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  Mercury - Gemini - Apollo
  Apollo launch windows: Moon at perigee? (Page 2)

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Author Topic:   Apollo launch windows: Moon at perigee?
Peter downunder
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posted 08-26-2013 05:58 AM     Click Here to See the Profile for Peter downunder   Click Here to Email Peter downunder     Edit/Delete Message   Reply w/Quote
The original question has been one that has intrigued me for years.

Without going back to check, I understand Apollo 8 had a launch window that was a couple of weeks (a month?) ahead of a potential Russian launch. My question is: without a concern for lighting conditions at the landing site, what is taken into consideration for a lunar orbit launch? After all, in 1968 it was paramount to be the first into orbit. Would finding the correct lighting conditions on arrival even be considered if losing the 'race' was a possibility?

With my limited understanding, I would have thought once in orbit whether were launched from Russia or New Zealand, on the 1st of the month, or the 29th, the TLI burn could be adjusted to find the moon three days later. Why did it matter what time of the month we took off?

Jim Behling
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posted 08-26-2013 06:49 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
There was a lighting concern. They wanted the Apollo 8 crew to be able to see the potential landing sites while lit by sunlight.

Headshot
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posted 08-26-2013 07:24 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
The Soviet lunar launch window actually opened on Dec 7 1968., several weeks ahead of Apollo 8's launch window. Technical problems with the unmanned Zond 6 test mission in Nov. 1968 prevented the Soviets from launching their manned mission around the moon ahead of Apollo 8.

Peter downunder
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posted 08-27-2013 01:57 AM     Click Here to See the Profile for Peter downunder   Click Here to Email Peter downunder     Edit/Delete Message   Reply w/Quote
Okay, this is the crux of my question: Why is there a launch window for Moscow (or thereabouts) and KSC's opens days/weeks later? Keep it simple! I failed high school physics.

Andy Anderson
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posted 09-03-2013 09:57 PM     Click Here to See the Profile for Andy Anderson   Click Here to Email Andy Anderson     Edit/Delete Message   Reply w/Quote
The following is quoted from NASA MSC Internal Note No. 70-FM-78 - Apollo 13 Lunar Trajectory Notes.
LAUNCH WINDOWS.

There are a number of considerations which determine the unique time period called the launch window from which the lunar mission is flown. These considerations are as follows.

  1. Daylight launch from Kennedy Space center
  2. Launch azimuth (direction) from Kennedy Space Center restricted to 72 (degrees) to 108 (degrees) from north
  3. Translunar injection to occur over the Pacific Ocean (as opposed to the Atlantic Ocean)
  4. Low sun elevation at the lunar landing site
  5. Goldstone, California, radar coverage of the lunar landing phase
  6. Daylight earth landing in the prime recovery area.

By the time Apollo 17 launched, the location of the lunar landing site combined with item .4 and confidence in the launch procedure was probably the reason that item .1 was not complied with.

As for...

quote:
Originally posted by Dietrich:
A further question to this: On the chart the lunar orbit is clockwise. What is the reason for this?
My uneducated guess is that, to have a Free Return trajectory in case of a translunar abort (as occurred on Apollo 13) you have to approach the moon from the leading edge of the orbit (clockwise around the moon) in order to get a moon gravity assist to slow your approach speed and "curve" the trajectory back towards earth.

Finally as to what goes round what, from the same document, NASA says,

The moon rotates 360 (degrees) about the earth in a near circular orbit every 27.32 days. The distance from the earth to the moon varies between 222,000 miles at the nearest point and 253,000 miles at the furthest point, distances equivalent to 28 to 32 earth diameters.

The earth revolves about the sun every 365.25 day; leap year every fourth year compensates for the small difference. The earth's orbital plane about the sun, called the ecliptic plane, is inclined 23.5 (degrees) to the earth's equator, the latter defined by the earth's rotation about the north pole.

...and with a little help from Captain REFSMMAT, that seemed to work out OK for them.

moorouge
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posted 09-04-2013 01:09 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
MMMMMMMM - so, even NASA would seem to indulge in a 'lie for children' when it comes to simplifying the complicated orbital mechanics for flights to the Moon. In the above diagram, as I've previously tried to explain, the Earth is Sun centered whilst the Moon is Earth centered. The two are not compatible in real life.

