Email Question Bonanza

11 08 2009
The moon! Photo by Luc Viatour.

The moon! Photo by Luc Viatour.

Hello! I’m sorry for the drought of posts these past few days. Let me try to make it up to you with two great posts at once! Wow! I want to answer two questions I received in emails ( If I continue to receive emails on a regular basis, I’ll make this a regular sort of post. A kind of grab-bag. If I get emails with enough regularity, I might even be able to make it a weekly feature! Grab-bag Tuesday, or something. Anyway, here goes.


The first question is from Andy from Maine. Andy writes:

Hi Neil,

How come the moon doesn’t spin like the earth does? Or Does it?


An excellent question that demonstrates a keen eye for strange phenomena! Here’s the answer:

Andy’s question no doubt comes from the observation that the same side of the moon is always facing us. Any time you look at the moon, you can see the same familiar features in the same familiar places. An intuitive explanation would be that the moon doesn’t spin, but stays with its face locked towards the earth. However, this isn’t quite true. The moon does spin. But it spins exactly as fast as it orbits the earth. That is to say, if the moon rotates 10% of the way around, it orbits 10% of the distance around the earth during the same period of time. If it rotates 50%, it orbits 50% of the distance during that period of time.

Here’s an easy demonstration. Take some kind of spherical object, like a ball or a balloon. Make a mark of some kind on one side, and hold the object out in front of your face at arms length. Now, rotate it 50%. The mark should now be on the opposite side, and should no longer be visible. If this was the moon, it would have also traveled through 50% of its orbit during the same period. So without turning it, move the ball behind you. Now, holding the ball stationary (more difficult than it might seem!), turn around. You should see the mark staring right back at you, just as it was at the beginning of the demonstration. The same phenomenon is responsible for the fact that we only see one face of the moon. But why is this the case?

If an object’s rotation and orbit are linked directly, as in the case of the moon, the phenomenon is referred to as synchronous rotation, because the rotation of the moon is in sync with its orbit. There’s a 1:1 ratio. In the case of the moon, this synchronous rotation is due to the gravity of the Earth and a phenomenon known as tidal locking.

The powerful and mysterious Mike has made some great diagrams to help explain this phenomenon, which I’ll sprinkle through the rest of the post.  This part is not as simple as it might seem, so try to study these diagrams and understand each before moving on. I’ll try to make it as clear as possible.

Tidal locking occurs as a result of gravitational forces acting on the tidal bulge of the moon. The tidal bulge is a bulge (go figure) created by the gravity of the Earth acting on the surface of the moon. This gravitational force causes the surface of the moon closest to the Earth to bulge outward, which causes the moon’s shape to distort to an elliptical spheroid instead of the sphere shape it would normally have if unaffected by gravitational and rotational distorting forces. Please examine diagram #1.

This diagram shows how the Earth's gravity creates a tidal bulge, distorting the spherical shape of the moon into an ellipse.

This diagram shows how the Earth's gravity creates a tidal bulge, distorting the spherical shape of the moon into an ellipse. Note that this is a top-down view. We are looking down on the North Pole of the Earth and Moon.

Now, if there was no rotation of the moon, this diagram would be accurate – but there is rotation. Let me emphasize again that the above diagram is a top-down view. This is very important for what follows. So imagine that in the middle of the Earth up there, you can see the North Pole there in the middle. Now let’s imagine that the moon is rotating clockwise. Got that in your head? Okay great.

So if the moon rotates clockwise, then this bulge gets moved along with the clockwise rotation until it reaches a sort of equilibrium. The result is a bulge out of line with the Moon-Earth axis (the dotted line above). Examine the next diagram for an illustration.

tidal 2
This diagram is a two-for. First, notice how the bulge in the first diagram has shifted clockwise in the direction of the rotation. Now notice how the two sides of the bulge are different distances from the Earth. This results in a stronger pull on the closer bulge, because gravity is stronger the closer you are to the body pulling on you. The stronger pull is marked with the thicker red arrow.

The fact that the bulge is not in line with the Earth-Moon axis means that one part of the bulge is closer to the Earth than the other part. Because gravity is weaker on far away bodies and stronger on close objects, the Earth pulls on the close part of the bulge more than it pulls on the far part of the bulge. Here’s another diagram that I made to help explain what’s going on. Make sure you click on it so you can see it in full size.


The type on this picture is too small to read like this - Click on the picture to blow it up!

