The Limits of Chemistry

In the last post, I talked about how it was basically impossible for humanity to get to another star using modern technology. For this post, I would like to talk about why that is, and why we don’t have space hotels or moon bases yet.

The whole reason comes down to chemistry. The vast majority of rockets that exist and all rockets that take anything into outer space use chemistry to make the rockets go.  A few posts ago, I talked about thrust. Thrust is a pretty simple concept – basically, a rocket moves forward by expelling things quite quickly out the back.  There are two terms in the thrust equation, the mass flow rate (how much stuff the rocket spits out), and the exhaust velocity (how fast it spits it out).  Simple.

The mass flow rate is pretty easy to understand also.  It basically is just how much fuel the rocket uses per second.  In some ways, it is like hitting the gas pedal on your car: the harder you push on the gas pedal, the more gas flows into the engine and the faster you go.  That is a pretty simplified version, but it is about right.  A larger rocket really just has a larger mass flow rate.  The space shuttle had pipes that fed into the main engines that were about a foot in diameter.  That is a LOT of fuel!  The Saturn V used roughly 1000 gallons of fuel per second.  They actually had a very hard time mixing the fuel with oxidizer on the Saturn V, since the flow rate was so high (they didn’t have great fuel injectors in the 60s!), and they would get explosions in the engines.  Instead of giving up, they simply made the combustion chambers more sturdy to handle the explosions.

Anyways, the mass flow rate is how much fuel the rocket uses per second.  This is set by how big the engine is, and there is no real limit, except how big you can build the engines (or how many engines you can stick on a rocket – yes, I am talking to you Space-X with your 27-engine Falcon Heavy rocket).

The other term in the equation is the really tricky one – this is the exhaust velocity, which is how fast you can expel the mass out the back. Simplistically, you would think that this would be easy to turn up, but it is not. There has not really been any big revolutions in the exhaust velocity in a long time (like the 60s). The most common way to make a large exhaust velocity is to make an extremely hot gas, and direct it into a nozzle.  You mix fuel with an oxidizer, and you get an explosion. Then you turn the explosion into directed energy using a nozzle.

We can design pretty good nozzles.  They can be something like 90%+ effective at turning the thermal energy into kinetic energy.  That is great.  There is no factor of 10 improvement or anything that can be gained from nozzles.

The big problem behind this is chemistry. Let’s take the space shuttle’s main engine. This engine used two of the most abundant elements we have on Earth: Hydrogen and Oxygen.  You cool them both down until they are liquids, store them until the rocket is ready to fly, then combine them in the engine.  What is the result?  Water!  The space shuttle’s main engine exhaust is water!  Crazy, eh?  The amount of energy that is released when 2 molecules of Hydrogen are introduced to one molecule of Oxygen is exactly the same every time – about 6 eV, which is a tiny bit of energy.  The fundamental issue here is that we get only a very specific amount of energy out of the reaction.  If we take the 6 eV of energy and we turn that into an exhaust velocity, it ends up being about 3,000 m/s.  This is very fast at first glance, really it is not.

Space Shuttle Columbia taking off for the first time.  There are really 5 engines that you can see if you look really closely.  The big white stick things are solid rocket boosters – they don’t burn hydrogen and oxygen). On the back of the shuttle proper (orbital vehicle, to be more precise), you can see three engines.  The huge white thing that the shuttle is attached to os a gigantic fuel tank.  That is where the hydrogen and oxygen are located.

This small amount of energy totally limits us so that rockets have to be huge.  If the chemistry were such that these elements released 10 times more energy, then we could (in theory) make rockets that were much smaller (more than 10 time – by a lot). In fact, we play around with different chemicals to try to make a larger exhaust velocity, but the problem is that the chemicals that produce the most wickedly large exhaust velocities are horrific to work with – like super caustic and really, really bad for humans. So, there has to be a balance between safety (which costs a LOT of money or lives) and exhaust velocity too. This huge Russian rocket explosion that killed over 100 people, was while they were trying out new fuels that would have larger exhaust velocities.

We have not invented a better way to get off the ground than using a chemical rocket engine.  There are a TON of other ideas out there, but it is this fundamental limitation of the exhaust velocity that limits our ability to actually go very many places far away from Earth.