Jim Behling
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posted 09-04-2013 06:33 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
Stop with this 'lie for children' nonsense. They are compatible. The moon orbits the earth no matter what frame is used.

moorouge
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posted 09-08-2013 10:14 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
I'm beginning to understand how Galileo felt. So, here goes for a final time.

Does the Moon orbit the Earth? Yes, if you like the simple life and want to live in an Earth-centered universe and are happy with a 'lie for children'.

However, in the real universe and the adult world, the Moon only appears to orbit the Earth, something like this diagram shows.

As Heather Couper, a leading UK astronomer explains -

"When we regard the Sun as the centre of our frame of reference, then the Moon follows a path that weaves in and out of the Earth's orbit. It's a bit like the sides of a 50p piece, but with 12 or13 sides instead of 7."[/i]
The mechanics and maths of this are better described here.

Now, there you have it. "To be(lieve) or not to be(lieve), that is the question," — as someone once famously said.

Kite
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posted 09-08-2013 01:46 PM     Click Here to See the Profile for Kite     Edit/Delete Message   Reply w/Quote
Well put. You have worked very hard on this thread and explained it well. Seems perfectly clear to me.

Jim Behling
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posted 09-08-2013 05:11 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 moorouge:
I'm beginning to understand how Galileo felt.
Don't put yourself in the class of Galileo, especially when you are completely wrong. It is not a "lie for children." The moon is in an orbit of the earth. That is a plain and simple fact and is not changed by the different paths it follows in other frames. The weaving path in solar frame does mean it is not in an earth orbit.

You can't not mathematically describe the path of the moon in a solar reference frame without it being defined in a earth orbit.

Robert Pearlman
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posted 09-08-2013 05:12 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:
Yes, if you like the simple life...
There is nothing simple about the mathematics between the three-body problem, and for you to try to dismiss those calculations — that do indeed establish the moon orbits the Earth and the sun — is childish.

Eddie, we all get it. We simplify the nature of the moon's orbit when we describe it to those without a degree in physics.

Blackarrow
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posted 09-08-2013 08:39 PM     Click Here to See the Profile for Blackarrow     Edit/Delete Message   Reply w/Quote
Taking Eddie's (moorouge) argument to its logical conclusion, must we conclude that Phobos does not orbit Mars; that Europa does not orbit Jupiter; that Titan does not orbit Saturn; and that the millions of individual particles in Saturn's rings do not actually orbit Saturn?

canyon42
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posted 09-08-2013 08:50 PM     Click Here to See the Profile for canyon42   Click Here to Email canyon42     Edit/Delete Message   Reply w/Quote
When we regard the Sun as the centre of our frame of reference...
The quote you are referring to isn't really making the iron-clad case you think it is.

Yes, WHEN the frame of reference is sun-centered, the moon (and the Earth) orbits the sun. However, there is no single frame of reference to begin with — as I noted before, your reasoning means we might as well say that the Earth doesn't orbit the Sun, but instead orbits the center of our galaxy — except that our galaxy has its OWN motion, so even that isn't definitive. Referring to "the real universe" is just silly — there IS no "real" frame of reference. This "lie for children" nonsense is just that, nonsense.

Galileo? Jeesh.

Oh, and incidentally, that diagram you posted sort of works, except that it is WAY off-scale. Stretch it out to its true extent and you'll see that at no point at all during the moon's path throughout a year IN A SUN-CENTERED FRAME OF REFERENCE is its path not concave. Its concavity just becomes a little less pronounced as it nears the new moon phase.

moorouge
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posted 09-09-2013 02:36 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
On some matters of clarification.
  1. "A lie for children" is a phrase coined by Prof. Ian Stewart to describe a simplification of complicated things to provide a basic understanding. A good example of this is the responses given to four or five year olds when they ask where babies come from. It is equally applicable when used to describe our relationship with the Moon and it is quite alright to say that our nearest neighbour goes round the Earth. However... that isn't quite the whole truth as I've been trying to point out.

  2. In response to Jim. Why do people put words into my mouth that I never said or implied just to make a rather hurtful point? I never have, nor would I ever dream of, putting myself in the class of Galileo. All I said was that I was beginning to understand how he must have felt. Two sentences in Jim's latest post seem to prove my point. In one he says, "The moon is in orbit round the Earth." and a couple later says, "The weaving path in solar frame does mean it is not in an earth orbit." That's what I've been trying to point out all along.

