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  Mercury - Gemini - Apollo
  Apollo launch windows: Moon at perigee? (Page 1)

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Author Topic:   Apollo launch windows: Moon at perigee?
Tykeanaut
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posted 08-15-2013 01:33 PM     Click Here to See the Profile for Tykeanaut   Click Here to Email Tykeanaut     Edit/Delete Message   Reply w/Quote
Did the Apollo moon missions coincide with the moon at perigee?

SpaceAholic
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posted 08-15-2013 01:42 PM     Click Here to See the Profile for SpaceAholic   Click Here to Email SpaceAholic     Edit/Delete Message   Reply w/Quote
The launch window was not established contingent upon earth/moon distance.

Robert Pearlman
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posted 08-15-2013 02:22 PM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
Here is a 1967 NASA film explaining the Apollo launch window constraints:

YankeeClipper
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posted 08-16-2013 11:35 AM     Click Here to See the Profile for YankeeClipper   Click Here to Email YankeeClipper     Edit/Delete Message   Reply w/Quote
If you seek out the various Apollo TransLunar/TransEarth Trajectory Plotting Charts (ATT) you will see great diagrams for each mission showing lunar orbital positions during possible launch dates, mission trajectories, distances etc.

eBay Item 111138447084 Apollo 13 ATT Chart is an example.

Dietrich
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posted 08-16-2013 03:29 PM     Click Here to See the Profile for Dietrich   Click Here to Email Dietrich     Edit/Delete Message   Reply w/Quote
A further question to this: On the chart the lunar orbit is clockwise. What is the reason for this?

The moon's rotation is anti-clockwise in this chart (to face always the same side to the earth). Would not the braking impulse and the re-start to lunar orbit by the LM be less if these two maneuvers are performed not counter the movement of the lunar surface but in the same direction of the moon's rotation?

Practically all rockets are launched towards East on earth (except retrograd and polar orbits and launches from Israel) to utilize the speed of the earth' rotation.

moorouge
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posted 08-17-2013 12:40 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
Just to confuse the issue folks. The conception we all have of the Moon (or satellites for that matter) in nice, tidy paths going round our planet is what may be termed a 'lie for children'. In other words, an over simplification to make it understandable.

In fact, the Moon weaves in and out of the path the Earth follows as it seems to go round the Sun and does not 'go round' us at all.

Another 'lie for children' I sometimes used was comparing the difficulty of getting to the Moon to someone on a carousel trying to hit a rabbit running in the opposite direction about 100 yards away.

Jim Behling
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posted 08-17-2013 07:40 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by moorouge:
In fact, the Moon weaves in and out of the path the Earth follows as it seems to go round the Sun and does not 'go round' us at all.
No, that is not true. The moon does orbit the earth and does 'go round' it, just as the earth orbits the sun. However, when looked at from the reference of the sun, the moon weaves in and out of the path the Earth follows as it goes round the Sun. Looking at the solar system from outside it, and the motion of the solar system makes the moon's path is even more complex.

The point of reference in inertial space is very important when describing motion of objects in relation to each other.

moorouge
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posted 08-17-2013 09:43 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Jim Behling:
However, when looked at from the reference of the sun, the moon weaves in and out of the path the Earth follows as it goes round the Sun.
I thought that's what I said. So, you do agree after all!!

Jim Behling
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posted 08-17-2013 02:43 PM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
No, we don't because you said it is a lie for child, when it does in fact goes around the earth. If we are talking the 'truth', then the moon doesn't weave around the earth, it has a much more complex path.

moorouge
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posted 08-18-2013 01:07 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
You're misinterpreting what I said and meant. It's an illusion that the Moon appears to orbit the Earth. As I said in my original post, the 'lie for children' is a simplification of the facts to enable an initial understanding of the relationship between us and our nearest neighbour.