Next time, I will go through a simple formula that was invented in the early 1900s that predicted this whole problem. It was a good 40 years before modern rockets were even invented! And then, I will start posting about all of the absolutely crazy ideas that could possibly get us to the stars. Well, ok, maybe not.  But, they are awesome anyways!


Traveling To Another Star: The Idea

Given the news of the finding of an Earth-like planet around one of our nearest stars, as described here, it is interesting to see if we could ever actually get to this planet.  I am going to start a small series on the idea of getting to another star. I will cover a few topics such as the problem with getting to another star with our current technologies, what new technologies we could use, and why getting some satellites to another star might not even help.

Right then.  Why can’t we get to another star now?

Let’s say that we want to get to a star that is about 4.5 light years away.  A light year is actually a distance – it is the distance that light travels in an Earth year (as opposed to a Martian year). It is roughly 9.5×10¹² km. That is a long distance. To give some perspective, it takes just over 8 minutes for light to get from the sun to the Earth. To reach Pluto, it takes light 5.3 hours. Given that it took New Horizons 9.5 years to get to Pluto, you can see that it will take us a fair bit of time to go all the way to another star.

But, let’s try.

How would you get a satellite to another star?  Well, you would have to accelerate it up to some speed, then cruise for a long time, and then decelerate it once it gets to the other star. Ideally.

Since we would want to actually see what this thing measures in our lifetime, let’s assume that we can get it up to about 1/3 light speed (a nice round number of 100,000,000 m/s), and let’s assume that we can accelerate at roughly the gravity of Earth, which is about 10 m/s². If we do a tiny bit of math, we can see that it will take us 10,000,000s to reach this speed, which seems like a huge amount of time. But, considering that there are 86,400s in a day, this is only about 116 days or about 4 months.

So, if we wanted to get to another star, we would take about 4 months to accelerate up to about 1/3 the speed of light, cruise for about 12 years, and then decelerate for about 4 months, with a total trip time of about 13 years total. Not that long! Why aren’t we packing our backs now (or building a satellite for the 13 year journey)?

The problem with this is that we don’t have the technology to accelerate something at 10 m/s² for 4 months.  If you watch a rocket launch, you will see that the rocket accelerates for only a few minutes – like 10.  In fact, to reach orbital speeds (7,600 m/s) , if you accelerate at 10 m/s², it will take about 13 minutes. To break away from the Earth, which is much harder, it will take about 20 minutes. Even with math, that is significantly shorter than 4 months.

One of the reasons that I brought up New Horizons before is because it is one of the fastest satellites ever launched. It left our orbit going about 36,000 MPH, which is about 16,260 m/s.

Let’s put this speed in the context of getting to that other star which is 4.5 light years away. 16,260 m/s is about 0.0000542 times the speed of light. So, to get to another star, New Horizons would take about 83,000 years. That, my friends, is a long time, and why no one is packing any bags.

It will be a long time before we can make this journey in any reasonable amount of time.  Next time, let’s talk about why we will never be able to get to another star using our current rocket technology.  I am not even joking here. Using modern rocket technology, it would more mass for fuel that there is matter in the entire universe to accelerate us up to anywhere close to the speed of light. But, let’s talk about that next time.

How Much Fuel Does an Airplane Use?

One of the interesting things about learning about thrust for a rocket, is that we can use the same types of formulas for airplanes. So, let’s take a few minutes and figure out how much fuel an airplane uses to travel from one place to another. We can also look at why airplanes fly at the altitudes that they do, and how the wind affects the fuel used.

How to actually do this?  Well, we need two equations that we have talked about on different posts: (1) the equation for thrust; and (2) the equation for the drag force. When a plane is traveling between two places at a constant altitude, we can ignore the forces in the vertical direction, since the gravity of Earth is balanced by the lift from the wings.  In the horizontal direction, the forces are also balanced (since we are traveling at a constant velocity), namely the thrust of the airplane is balanced by the drag force from the airplane moving through the air.

A Boeing 747 for Delta Airlines.