As you can see in the diagram above, pulling on the B bulge causes the Moon to rotate counter-clockwise around the pivot (the “vertical” pole axis down the middle of the Moon). This counter-clockwise rotational force (called a torque) opposes, slows, and eventually reaches an equilibrium with the natural clockwise rotation of the Moon. The equilibrium or balance point is reached when the Moon’s rotation is slowed to the point where the bulge is no longer skewed. Once the bulge is in line with the Earth-Moon axis  (the dotted line in above diagrams), there is no net rotational force produced by the Earth’s gravity on the different bulges. As a result the counter-rotational force ceases, and the Moon is no longer slowed down. This is how the Earth can slow down the Moon to the point that it is tidally locked but doesn’t slow it down any more than that. Examine two more diagrams to see what the system looks like when tidal lock is achieved. First is Mike’s, then mine.

tidal 3

Text reads: Once the moon's rotation balances its orbital motion the tidal bulges remain aligned with the Earth, and therefore the Earth's gravitational pull on them no longer produces a torque (rotating force).

Again, click for bigger.

Again, click for bigger.

Tidal Lock Achieved! Booyah! Now the same face of the Moon is always present in the sky. This is the present condition of the Moon. Now tidal locking is a force that applies to all bodies, including the Earth. In fact the Moon is in the process of tidally locking the Earth so that one face of the Earth will always point toward the moon. This will take billions of years, so our planet may not survive long enough for this to happen, but the effect is that our rotation is constantly being slowed down. As a result, each day is slightly longer than the last one. Today is longer than yesterday, and tomorrow will be longer than today. Here’s another diagram of mine to help illustrate. Notice how my diagrams are not nearly as shapely as Mike’s… I’ll work on it. Anyway!

The Moon also exerts tidal forces!

The Moon also exerts tidal forces!

And that’s tidal locking! Pretty cool, isn’t it?

The next question is from Jim, also from Maine. Those Mainers, I tell you what. They have a knack for this sort of thing. Jim writes:

I have a science question for you. So living beings started out as bacteria right? Did plants also start as bacteria? Where did plants come from?!


The consensus is that the first lifeforms resembled today’s bacteria. The earliest fossils we’ve found are about 3,500,000,000 years old and closely resemble current bacteria.


Here is a picture of stromatolites on a beach in Shark Bay, Western Australia. These rock-like formations are the result of little grains of sand and rock getting trapped between layers of bacteria. These modern stromatolites closely resemble fossils found in other parts of Western Australia, which have been dated to 3.5 billion years ago.


Here’s a simplified diagram of a prokaryotic cell. Keep in mind that while prokaryotes are simpler than eukaryotes, they are still exquisitely complicated beings.

The fact that such complex lifeforms as bacteria existed 3.5 billion years ago suggests an earlier beginning of life – since it is hard to imagine the components of the “primordial soup” spontaneously assembling themselves into beings as complex as bacteria. More likely, life would have began with “proto-lifeforms” consisting of a lucky assortment of amino acids, enzymes, and nucleosides bound up within a lipid membrane. Lipid membranes can form spontaneously in water, so if the water contains enough of these components and there was enough forming and reforming of lipid membranes, eventually we should get a few that have the right stuff inside. Anyway, as a result of the relative complexity of the first fossil evidence of life and the likelihood that life began earlier with simpler forms, biologists tend to put the “beginning of life on earth” at about 4 billion years ago. Of course this is just an estimate since there is no evidence of life before 3.5 billion years ago.

Over time, some of these bacteria evolved into eukaryotes, cells that have specialized organs (they’re called organelles) that do different tasks for the cell, like recycling broken down parts and digesting food, making energy out of food, producing basic parts, or storing genetic information. The origin of eukaryotes probably involved the folding of the bacterial membranes into little pockets inside the cell, as well as transfer of DNA between different bacteria. In any case, these first eukaryotes are the ancestors of both animals and plants, and the first fossil evidence of eukaryotes is from 2.1 billion years ago. It’s amazing to think that for almost 1.5 billion years (about 23 million human lifespans, to put that in perspective!), there were only bacteria living on earth.


A simplified diagram of a eukaryotic cell shows the diversity of specialized organelles that characterize the eukaryote. Make sure you click this one so you can see it in more detail. Image from Encyclopedia Britannica copyright 2008.