Another 'lie for children' is to say that the refraction of light though raindrops causes rainbows. This is good enough an explanation for children but it goes nowhere near the complexity of truly understanding the formation of a rainbow.

Robert Pearlman
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posted 08-18-2013 01:12 AM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by moorouge:
It's an illusion that the Moon appears to orbit the Earth.
It is not an illusion that the moon orbits the Earth. The moon orbits the Earth.

The shape of that orbit is dependent on the frame of reference of the observer.

moorouge
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posted 08-18-2013 02:05 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
To us, sitting here on Earth, the Moon appears to go round us. In fact, the truth is that the Moon, like us, is in its own orbit round the Sun. That this orbital path is influenced by the Earth's gravity causing it to weave in and out of the Earth's orbital path creates the illusion that it goes round us. Find an open space with some friends to be Earth and Moon and you'll understand what I mean. *

Whilst on the vagaries of orbital mechanics, and getting back to somewhere near the original query, the furthest humans have been from Earth are the crews of Apollo 8 or Apollo 13 depending on which source one looks at. However, because the distance between the Earth and Moon is constantly changing, even during the course of a flight, this is a figure it's almost impossible to compute.

For the same reason of changing orbits, the closest humans have been to the Sun are not the Apollo astronauts. This distinction belongs to the inhabitants of our planet who lived some 70,000 years ago.

* if you're going to do this don't forget to factor in the rotation of both Earth and Moon.

Robert Pearlman
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posted 08-18-2013 02:57 AM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
To cite Phil Plait from the excellent Bad Astronomy:
Every now and again I get an email from someone who tries to tell me that the moon orbits the Sun more than it orbits the Earth.

On the face of it, their claim appears to hold water. For example, they'll say that if you compare the orbital velocity of the Moon around the Earth (about 1 km/sec) to the orbital velocity of the Moon around the Sun (the same as the Earth's velocity around the Sun, about 30 km/sec), you'll see that the Moon's orbit is always convex to the Sun; in other words, it doesn't make a loopy pattern around the Sun, as you might expect...

Moreover, if you calculate the force of the Sun's gravity on the Moon, you find it's more than twice the force of Earth's gravity on the Moon!

Whoa. So does the Moon orbit the Earth, or the Sun?

Turns out, it orbits the Earth, despite these claims. The above claims are true, but are not important in this argument. Instead, you have to look at something called the Hill sphere. Basically, it's the volume of space around an object where the gravity of that object dominates over the gravity of a more massive but distant object around which the first object orbits.

OK, in English — and more pertinent to this issue — it's the volume of space around the Earth where the Earth's gravity is more important than the Sun's. If something is orbiting the Earth inside Earth's Hill's sphere, it'll be a satellite of the Earth and not the Sun.

The derivation of the math isn't terribly important here (and it's on the Wikipedia page if you're curious), but when you plug in the numbers, you find the Earth's Hill sphere has a radius of about 1.5 million kilometers. The Moon's orbital radius of 400,000 km keeps it well within the Earth's Hill sphere, so there you have it. The Moon orbits the Earth more than it orbits the Sun. In reality it does both, and saying it orbits one and not the other is silly anyway....

moorouge
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posted 08-18-2013 06:48 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
OK folks - let's stick with the 'lie for children' until we're older and wiser.

Kite
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posted 08-18-2013 08:22 AM     Click Here to See the Profile for Kite     Edit/Delete Message   Reply w/Quote
The way I have always understood is that the Earth and Moon revolve around their common centre of gravity, which is well underneath the Earth's surface, which explains the bobbles in the Moon's and Earth's orbits.

canyon42
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posted 08-18-2013 09:53 AM     Click Here to See the Profile for canyon42   Click Here to Email canyon42     Edit/Delete Message   Reply w/Quote
It's really not that complicated. Does the moon orbit the Earth? Yes. Does it orbit the sun? Yes. Does it orbit the Milky Way? Yes. Does it orbit some ridiculously larger cluster of galaxies/objects than that? Undoubtedly, yes.