So, what is the drag force on an airplane?  Well, we can calculate it using the formula: F=0.5*Rho*Area*DragCoefficient*Speed², where Rho is the mass density of air, Area is the frontal area of the airplane, DragCoefficient is the Drag Coefficient of the airplane, and Speed is the speed of the airplane (with respect to the wind). Let’s do a simple example, and take the Boeing 747, like the plane shown above. Some assumptions about the 747:

Area = 158.3 m² (that is pretty big!)

Drag Coefficient = 0.05 (that is pretty small!)

Velocity = 562 miles per hour = 250 m/s

I basically found these by looking around on the web.

The surface mass density is 1.23 kg/m³. The density decreases pretty rapidly as you go up in the air. At 30,000 ft, the density is roughly 38% of the surface density (0.467 kg/m³).  At 40,000 ft, the density is about 25% (0.308 kg/m³).

Ok, that was a lot of numbers.  Sorry.  What does this mean?  Well, we could talk about the drag force that the 747 experiences. If we do all of the math, and we assume that the 747 is cruising at 40,000 ft, we get a force of 75,750 N, which is 17,030 lbs. If the airplane were to be flying just 10,000 ft lower, the force would be 25,885 lbs, which is much (50%) larger, showing that the altitude that the airplane flies is pretty important.

Now, let’s calculate how much fuel is used during a 6 hour flight (say New York to London). If we assume that it is all cruise (which is a bad assumption, since a lot of fuel is used to take off), how much fuel does the 747 use?

Well, we have to calculate how much fuel a 747 uses each second at cruise speed and at altitude.  Remember that Thrust = MassFlowRate * ExhaustVelocity.  For a rocket engine, the Exhaust Velocity is really the speed at which the gas comes out of the engine.  For a jet engine, that is not really the case, and it is a bit more complicated.  But, let’s skip over that and just take my word that the “ExhaustVelocity” of a jet engine is about 35,000 m/s. (If the exhaust velocity were really that large, it would be pretty dangerous to be around the backend of an airplane!)

To get the MassFlowRate, we can just divide the thrust by the ExhaustVelocity.  At 40,000 ft altitude, the MassFlowRate would be 2.16 kg/s.  A gallon of jet fuel is about 2.7 kg.  So, a 747 uses just under (80%) a gallon of jet fuel every second.  Depending on your point of view, this is either a lot (a car uses a gallon every few hours), or a tiny bit (a rocket uses a hundreds of gallons each second).

Over the course of a 6-hour (6*3600 seconds) flight, the airplane would  use about 17,300 gallons of fuel (not counting takeoff and landing) if it flew at 40,000 ft.

If the airplane were to fly at 30,000 ft and keep the same exact speed (562 mph), the airplane would use 26,300 gallons!  That is 9,000 gallons of jet fuel more, just for flying at 30,000 ft.

Hopefully this helps you understand why airplanes fly as high as they can.  If you are on a very large airplane that is flying a long way, then the airplane may raise the altitude a couple of times as it uses fuel.  A super heavy 747 can’t fly at 50,000 ft, since its wings can’t support the lift at 50,000 ft.  As the 747 uses fuel and is less mass, it can fly at higher and higher altitudes.  The best track would be to fly at the highest altitude all of the time, increasing altitude all of the time, but rules stop this – there are certain altitude “lanes” that planes can fly in.

Just for fun, if the airplane is at 40,000 ft, it gets about 0.195 miles per gallon. At 30,000 ft, it gets about 0.128 miles per gallon. If the flight had 400 people on the 747, and it flew at 40,000 ft, then each person would get the equivalent of about 78 MPG. Not really that bad! It would be hard to drive somewhere for this type of fuel economy!

Interestingly, if a 747 were to fly at ground level the entire flight, it would use 69,000 gallons of fuel to fly from New York to London, or would get about 0.05 miles per gallon. Yikes!

Finally, how does wind effect the amount of fuel used?  Well, a 747 goes 562 MPH not with respect to the ground, but with respect to the background wind.  So, if the 747 is flying in the jet stream, which can be about west-to-east at 100 MPH, then the ground speed of the 747 flying from New York to London would be 652 MPH, but coming back from London to New York, the ground speed would be 452 MPH.  This doesn’t cause the amount of fuel used per second to change, but it changes the number of seconds the airplane is in the air. From New York to London, the flight would be shortened to 5:10, and back to New York, it would be lengthened to 7:30.  The amount of fuel used would be 14,700 gallons (saving 2,600 gallons, NY to London) or 21,000 gallons (costing about 3,700 more gallons, London to New York).