  3. Robert - where have I said or implied that the maths of the three body problem was simple? It most certainly isn't childish. In fact, in the 'Unsung Heroes' thread I cite the people who solved it to enable the Voyager missions to succeed.

  4. Yes Canyon42 - one can make it even more complicated. To quote Heather Couper again, "Then again, if we think of how the Sun is moving - and make the Milky Way our reference frame - then the Earth and Moon are both following spiralling (helical) paths through space!!"
Finally, my thanks to Kite who seems to have got the point.

Jim Behling
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posted 09-09-2013 06:25 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:
"The weaving path in solar frame does mean it is not in an earth orbit." That's what I've been trying to point out all along.
Your point is still wrong. I left out a word. "The weaving path in solar frame does not mean it is not in an earth orbit."

Or better said without double negatives: The weaving path in solar frame has no bearing on the fact that moon is still in an earth orbit.

In all frames, the moon is in orbit of the earth.

LM-12
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posted 09-09-2013 07:05 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The Moon orbits the Earth. The weaving path is the orbit of the Moon around the Sun, not the Earth. That is how I understand it to be.

moorouge
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posted 09-09-2013 07:39 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by LM-12:
The Moon orbits the Earth. The weaving path is the orbit of the Moon around the Sun, not the Earth. That is how I understand it to be.

To quote from the link two postings ago which you should read -

Imagine you're driving on a circular race track. You overtake a car on the right, and immediately slow down and go into the left lane. When the other car passes you, you speed up and overtake on the right again. You will then be making circles around the other car, but when seen from above, both of you are driving forward all the time and your path will be convex.

Kite
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posted 09-09-2013 01:03 PM     Click Here to See the Profile for Kite     Edit/Delete Message   Reply w/Quote
In reply to Geoffrey (Blackarrow) you must take into account that the Earth and Moon are a binary system, even if the balance of which is under the Earth's crust. In comparison to the examples you cite our Moon is much larger to its sister planet and would have much more gravitational effect. Although the Moon appears to orbit the Earth they really do revolve around their centre of gravity which I explained in my much earlier post.

Gonzo
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posted 09-09-2013 02:57 PM     Click Here to See the Profile for Gonzo   Click Here to Email Gonzo     Edit/Delete Message   Reply w/Quote
Eddie - I fully understand your frustration. I piped in a long time ago on this and tried to make your case as well. I fully understand, and agree with, your point. However, there are those that don't. The truth is, from the different viewpoints of both camps, both are right.

On one hand, to state that the Moon orbits the Earth, is a simplification, hence, a "lie for children" as the abstraction makes sense here. On the other, we all know that it is much more complicated than that when we start looking at the actual orbital mechanics. However for those who see this and disagree, the abstraction doesn't make sense. So it's all a matter of view and whether the abstraction is valid.

I guess what I'm saying is that you're beating a dead horse. There are those that understand your point. There are those that don't.

Jim Behling
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posted 09-09-2013 03:13 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:
On one hand, to state that the Moon orbits the Earth, is a simplification
It isn't a simplification, it is a cold hard fact.

Blackarrow
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posted 09-09-2013 03:19 PM     Click Here to See the Profile for Blackarrow     Edit/Delete Message   Reply w/Quote
Kite, with all due respect, you are as mistaken as Eddie. The fact that the Earth and Moon are sometimes thought of as a binary planet is neither here nor there. Yes, the Earth-Moon system revolves around its centre of gravity (or is that centre of mass?) which is within the Earth, but that is irrelevant. The centre of gravity for Mars and Phobos is within Mars, just a lot closer to the centre. [Bear in mind that Phobos, although tiny, still has enough mass to raise a tide in the Martian crust]. The set-up is still the same. Phobos orbits Mars while Mars orbits the sun, while the sun orbits the galactic centre, and so on.

It was, I believe, Carl Sagan who told us that "extraordinary claims require extraordinary proof." Apart from the "Apollo hoax" claims, I can't think of a more extraordinary claim than: "The Moon doesn't orbit the Earth."