You have to determine what frame of reference you are specifying. The simplest one — and the one that matches our own everyday experience — is that the moon does indeed orbit the Earth (or as was previously noted, they both orbit the pair's center of gravity). The fact that it also "orbits" other objects in other frames of reference does not nullify this.

The claim that its "actual" path through space is much more complex is actually sort of meaningless in the sense that there is no ultimate backdrop against which it can be measured — it all depends on the viewpoint one selects, and that is always going to be somewhat arbitrary.

Blackarrow
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posted 08-18-2013 12:59 PM     Click Here to See the Profile for Blackarrow     Edit/Delete Message   Reply w/Quote
Does it help to look to the examples of Apollo SIV-B stages that have slipped from Heliocentric orbit to Earth orbit and back again? I'm thinking in particular of the Apollo 12 SIV-B which was spotted a few years back.

Lou Chinal
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posted 08-18-2013 01:50 PM     Click Here to See the Profile for Lou Chinal   Click Here to Email Lou Chinal     Edit/Delete Message   Reply w/Quote
Thanks, Robert for the link. I never saw that film before.

Gonzo
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posted 08-19-2013 06:29 PM     Click Here to See the Profile for Gonzo   Click Here to Email Gonzo     Edit/Delete Message   Reply w/Quote
First, thanks Robert for the film. While it reminds me of all the bad science films I had to watch in high school (many years ago), it also very elegantly explained something that has puzzled me for those years as well. I never really understood why the launch windows were where they were and why they were so short. I mean, from one point of view, the moon isn't going anywhere. It's been there for billions of years and will continue to do so for the foreseeable future! So thanks for the education.

Second, whether the Moon revolves around the Earth or the Sun is relative. Calling it a "lie for children" is exactly as described, an over-simplification so it is understandable to start with. It is only then, as you learn more about it's movements that you can start throwing in the more complicated factors of orbital mechanics.

So both statements are correct if taken from the proper relative view. Because in it's simplest form, the Moon goes around the Earth, and the Earth goes around the Sun. However, when you start understanding orbital mechanics, the fact is, the Earth and the Moon orbit around a central point due to their gravities and that point orbits around the Sun as a system.

You can argue definitions all you want, but the fact is, the Earth and the Moon are a system. What one does effects the other.

Look at it this way, how would Earth's orbit around the Sun change if the Moon wasn't there? How would that path change? What about the Moon's orbit if the Earth wasn't there? (And by extension, the path of the Sun itself if the Earth-Moon system didn't exist?)

While these perturbations may be small, you can't dismiss them either. If you can show that nothing would change (which you can't), only then could you claim that one orbits the other.

SpaceAholic
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posted 08-19-2013 09:21 PM     Click Here to See the Profile for SpaceAholic   Click Here to Email SpaceAholic     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Gonzo:
the Earth and the Moon orbit around a central point due to their gravities and that point orbits around the Sun as a system.
Since the barycenter continuously resides within the earth's radius the Moon still revolves around the Earth regardless of frame of reference.

moorouge
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posted 08-20-2013 02:02 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Gonzo:
Calling it a "lie for children" is exactly as described, an over-simplification so it is understandable to start with.
At last - someone understands a 'lie for children'.

Now I wait for someone to work out why the concept of a giant barbell whizzing through space doesn't work either. Unless of course between us we've discovered a new law of physics.

Jim Behling
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posted 08-20-2013 06:03 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by SpaceAholic:
Since the barycenter continuously resides within the earth's radius the Moon still revolves around the Earth regardless of frame of reference.
And that is why it is not a "lie for children".

SpaceAholic
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posted 08-20-2013 01:20 PM     Click Here to See the Profile for SpaceAholic   Click Here to Email SpaceAholic     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by moorouge:
Now I wait for someone to work out why the concept of a giant barbell whizzing through space doesn't work either.
How is this germane to the discussion at hand? (The dynamics of a spinning barbell are totally different since the ends are not gravitationally bound.)