Ah, physics. I love you.

Oh, on a side note, think about in the first Iron Man movie when he (Iron Man) flew from Los Angles to the Middle East in his suit.  Obviously it must have been pressurized, since he would have to fly at incredibly high altitudes. Iron Man is quite a big smaller than a 747, but he probably was flying about twice as fast as a 747.  So, if you do the calculations, he would have had to use about 147 kg of fuel.  If this was jet fuel (which it was not, but that is a separate discussion) it would be about 55 gallons.  Where did he put all of this fuel??? Marvel Fans want to know!


What I do. Simply.

I was asked to give a five minute talk describing what I do at a conference of people who study similar things as I do. Specifically, I was asked to give a talk describing modeling using only the 1000 most common English words used.  They gave two web sites, which don’t match each other exactly, but one is the list and one is a page in which you can enter text and it will tell you which words are “less simple”. (I copied and pasted all of the words from the list into the simplewriter web page and it spit out a ton of them.  So, I made sure that all of my words were not “less simple”.  This is a new fad in science, since it is sort of fun to try to explain what you do using vocabulary that everyone understands.  The problem is that sometimes you have to jump through some very big hoops to say simple things like “model”. The original idea creator of this has a book called Thing Explainer.

Anyways, hopefully you get the idea.  Here is a description of my occupation, only using some of the most common words in the English language:

My Job

I like to explain the hot air in space with a computer: space whether (funny because it is the wrong word!🙂

It is fun, but hard. When the computer does not agree with another approach, it does not make me happy (like sh*t).

One thing to note: the sun does knock off some tiny bit from some air and does give it power. Other air – not so much. Air down low doesn’t have much power. Air up high does. Tiny air bit from space does hit air and does give it power too. Air does move fast. It is hot.

How to explain the hot air and space with a computer?

Break the hot air way up in the sky (or space) into a box and another box and another box and another box and another box…

Then idea is: First consider how fast air does move in one direction, then another, and another. Then consider how much air is fast and slow at the same time (hot or cold). Consider how much air will hit other air and air with a tiny bit taken off. Look at how some air does turn into other air. Then how thick the air is and how much has a tiny bit taken off. Pass to another computer. Repeat. Repeat. Repeat. Like. A Lot. Write out. Draw picture. Another picture. Another.

Do a dance when it does work. Good me.

Give a little cry when it doesn’t. Stupid computer.

Say “My air and space thought computer is the best!”

Show movie of air to work people.

Ask for money. Cry. Because no money.


A couple of posts ago, we discussed the idea of Newton’s Third Law: for every action, there is an equal and opposite reaction.  Let’s figure out how to mathematically apply that to getting a rocket off the ground and investigating how airplanes fly!

The main idea with thrust is that if you throw enough stuff out the back at a high enough velocity, it will propel you forward. It is therefore likely that the thrust that is experienced is somehow related to the velocity at which the material is expelled and the mass of the expelled material. This is almost exactly right, except that when we are talking about a rocket, or even an airplane, the stuff that is coming out is a stream of material, and so the mass is not really just a mass, but a mass flow rate (i.e., the amount of stuff per second). In other words, thrust is equal to the mass flow rate times the exhaust velocity, or:


For a normal, everyday rocket, the exhaust velocity is roughly equal to about 3,000 m/s (meters per second, or about 6,700 MPH).  This exhaust velocity doesn’t vary very much, and it could be as low at 2,000 m/s or as high as about 4,000 m/s, but those are about the limits for a “normal” rocket engine. I will talk about why in further posts, but it mostly has to do with chemistry.

On the other hand, the mass flow rate can vary a large amount from rocket to rocket. That is because the only knob the rocket manufacturers have (if the exhaust velocity is about fixed) is the mass flow rate.  A smaller engine will have a smaller mass flow rate, and there won’t be able to lift large things off the ground.  A large rocket, like the Saturn 5, the largest successfully flown rocket ever, has a huge mass flow rate, and was able to lift a gigantic amount of stuff off the ground (elephants in space?).