Gonzo
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posted 09-09-2013 08:12 PM     Click Here to See the Profile for Gonzo   Click Here to Email Gonzo     Edit/Delete Message   Reply w/Quote
I don't think the issue is whether the Moon orbits the Earth. Rather, the question and the point of contention is that the statement is a simplification of the facts. Yes, I will agree that the Moon orbits the Earth as a simple statement. On the surface (no pun intended) the statement is correct by most common definitions.

However, the point that is trying to be made (so far unsuccessfully so), is that statement is in and of itself a simplification for the much more detailed and much more complicated orbital and planetary mechanics involved. What I believe Eddie and others are trying to point out is that technically, when you really look at all the math and theory involved, the Moon in fact (technically speaking, not in the common vernacular) does NOT orbit the Earth insofar as that the two work together and orbit a center of mass/gravity.

Now hear me out. As I said, by most definitions, because that center of mass/gravity is within the Earth's radius, it is accepted as a common statement that the Moon orbits the Earth. However, technically speaking, they do no such thing. To state scientifically that they do is a misconception in that in order for that to happen, one must ignore the effects of gravity. Because I think what is being pointed out here is that SCIENTIFICALLY, there is no such thing as a true orbit. Rather, the bodies involved revolve around a common center of mass/gravity.

Care for yet another example? You and your friend about the same size/weight as you lock hands facing each other. I'm sure you've done this as a kid. You then start spinning with your feet being the center of rotation. You spin fast enough that if either of you were to let go, you'd both go crashing outward (and probably fall). The reason the center of rotation is the axis coming up from your feet is due to the two masses being approximately equal. Follow me so far?

Now let's say that your friend weighs 100 times as much as you do. Now lets repeat the exercise. The center of rotation is now within your friends body because he weighs so much more than you. Can you now say that you orbit your friend when you spin? Of course not. With this example you can see that your weight is not enough to make an observable difference in your friends location when you spin. Yet the only thing we changed here was his mass. So how can you say that the idea of you orbiting your friend is now valid?

That is what is being argued here. Only on a much larger scale. Look at it this way. If the Moon and Earth were closer to being the same size and were still locked in rotation, would the Moon still orbit the Earth or would they spin as a system around a point in the middle somewhere?

Be careful how you answer. To say that the Moon doesn't orbit the Earth anymore is only valid if you take into account the gravities involved. And that negates the argument that it does to start with.

On the other hand, to say that it does simply no longer makes sense. The point is that the common definition of an orbit requires that center of rotation is within one of the bodies radius. But that is a common definition in that the facts of orbital mechanics is much more complicated and shows the falsehood of the concept of an orbit.

Robert Pearlman
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posted 09-09-2013 09:34 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 Gonzo:
...when you really look at all the math and theory involved, the Moon in fact (technically speaking, not in the common vernacular) does NOT orbit the Earth...
And yet, when you look at all the math and theory involved, it does.

I've raised this twice so far but it seemingly keeps being ignored. So maybe the third time is the charm.

The Hill sphere defines the the region around an astronomical body in which it dominates the attraction of satellites. The Earth's Hill sphere extends out 1.5 million kilometers, hence:

The Moon's orbit, at a distance of 0.384 million km from Earth, is comfortably within the gravitational sphere of influence of Earth and it is therefore not at risk of being pulled into an independent orbit around the Sun.
Ipso facto, the Moon does orbit the Earth, without any disclaimers needed.

Don't believe me? That's fine, read Phil Plait's answer on this very question.

moorouge
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posted 09-10-2013 02:48 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
Let me repeat, as Robert has, in the real world the Moon only APPEARS to orbit the Earth. To say that this means that the Moon actually is in orbit round the Earth may well be good enough for most people. However, it just isn't the whole story.

I wonder how many of those who disagree have taken on board what Heather Couper has said and how many have bothered to read the link I provided earlier.

Robert - Phil Plait's article you quote in the first of his diagrams 'Not This' admits that the Moon does not physically orbit the Earth. However, his 'This' diagram is way off the mark. As far as I'm aware planetary bodies don't suddenly stop and change direction.