Gonzo
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posted 08-20-2013 01:21 PM     Click Here to See the Profile for Gonzo   Click Here to Email Gonzo     Edit/Delete Message   Reply w/Quote
But I will stand by my view that it is a "lie for children". As I stated before, looking at it from a very simplistic view, the Moon orbits around the Earth. Simple enough. But that's not entirely true.

What that suggests is that the Earth is a stationary object (from the Moon's point of view) and that the Moon orbits around it uniformly. We all know that isn't true. That is the part that's not accurate. It's not uniform and the fact is, the two work together.

I agree that the center of their gravities is within the radius of the Earth (and by convention defines what orbits what), but as I stated earlier, take the Moon away and look at the effect on the Earth's path. Likewise, take the Earth away and the Moon takes an entirely new path as well. So they are joined in their movements.

The "lie for children" is exactly as implied, a very simplistic view to make it easy to understand. Can you imagine the difficulties you'd encounter trying to explain orbital mechanics to children by discussing interacting gravities?

Robert Pearlman
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posted 08-20-2013 01:31 PM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
This is a rather unncessary argument given the amount of computational work that has gone into answering this question and is available for anyone via a simple web search.

It is only a "lie for children" if you say that the moon only orbits around the Earth. The statement, "The moon orbits around the Earth" is categorically true.

What we're all discussing here is a what is generically referred to as the n-body problem, which "is the problem of predicting the motion of a group of celestial objects that interact with each other gravitationally."

In particular, we're referring to the three-body problem, in this case the Earth, moon and Sun. Even more specifically, it is the "circular restricted three-body problem," where two of the bodies are in circular orbits (i.e. the Earth and the moon).

And while it is true the three-body problem is not entirely solved, for the narrative purposes of explaining the Earth-moon relationship to another person of any age, the Hill sphere — the the region around an astronomical body in which it dominates the attraction of satellites — establishes that the moon circles the Earth.

That is not a lie. It is based on accepted mathematical and physical calculations.

Jim Behling
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posted 08-20-2013 04:56 PM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Gonzo:
But I will stand by my view that it is a "lie for children".
It is a wrong view. If you follow your view, then everything is a lie. Nothing in nature is uniform, there are always slight deviations. You go to a tire store to get your tires balanced. Guess what? They are not exactly balance. You go to the store and buy a pound of cheese and guess what? It is not exactly a pound. You will say they are within the accuracy of the balance machine or market scale. Guess what? The same applies with the perturbations cause by the moon on earth's orbit. They are too small to represent visually. You would not be able to see them if earth's solar orbit was the size of a car tire. And it would actually be more smooth than the car tire.

Also, It is very easy to explain orbital mechanics to children. Look up orrery.

moorouge
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posted 08-20-2013 10:16 PM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Jim Behling:
Look up orrery.
But the orrery is a lie. Perhaps a 'lie for adults'.

Going back to my original post, try it for yourselves. Mark out a line and get a friend to walk it. Now you try to walk round your friend keeping roughly the same distance away without having to speed up and slow down. The Moon doesn't do this. It can't be done. The only way it's possible is to weave in and out of the path followed by your friend's Earth. If you now add in the rotation of the Earth about its axis - providing your friend doesn't get giddy - you generate the illusion of the orrery that the Moon is in orbit round the Earth.

Once you've grasped the basic concept — the 'lie for children' — you can begin to add in all the other mathematical complications that Robert talks about.

Yes, the Moon as far as we are concerned from our limited perspective does orbit the Earth.

Rocketman!
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posted 08-21-2013 12:23 AM     Click Here to See the Profile for Rocketman!   Click Here to Email Rocketman!     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Dietrich:
A further question to this: On the chart the lunar orbit is clockwise. What is the reason for this?
I find these kinds of questions fascinating and fun to think about... so thanks for posing yours.