My favorite example of a rocket is the V2, which was the first really “useful” liquid propulsion rocket ever made.   This was the rocket build by the Germans in WWII to bomb the Allies.  I will discuss these in more detail in another post also.  But for now, I would like to use this as an example to look at some aspects of thrust and getting a rocket off the ground. So, let’s get the stats on a V2 rocket:

  • With no fuel and no warhead, it had a mass of about 12,500 kg;
  • The warhead was about 1,000 kg;
  • It carried about 8,800 kg of fuel;
  • The mass flow rate about 110 kg/s; and
  • The exhaust velocity of the engine was about 2,400 m/s.
A V2 Rocket. It was extremely advanced from about 1944 to 1957.

With this information, we can calculate some things about the rocket.  For example, the most important thing to calculate is whether the rocket can actually lift itself off the ground. To do that we calculate:

  1. The total mass of the rocket at launch, which is equal to the mass of the rocket, the warhead, and the fuel, which is 22,300 kg.
  2. The weight of the rocket, which is just the mass times the acceleration due to gravity (-9.8 m/s²), which would then be -218,540 N.  It is negative, since gravity is pulling down on it. In order to lift off the ground, the rocket engine has to produce more than 218,540 N of thrust.  That will just overcome gravity, and cause the rocket to lift into the air.
  3. The thrust of the rocket, which is given by the formula above.  We know that the exhaust velocity is 2,400 m/s, and the mass flow rate is 110 kg/s. The thrust is therefore 2400*110 = 264,000 N, which is larger than 218,540, so the rocket will definitely move upwards.

In order to actually calculate that rate of acceleration, we can do a bit more math, using Newton’s second law, which is F = m * a, or force equals mass times acceleration, but we can rearrange it to calculate the acceleration (a = F/m).  The force is the total of the weight of the rocket (-218,540N) plus the thrust of the rocket (264,000N), which is 45,460N.  The total mass of the rocket is 22,300 kg, so the acceleration is 45460/22300 = 2 m/s² upwards.

Now, the cool thing about this has to do with the mass flow rate.  The mass flow rate literately means that the rocket is becoming less massive every second. For the V2 rocket, it is becoming 110 kg less massive every second.  So, after one second, the rocket has a mass of 22,190 kg.  After two seconds, it has a mass of 22,080 kg. And so on, for about 80 seconds, until the rocket has a final mass of 13,500 kg. During the final second of thrust, the acceleration will be quite different than the first second.  In the last second, the rocket weighs about -132,300N, while the thrust is exactly the same as before (264,000N), so the difference is 131,700.  The mass of the empty rocket is 13,500 kg, so that the acceleration just before the thrust cuts off is about 9.8 m/s² upwards, about 5 times larger than the starting acceleration!  This increasing acceleration effect happens with all rockets that have a constant mass flow rate – as they use more fuel up, the accelerate upwards faster and faster.  It is no real surprise then that when rockets are sitting on the launch pad and fire their engines, they look like they are just crawling upwards – because they are!  As the rocket loses more and more fuel, it gains speed rapidly.

An illustration of a rocket shooting mass (80 kg – should be 110 kg for the V2 – oops!) at a given velocity (2200 m/s) every second. In 80s, the rocket is empty, but in that last second, it is accelerating upward fastest.

For manned missions, it is important that the acceleration not become too large, since people can’t withstand huge accelerations.  People tend to pass out when they are accelerated at rates that are about 5 times Earth’s gravity.  And the point of maximum acceleration doesn’t happen at the beginning, but near the end of the thrust.

Ballistic Motion

Ballistic motion is an important concept in our path to understanding how rockets get up into orbit. Really, there are a few types of rockets: (1) rockets that just go up and then come right back down, otherwise known as ballistic missiles; (2) rockets that put something into orbit; and (3) rockets that take something away from the Earth and put it on a trajectory to somewhere else.  Each of these requires more energy than the last one, with the ballistic missile requiring the least amount of energy.

But, what is ballistic motion?  It is the motion that something feels when the “only” force acting on it is gravity. (I say “only” because often atmospheric drag is acting on it also, but we will ignore this for now.)