Some time after the origin of the eukaryotes 2.1 billion years ago, these guys started picking up bacteria, which then lived inside of them and became a part of them. It’s similar to the way you have lots of bacteria (billions!) that live in your gut and help you digest food. Eukaryotes had bacteria living inside of them (referred to as endosymbionts) that helped them do things like turn light, carbon dioxide, and water into sugar, or produce energy from that food. Over time the organisms became so enmeshed that they became one. The energy-processing mitochondria possessed by all plants and animals, and the chloroplasts that plants use for photosynthetic food production both had their origins in bacterial endosymbionts.*

The divergence of plants and animals occurred when some of these single celled eukaryotes picked up chloroplasts (photosynthetic bacteria) from their environment, while others did not. Those that picked up the chloroplasts went on to become the plants we all know and love. The ones that didn’t pick up chloroplasts became animals and fungi. Yep – you’re more closely related to a mushroom than you are to a tree! This divergence of plant and animal lines occurred about 1.6 billion years ago.


This is algae of the genus Nostoc. Notice the long filaments of cells. A few of these are differentiated into heterocysts which serve to supply other cells with nitrogen, but do not reproduce. Similar relationships exist among the organisms today classified as multicellular.

After this point, you soon start to see multicellular organisms. These are organisms that probably began as colonies of single celled organisms. Today for instance we see a number of organisms that are single celled but really behave quite like simple multicellular organisms. For instance there are algae (single celled photosynthetic bacteria) that form long chains (or filaments) of cells. A few of the cells in a chain transform into cells dedicated to taking nitrogen up from the environment and giving it to the other cells. The really interesting part is, these transformed cells can no longer reproduce! So you see the same kind of thing that you have in multicellular organisms, where you have cells that exist not to reproduce themselves but to help other cells reproduce.

Over time, these multicellular eukaryotes got more and more and more complicated until you get the wonderful lifeforms we call plants and animals, which are each made up of trillions of eukaryotic cells each. You, for instance, are composed of about 10,000,000,000,000 (ten trillion) eukaryotic cells, with a tremendous variety of cell types and functions among those. Pretty cool, huh? And you also have something like 100,000,000,000,000 (one hundred trillion) bacteria from hundreds of different species living in you and on you! That means you have 10x more cells that are “not you” than those that are you! If you are ever feeling down, just remember that you and only you are the reason a hundred trillion lives continue to exist – and given the rapid pace of bacterial reproduction, a human being like you is likely to be the habitat for quadrillions of unique beings. Every one of us has a whole ecosystem that depends on us and us alone!

So I hope that clarifies the origins of plants and animals as well as the reason we always see one face of the Moon. Keep those questions coming. You, too, could reach the levels of internet celebrity and adulation that Andy and Jim now enjoy!

*For a really cool illustration that mitochondria are really captured bacteria, check out some video. Here’s a link. The red rods creeping about are mitochondria within a eukaryotic cell. Nifty, huh? Many more similarly amazing videos can be found here .




3 responses

11 08 2009
Sam T

Hey Neil,

Great post! I found some of the tidal locking stuff a little tricky- the process of torque and the delicate balance to reach a tidal lock point is something that HAPPENED, and not a regular occurrence, right? How long did that take? When did it happen?

I liked the origin of life stuff- very interesting. I think the point about mushrooms could have been better articulated by saying a mushroom is closer to YOU than it is to a TREE! Makes me think twice about ordering them on pizza…Fungi is a whole ‘nother kingdom, man.

14 08 2009

Once a body is tidally locked to another body, it stays that way forever, unless some outside force disturbs things. It only took a few hundred million years for the Moon to be tidally locked to the Earth, but since the Earth is so much more massive the reverse process (the Earth getting tidally locked to the moon) is still ongoing. Tidal locking in general is pretty common though- any two bodies in orbit around each other, anywhere in the universe, will eventually become tidally locked. However, if the orbit is very elliptical they might not get locked into a 1:1 spin:orbit ratio. Mercury, for example, is tidally locked to the sun with a 3:2 ratio. That means that a Mercurean day is 2/3 as long as a Mercurean year, and that someone standing on the surface of Mercury would experience a double sunrise.

Also, I don’t think they actually found fossil stromatolites in Montana. I know they found them in Australia and South Africa, and the only other places where old enough rocks even exist are Canada and Scandanavia. Finding fossils of the earliest life is very difficult, because very few rocks from back then have survived unaltered to the present day.

15 08 2009

Mike/arfalarf is right! The stromatolites found in Montana were dated to only (only!) 1 billion years ago. My bad! Fixin’ this.

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