First, I'm no orbital mechanics expert, so if anyone reading this is one please chime in with any necessary corrections. I believe your insight is correct regarding the additional braking force, and subsequent additional launch acceleration, that the Lunar Module would need to attain as a result of its east-to-west orbital direction. But, the amount of the disadvantage is very small compared to launching east-to-west on Earth, from what I have been able to find out.

The relevant facts that follow are gathered from a quick survey of reliable internet sources...

Spacecraft launched from Cape Canaveral, FL, Earth, attain a "speed boost" by launching easterly which contributes 915 mph to their orbital speed. Thus, to attain low Earth orbit, the rocket need only add 16,585 mph to get to 17,500 mph. In this case, the Earth's rotation contributes about 5% of the total required orbital speed.

The speed of a point on the Moon's equator moves at about 10 miles per hour due to its rotation. (Circumference divided by 27 day period of rotation divided by 24 hours per day.) Since Apollo LMs landed and launched in a lunar westerly direction, it follows that they had to overcome about 10 miles an hour due to the Moon's rotation in the direction opposite to travel. To orbit the Moon at 60 miles above the surface, a speed of 3330 mph is needed. That figure is derived from the Apollo 8 orbital period of 129 minutes at 60 miles. So, the 10 mph surface motion is only a 0.3% disadvantage relative to the required lunar orbital speed.

As I have come to understand it, and as shown in the video Robert shared, the primary reason they approached the Moon during landing in the direction they did is that they wanted a low Sun angle to help highlight the lunar terrain, and they also wanted the Sun at the pilot's back for visibility. I believe there may also be a secondary advantage to orbiting in the direction the CSMs did that uses the Moon's orbital motion to help decelerate when entering lunar orbit (the spacecraft moving opposite the direction of the Moon's orbit). The effect would be somewhat opposite to the "slingshot" trajectories used by the Voyager and Pioneer probes to accelerate. This last notion is mainly speculation on my part, and I again invite those who may have more orbital mechanics knowledge to weigh in if I am in error.

Jim Behling
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posted 08-21-2013 09:36 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by moorouge:
But the orrery is a lie.
No, it is not. That is the actual motion of the moon around the earth, except for the eccentricity of the orbit.

No, it is not from our limited perspective nor is it an illusion, it is a physical fact that the moon does orbit the Earth. Just like all the satellites like the ISS.

moorouge
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posted 08-21-2013 09:59 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Jim Behling:
No, it is not. That is the actual motion of the moon around the earth, except for the eccentricity of the orbit.

No it isn't. Try my demonstration. If you can make the Moon orbit the Earth without changes in direction and speed you've outdone Newton.

Jim Behling
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posted 08-21-2013 10:04 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
No, your example shows that you don't understand orbital mechanics. The moon does orbit the earth at constant speed but depending on your frame of reference, the moon does "speed up and slow down." And yes, the orrery exactly represents the path of the moon around the earth.

Your friend is maintaining a constant speed circling you. What he is not doing is maintaining a constant speed with respect to the ground. This is the same as the moon around the earth from the frame of reference outside the earth/moon orbit. It also "speeds up" on one side of the orbit and "slows down" on the other side.

Take your example and put your friend on an air bearing surface (this removes him from the actual earth frame of reference). Take a rope and have him spin around you. He will go in a circle. Now start walking. Guess what? You and your friend are like the moon and earth going around the sun.

This video shows the moon going around the earth.

I am not talking from a hobbyist point of view, I have more that 30 years of experience in space related jobs, and more than 10 in spacecraft/launch vehicle integration which includes flight design.

Gonzo
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posted 08-21-2013 12:30 PM     Click Here to See the Profile for Gonzo   Click Here to Email Gonzo     Edit/Delete Message   Reply w/Quote
Jim - but your example is misleading. If you take your friend and hang him off a rope and then twirl him around you, yes, he is orbiting around you in a circular orbit that never changes - he is always the ropes length from you and from your center of rotation as well. And his speed (relative to you) never changes either.