Let’s take a person throwing a baseball as an example.  Figure 1 illustrates a person getting ready to throw a ball.  (My son Alan drew most of the images again!)

Ball 1
A person getting ready to throw a baseball.

The person then moves their arm forward, accelerating the ball up to some speed.  Typically, this speed is roughly parallel to the ground. Figure 2 illustrates the person’s hand accelerating the ball (wow, that is a beautiful hand!)  When the ball leaves the person’s hand, it is moving with a velocity of Vx parallel to the ground.  In addition, gravity is acting on the ball, so it starts, immediately, to accelerate towards the ground at a rate of 32 feet/sec per second.  The ball will follow an arched trajectory, with the velocity towards the ground growing and growing all of the time, but with the velocity parallel to the ground (Vx) staying the same all of the time.


Ball 2
A person throwing a baseball. The person accelerates the ball up to some speed, then lets it go. At that point, it starts falling towards the Earth, but still moves with a speed of Vx parallel to the ground.

Because I am in America, where we are lovers of guns (although I am not), we should use a gun example! Imagine a person shooting a bullet towards a target.  If the person is far enough away from the target, or the bullet is slow, gravity will have enough time to pull the bullet down, and the person could miss the target.  A person far away from a target with a low-muzzle-velocity gun, has to aim upwards to compensate for gravity.

A person shooting a gun directly towards a target will miss the mark, because gravity pulls the bullet down.

A much better example, in my opinion, is a catapult, which has an extremely slow speed, so that all objects need to have a very large upward velocity, in order to actually get the object to where you want it to go.

A catapult is a perfect example of a machine that relies on ballistic motion to crush enemies. With cows.

Ballistic missiles (or InterContinental Ballistic Missiles, ICBMs) operate on exactly the same principle as the baseball, catapult, or bullet.  Each goes through an acceleration phase, in which something is giving it an initial velocity (the rocket engine, which thrusts for a short amount of time). Then, the force cuts out, and the “only” force left is gravity. Gravity acts to decrease the upwards velocity down to zero, then causes the object to fall at faster and faster speeds. Just like the catapult.

Free Fall 1
A ballistic missile goes through an acceleration phase, then a free fall phase, where gravity is the only force acting on it.
Free Fall 2
A ballistic missile does not actually thrust through the vast majority of its flight!
Free Fall 3
When it lands, the ballistic missile is moving quickly, and typically causes quite a bang.

ICBMs are not the only types of ballistic missiles being developed right now.  There are many companies that want to take “space tourists” on a very fast ride (like 5 minutes).  These companies are creating reusable rockets that have a ballistic trajectory, taking the tourists to about 60 miles into the air, and bringing them back down safe and sound.

Ballistic Motion - Page 8
After the rocket engines turn off, the rocket is traveling under ballistic motion, so it is in free fall, and the people inside will be weightless.  That will continue until the rocket re-enters the atmosphere and the rocket is slowed down by atmospheric drag.  It is at this point, in which the people will experience the most acceleration! (This picture was drawn by me, and not by Alan. Notice the difference in quality. Which is vast.)

The rockets work exactly the same as ICBMs, in that they accelerate for a short amount of time (maybe 100 seconds), and then go into a free fall phase, where the only force acting on the rocket is gravity.

In reality, what happens next is that the rocket, which is well above the breathable part of the atmosphere, keeps going up for a while, reaches its maximum altitude, comes down, and re-enters the atmosphere.  At this point, the rocket is moving at very fast speeds, and starts to feel an incredible drag force.  The people inside the rocket actually have to lay down, since the forces acting on their bodies become so large.  The rocket is slowing down at such a fast rate that the people weigh about 3 times their normal weight.  Gravity is still acting to pull them down towards the ground, but the drag force is rapidly slowing them down.

The space tourists get a large force on them as they take off, and an even larger force on them when they re-enter the atmosphere.  It is truly a wild ride!

Lunar Eclipse

Tomorrow night, which is Sunday, September 27th, 2015, there will be a lunar eclipse that starts around 8 PM Eastern Daylight Time. I am sure that most people know what a lunar eclipse is, but I thought that it might be interesting to discuss the phases of the moon in relationship to the lunar eclipse.  Also, it is a good excuse to talk about the dark side of the moon.