Jim Behling
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posted 09-10-2013 05:38 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:
Let me repeat, as Robert has, in the real world the Moon only APPEARS to orbit the Earth.
Wrong, it is not an appearance, in the real physical universe, it does orbit the earth. Just like the ISS, HST and many other spacecraft including the shuttle. How is the moon different from them? Just because it is further away from the earth and hence orbits at slower rate, which makes it scribe a different path from other satellites in certain frames doesn't change the fact that it is in orbit of earth.

moorouge
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posted 09-10-2013 11:15 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
Jim - either you haven't read the article I posted or you don't understand what it says. Here it is again, so please read it.

Robert Pearlman
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posted 09-10-2013 11:28 AM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
Eddie, the page you're linking to describes (correctly) the path the moon takes around the Sun, but as it does not address other issues, including the Hill sphere, it does not negate that the moon orbits the Earth.

Find a valid mathematical refutation of the Hill sphere and then, maybe, you will have a case to be made.

Jim Behling
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posted 09-10-2013 12:40 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 moorouge:
Either you haven't read the article I posted or you don't understand what it says.
The path the moon follows around the sun does not negate the fact that it is in orbit around the earth. No loops, convex, etc. doesn't change that fact.

Your argument is no different than the following one I will make.

The shuttle and molniya spacecraft cannot be in earth orbit because the trace on the earth surface show paths that cannot possibly be happening. How can a spacecraft reverse itself.

Gonzo
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posted 09-10-2013 04:32 PM     Click Here to See the Profile for Gonzo   Click Here to Email Gonzo     Edit/Delete Message   Reply w/Quote
Robert, what you are saying about the Earth's Hill Sphere is correct. The Earth's gravity does indeed dominate the Moon as a satellite. All that means is that the Moon (a major catastrophe not withstanding) will never lose it's connection with the Earth. It will always be a satellite of the Earth.

However, stating that recognizes the gravities involved. As such, recognizing those gravities, you also recognize that they work together. The result is that while the Moon is within the Earth's Hill Sphere, that doesn't mean that the two do not interact. And that interaction is the central point in which they revolve around as a system.

I guess what I'm saying is that the two ideas are not mutually exclusive. That the Moon is in fact inside the Earth's Hill's Sphere (a.k.a. Roche sphere) while at the same time, the Earth is also still effected by the Moon's gravity. And I'd argue that you can't have one without the other.

Did this help at all or did I just muddy the waters further?

Blackarrow
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posted 09-10-2013 06:25 PM     Click Here to See the Profile for Blackarrow     Edit/Delete Message   Reply w/Quote
On further reflection (a "thought experiment" if you like...) I have come up with a possible answer to this puzzle. I am neither a mathematician nor an astronomer, so if my logic fails me I will happily retire from the fray and leave it to others.

I start on the basis that the Earth and the Moon obviously travel around the sun. As the Moon is in orbit around the Earth, it has no choice in the matter. It is a prisoner of the Earth's gravity.

Now look back at Eddie's diagram (posted on 8th September at 10.14am). The Moon and the Earth both make one circuit of the sun in a year, but on the diagram the Moon appears to travel a lot further. That must mean it travels faster (relative to the sun) than the Earth. Think of a water-skier following a zig-zag course. He must obviously be travelling faster than the boat which tows him because it is travelling in a straight line. Keep hold of that thought...

We know that in the Earth's distant past the Moon was much closer to the Earth. Gradually the Moon has retreated due to tidal acceleration so it is now orbiting about 240,000 miles from the Earth in about a month.

But what if the Moon recedes far enough from the Earth that its orbit around the Earth takes one year, the same time as the Earth's journey around the sun? (NB. I have no means of calculating how far the Moon would have to recede. It may be so far that its orbit would be perturbed by external gravitational influences. Never mind: bear with me and treat this as a hypothetical exercise to illustrate a point.)

My diagram illustrates the movement of the Moon over the course of a year. As the Moon orbits the Earth in a year, it would move from the "12 o'clock" position in December to the "3 o'clock" position in March, 90 degrees around the Earth's path around the sun. And so on.

Let us assume for the sake of the argument that the Moon has receded to a distance of one million miles from the Earth. That means that in one year the Earth travels 360 degrees around the sun at a distance from the sun of 93 million miles and the Moon also travels 360 degrees, but at a distance of 94 million miles from the sun.