The Earth/Moon system is different. The "rope" is the interacting gravities. That is, the Moon does not orbit around the Earth as your friend does around you. Your friend orbits around you from a never changing center of rotation at a never changing distance. The Earth/Moon system is different in that it (the Earth/Moon system) rotates around a common gravitational center between the two and that gravitational center is NOT the center of the Earth. So in that respect, the two are different.

As far as the change in speed that is mentioned, that is relative to the viewpoint (will we now delve into Einstein's theories as well?). Stating that the speed of the Moon is constant is misleading and also simplistic. It could be considered constant if it were in an exact circular path and the Earth never moved. But it's not. It's path is elliptical and the Earth moves in space as well. And from that, it speeds up (relative to the Earth) as it approaches and slows down as it degrades away. Granted the difference is small, but it is not constant. Nor is it constant from space. From an outside viewpoint it is even more evident as you could more readily see it approaching the Earth and then moving away as it follows its elliptical orbit.

Whether anyone wants to agree or disagree with the "lie for children" idea is meaningless. The point of the phrase is simply to show that the concepts at hand have been simplified to allow for an easier grasp as a basis for the complexities to come. That is, the concepts at hand have been simplified and are are much simpler than reality. Once you delve into the subject and start discussing it at length, then it becomes more complicated as you start seeing details left out of the simplification. In this case, it is a simplification to just say the Moon orbits the Earth. But when you start looking at it, you discover that the path of the moon is elliptical, that it changes speed and that the two bodies (the Moon and the Earth) actually rotate around a common gravitational center. That's why it's a "lie for children" to say the Moon orbits the Earth. At first look in its simplest form, that is true. But there is much more to it than that and once you start looking at it, studying it, you start to see that it is MUCH more complicated than the simple statement that "the Moon orbits the Earth".

Jim Behling
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posted 08-21-2013 01:05 PM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
It isn't misleading.
quote:
Originally posted by Gonzo:
The Earth/Moon system is different in that it (the Earth/Moon system) rotates around a common gravitational center between the two and that gravitational center is NOT the center of the Earth. So in that respect, the two are different.
The center of rotation is beneath the earth's surface.
quote:
It could be considered constant if it were in an exact circular path and the Earth never moved. But it's not. It's path is elliptical and the Earth moves in space as well. And from that, it speeds up (relative to the Earth) as it approaches and slows down as it degrades away. Granted the difference is small, but it is not constant. Nor is it constant from space. From an outside viewpoint it is even more evident as you could more readily see it approaching the Earth and then moving away as it follows its elliptical orbit.
And the eccentricity is barely visible when the size of the system reduced to the size of orrery.

All those nuances (eccentricity, sun influence, common CG, etc) disappear when the system is reduced for visualization to the size of a computer monitor or school blackboard.

quote:
But there is much more to it than that and once you start looking at it, studying it, you start to see that it is MUCH more complicated than the simple statement that "the Moon orbits the Earth".
That applies for all processes in life. Nothing is exact or clearly defined by simple equations.

Then defining the trajectory taken by a thrown ball as parabola is a "lie for children" since there is many more influences it than just earth's gravity.

Describing the earth as sphere is a "lie for children."

Measuring the true (not marketing) fuel efficiency of a vehicle is a lost cause.

Approximations and simplification of processes are prevalent everywhere and not just in spaceflight.

Here is a tool in spaceflight that would be "lies for adults", look up "patched conic approximation".