Let’s first discuss the phases of the moon. The illustration below shows four phases of the moon, which are spaced roughly one week apart for a month. The figure is obviously not to scale, but the relative positions of the Earth, the sun and the moon are roughly accurate.  When the moon is furthest away from the sun, so that the order of the bodies from left to right, go moon, Earth, sun, then the moon is completely full.  This is because the sunlit side of the moon is completely facing the Earth, allowing us to see the whole half of the moon.

An illustration of the phases of the moon, showing the moon in four different positions (roughly every week for a month)
An illustration of the phases of the moon, showing the moon in four different positions (roughly every week for a month)

When the moon is between the Earth and the sun we can’t see the moon at all, and it is termed a new moon. Half of the moon is still illuminated by the sun, but we just can’t see that side, since it is facing away from us.

When the moon is off to the side of the Earth, we can see a quarter of the moon, since we see roughly half of the part that the sun is shining on.

Interestingly, you can only see the moon during certain times of the day, depending on the phase.  For example, during a full moon, you can only see the moon when it is dark out (like the person standing on the dark side of the Earth in the illustration – they can see the full moon). That is because, if you are on the dayside of the Earth, then you can’t see the moon, since the Earth is between you and the moon.  If you are at dawn or dusk, the full moon will be on the horizon (opposite to the sun).

The exact opposite is true for a new moon (not that you can actually see the new moon, but when there is a tiny sliver of the moon you can see) – you can only see a new moon during the day – never at night. If you are on the night side of the Earth, the Earth would be between you and the new moon.  If you are at dawn or dusk, then you could see the new moon just on the horizon (close to the sun).

Now, the “dark side of the moon” is a complete misnomer. As you can see from the illustration, the dark side of the moon changes all of the time.  Really, the “dark side of the moon” is simply the side of the moon that we can’t actually see. Ever.

It is theorized that the Moon was once part of the Earth. A long time ago a gigantic asteroid slammed into the Earth, and ended up ejecting a chunk, which ended up being the moon.  As the moon was cooling down and orbiting around the Earth, a heavy part formed. The heavier side of the moon faces the Earth all of the time, so that the moon is considered “phase-locked” with the Earth, meaning that the same side of the moon faces us all of the time. This is illustrated below, with a little circle showing the heavy part of the moon, which constantly faces the Earth.

An illustration of the phases of the moon, with the same side of the moon facing the Earth highlighted.
An illustration of the phases of the moon, with the same side of the moon facing the Earth highlighted.

The side that is facing us is dark sometimes (new moon) and in sunlight sometimes (full moon).  The “dark side of the moon” is simply the side of the moon that we can’t see, since we only see one side of the moon ever.  It is dark about half the time and in sunlight about half the time, just like the side of the moon that is facing us. The first time that humans ever observed the side of the moon that we can not see is when the USSR’s Luna-3 orbited the moon in 1959 and sent pictures back to Earth.

Mercury is phase-locked with the sun, so that the same side of Mercury faces the sun all of the time.  This makes one side of Mercury super-hot, and the other side of Mercury super-cold.  It is theorized that if Earth were phase-locked with the sun, then no life on Earth would have formed, since one side would be extremely hot, and the other side would be extremely cold, making it an inhospitable hell-hole. Since the Earth doesn’t have any really heavy bits to tug one side towards the sun, we lucked out and have a nice 24 hour day. Perfect for humans!

A lunar eclipse is when the moon passes into the shadow of the Earth, as illustrated below.

A lunar eclipse!
A lunar eclipse!

One might imagine that a lunar eclipse would happen every single time that there is a full moon.  It would if the Earth and the moon were always in the same orbital plane, but they are not. The moon’s orbital plane is tipped with respect to the Earth’s orbital plane, so that when there is a full moon, it is almost always either a bit above or below the Earth’s shadow.  There are only a few hours of each month in which the moon is in the orbital plane of the Earth; and it happens that this month those few hours are when it is a full moon, and it will pass into the shadow of the Earth. A relatively rare treat.

Hopefully it won’t be cloudy where you are. We are supposed to be partly cloudy, which seems like the best you can ask for in Michigan.