Now remember the water-skier. By zig-zagging he can travel faster than the boat which tows him. The diagram shows that the Moon has travelled once around the sun in the same time that the Earth has travelled once around the sun, so the Moon MUST have been travelling faster than the Earth.

If, as has been argued, the Moon is really in orbit around the SUN, then by circling the sun faster than the Earth it would have to move into a more distant and slower orbit (like Mars) and would (again like Mars) quickly fall behind the Earth's position and become a planet in its own right, no longer gravitationally bound to the Earth.

The only way the Moon could make one circuit of the Earth in the same time as the Earth makes one circuit of the sun, as illustrated in the diagram, is if the Moon is in orbit around the EARTH, and if the Earth's gravitational hold on the Moon is the predominant influence on its journey through space.

moorouge
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posted 09-12-2013 05:34 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
I think that Geoffrey (Blackarrow) is to be commended on the thought he has put into his post and trust it hasn't caused him too many sleepless nights. However, he makes the same mistake that I have made and one that has confused all the discussions so far. Both he and I have failed to define exactly what we mean by an orbit of the Moon round the Earth.

Does the Moon orbit the Earth as it journeys round the Sun as demonstrated by an orrery? No it doesn't, notwithstanding Hill's Spheres and centres of mass mentioned in other postings, though these obviously have a bearing. This fact is clearly pointed out in the website quoted by Robert (Plait) and in that quoted by me. The first diagram on these sites respectively are clearly labelled as 'NOT TRUE'.

So what is true? All the astronomers I have consulted and both websites referenced by Robert and myself agree that the Moon follows a weaving pattern round the Earth as it orbits the Sun, though it has to be said that Robert's site is less clear about this than mine. This does NOT mean that the Moon does not orbit the Earth as shown in this diagram.

What it does mean is that the Moon doesn't orbit the Earth in the sense of being Earth centred as is ingrained in our conventional thinking about Earth orbits. It does orbit the Earth though as the diagram shows, taking about 29 days to do so. In its trip round the Sun it makes roughly 13 complete weaves about the Earth held in place and influenced by Hill's spheres, centres of mass and gravity.

This said, I'll hold up my hand and admit that I was wrong to use the word 'appears' in my original posts. What I intended to convey was that the Moon only appears to orbit the Earth in a conventional Earth centred sense. It still orbits the Earth as I hope I have illustrated above.

I apologise for the confusion.

On edit - going back to Geoffrey's post. The mind boggles at the thought of Apollo in a power-boat towing Diana behind him on water skis.

Peter downunder
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From: Lancefield, Victoria, Australia
Registered: Apr 2012

posted 09-12-2013 06:34 AM     Click Here to See the Profile for Peter downunder   Click Here to Email Peter downunder     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by moorouge:
...Moon follows a weaving pattern round the Earth as it orbits the Sun.
That's the problem I have. The moon follows a weaving pattern around the Sun as it orbits the earth.

This idea that the moon doesn't orbit the earth, or 'appears' to orbit the earth simply because it doesn't orbit around the exact centre of the earth, but somewhere within the earth's sphere is getting weird for me.

Bottom line: The moon orbits the earth. If we take the earth away and draw a line that describes the motion of the moon, it looks like it orbits (yes, wobbly) the sun. It doesn't really, it just looks like it. If you take the earth out of the equation, but you can't, because (insert exploding sound as my head comes apart here)...

Jim Behling
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From: Cape Canaveral, FL
Registered: Mar 2010

posted 09-12-2013 08:43 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:
What it does mean is that the Moon doesn’t orbit the Earth in the sense of being Earth centred as is ingrained in our conventional thinking about Earth orbits.
Yes, it does. In an earth centered frame, the moon goes in a circular path around the earth. The point of selecting a frame is other frames are ignored. Only in a solar frame does it go in a weaving path around the sun. And that solar frame ignores the path the moon follows in the galaxy based frame (which is not the same weaving path). Actually, those diagrams that you and Robert linked are wrong, they are "lies for children", the earth doesn't "orbit" the sun, it "weaves" around it and the earth/moon path is too complex to show.

Gonzo
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From: Lansing, MI, USA
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posted 09-12-2013 11:46 AM     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:
Yes, it does. In an earth centered frame, the moon goes in a circular path around the earth. The point of selecting a frame is other frames are ignored.