Gonzo
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From: Lansing, MI, USA
Registered: Mar 2012

posted 08-21-2013 02:01 PM     Click Here to See the Profile for Gonzo   Click Here to Email Gonzo     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Jim Behling:
The center of rotation is beneath the earth's surface.
I agree. It is below the Earth's surface. But it is not at the center of the Earth either. That is what I was trying to point out. The two rotate around a common center of gravity, not the Earth's center. Your example assumes you (the center of the rotation) is also the center of the rotation for your friend, much like the center of a giant bicycle wheel. The Earth and Moon don't rotate that way. Their rotational center is NOT the center of the Earth. Rather, it is a bit off-center from the center of the Earth.
quote:
All those nuances (eccentricity, sun influence, common CG, etc) disappear when the system is reduced for visualization to the size of a computer monitor or school blackboard.
Again, I agree. But we're not talking about it being reduced. We're talking about reality — the system in real life size.
quote:
Approximations and simplification of processes are prevalent everywhere and not just in spaceflight.
And yet again, I agree. There is nothing that can't be looked at more closely only to discover that there is more to what you are examining. That's the process of learning and expanding your knowledge (and hopefully, your wisdom as well).

SpaceAholic
Member

Posts: 3059
From: Sierra Vista, Arizona
Registered: Nov 1999

posted 08-21-2013 02:29 PM     Click Here to See the Profile for SpaceAholic   Click Here to Email SpaceAholic     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Gonzo:
Jim - but your example is misleading. If you take your friend and hang him off a rope and then twirl him around you, yes, he is orbiting around you in a circular orbit that never changes - he is always the ropes length from you and from your center of rotation as well.

Both static and dynamic rope possess varying degrees of elasticity so distance perturbation and as a result orbital changes will be present unless the velocity is absolutely constant.

moorouge
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Posts: 1547
From: U.K.
Registered: Jul 2009

posted 08-21-2013 02:56 PM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
Look up Wikipedia 'Orbit of the Moon'. There is a diagram there of the Moon weaving in and out of the Earth's path round the Sun and an explanation as to why this is.

Blackarrow
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Posts: 2091
From: Belfast, United Kingdom
Registered: Feb 2002

posted 08-21-2013 07:04 PM     Click Here to See the Profile for Blackarrow     Edit/Delete Message   Reply w/Quote
When we start relying on Wikipedia, we are surely lost.

Robert Pearlman
Editor

Posts: 27717
From: Houston, TX
Registered: Nov 1999

posted 08-21-2013 07:13 PM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by moorouge:
Look up Wikipedia 'Orbit of the Moon'.
The irony of your citing that page is illustrated by these excerpts...
The Moon completes its orbit around the Earth... The Moon's orbit around the Earth... The Moon orbiting Earth... the Moon orbits the Earth... the Moon circles around the Earth...

moorouge
Member

Posts: 1547
From: U.K.
Registered: Jul 2009

posted 08-22-2013 12:41 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
If you've only found these quotes, then you're looking at the wrong entry.

The basic problem is that ingrained into our thinking is that representations of the solar system are Sun centered and those of the Moon are Earth centered. And that Jim is why the orrery is not accurate. The two are not compatible.

moonguyron
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Posts: 44
From: salado, tx, usa
Registered: Jan 2011

posted 08-24-2013 02:06 PM     Click Here to See the Profile for moonguyron   Click Here to Email moonguyron     Edit/Delete Message   Reply w/Quote
Boy you guys are about to make my head explode. But I think I may have stumbled onto an explanation that might satisfy both views. It is a matter of reference of the observer.

Consider for a moment the path of a tank tread in motion on a tank. Assume the tank is moving at 10 mph. If I hold the tank off the ground and look at the tread it is obviously moving around the wheels at 10 mph. Then I put the tank on the ground. From the perspective of the ground the tread is not moving at all on the portion that touches the ground while the top of the tread seems to move in the opposite direction of the tank at 20 mph. All these observations are true and exist at the same time. It just depends on the frame of reference of the observer.

Now let's put this to rest 'cause I am having trouble sleeping knowing the moon might not in fact orbit the earth.


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