoe at NASA's JPL
Fold Mountain - by National Geographic
Earthquake Facts & Earthquake Fantasy - by USGS