And that's the whole point of this conversation. Ignoring all the other frames of reference, that is if you ONLY look at the Earth and Moon, the Moon does NOT make a circular path around the Earth. If you could look at only these two, you'd notice, however slightly, the Earth would seem to wobble back and forth as the Moon makes it's rounds. And again, that is because they work together and rotate as a system around a common point. And that point is NOT the center of the Earth. It is WITHIN the Earth, but not at it's center (and that is what would cause it to appear to wobble).

Now if you stabilized the Earth somehow so that it didn't wobble, the Moon would make an elliptical path. And that ellipticality (?) would be very noticeable.

Jim Behling
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From: Cape Canaveral, FL
Registered: Mar 2010

posted 09-12-2013 02:44 PM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
Forget I said "goes in a circular path around the earth" (that isn't the point) and instead said "goes around earth."

Blackarrow
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From: Belfast, United Kingdom
Registered: Feb 2002

posted 09-12-2013 04:09 PM     Click Here to See the Profile for Blackarrow     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by moorouge:
I think that Geoffrey (Blackarrow) is to be commended on the thought he has put into his post and trust it hasn't caused him too many sleepless nights.
I can assure Eddie that my diagrammatic contribution to this debate arose from about 10 minutes perusal of his "weaving Moon" diagram. It took rather longer to type the post...

Nobody seems to have grasped my point: if you want a sun-centred explanation, look no further than my diagram. You are looking down on the Sun - Earth - Moon arrangement from above the sun. The Earth is ORBITING the sun, obeying Newton and Kepler. The Moon is ORBITING the Earth, again obeying Newton and Kepler. The Moon APPEARS to be orbiting the sun, but as my previous post explains in greater detail, it is travelling too fast and is NOT in a solar orbit. It is only by raising the Moon's orbital period to 365 days that it is possible to see this without the unnecessary and confusing distraction of a Moon weaving in and out of Earth's orbital path.

The diagram also works if the Moon is in a 365-day orbit of the Earth, but circling one million miles closer to the sun. It is in orbit around the Earth, but NOT in orbit around the sun: it is travelling too slowly, at a speed which (if it was free of Earth's gravity) would cause it too move much closer to the sun.

The key point is that the diagram shows how the Moon can circle the sun smoothly (none of this weaving around) without being in orbit around the sun.

I notice no-one has attempted to disprove my point, but if anyone thinks they can, go for it! Knowledge advances by disproving previous theories.

Ronpur
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Posts: 275
From: Brandon, Fl
Registered: May 2012

posted 09-12-2013 04:11 PM     Click Here to See the Profile for Ronpur   Click Here to Email Ronpur     Edit/Delete Message   Reply w/Quote
Wow, this has been fascinating to follow. (And giving me a slight headache.) However, to me, it seams to about frame of reference.

It reminds me of an exercise we had in physics trying to understand frame of reference:with a person tossing a ball up and down in a moving train and the ball would follow the path of a wave to an outside observer.

But how do you want to pick your frame of reference?

If you say the moon orbits the sun, then wouldn't ISS as well?

Are you in the center of the orbit looking out or outside looking in?

So, aren't both observation points correct?

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

posted 09-13-2013 02:50 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:
The point of selecting a frame is other frames are ignored.

Yes - it does depend on a frame of reference. The key is selecting which gives the most accurate picture of what actually happens without confusing everyone too much.

One can make a case that the Earth is really in orbit round the Moon. It must be, or how else was the Apollo 8 'earthrise' photo taken?

Jim Behling
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Posts: 587
From: Cape Canaveral, FL
Registered: Mar 2010

posted 09-13-2013 06:40 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:
The key is selecting which gives the most accurate picture of what actually happens without confusing everyone too much.
Wrong. There is no frame that "gives the most accurate picture." If that were true, then your solar based one with the weaving path is not "the most accurate picture." It is a "lie for children" also since the earth does not orbit the sun.

You just don't get that you are wrong in this. The issue that you say is wrong with the earth frame is also applicable to your solar frame.

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

posted 09-13-2013 06:49 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
Sorry - it's you that doesn't get it. I'm quite happy to believe the Royal Astronomical Society and other professional astronomers that I've consulted.


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