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### Inclination of an Ellipse's Major Axis ... in Three Dimensions?

```Date: 01/11/2011 at 14:58:41
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

I want to build an ellipse given the minimum n points, and knowing that
one of the foci has the coordinates (0,0). The other one can be elsewhere.

My questions are:

1) How many points are needed? Uniquely determining an ellipse requires a
minimum of five points. But in this case, I know one of the foci.

2) How do I calculate the formula of that ellipse? Is the polar form a
better way?

3) How do I calculate the inclination of the major axis regarding the
x-axis? This is the goal.

So far, I'm a little confused because the ellipse must be unique and has
an inclination regarding the x-axis.

Please give me the complete solutions, if possible.

Kind regards, and thanks,

Rui Miguel Fernandes

P.S. I would like to add that this question is, in fact, an astronomy
question. By solving it, you'll be helping a very useful project: the
public DLL of our website!

```

```
Date: 01/11/2011 at 16:16:53
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Hello Rui,

Thanks for writing to Doctor Math.

This problem is not at all simple to solve in closed form, as you have
found. It would be difficult to even write the closed form solution in
plain text.

Since you are writing a computer program, can we instead develop a
procedure that you can turn into code? I think I could walk you through it
that way.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/11/2011 at 17:17:22
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

Thank you very much for your reply.

Yes, a piece of C/C++ code would be perfect. I would adapt it to the DLL,
which is based in the same language.

Thank you very much for your time and kindness.

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/11/2011 at 22:10:42
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Dear Rui,

There is a lot to do.

I assume that you already know quite a bit about ellipses, so I will focus
on the main flow of the development. If you need me to fill in details,
please let me know where you need more input.

Let's call the other focus (xf, yf) and the major axis length 2a. From the
definition of an ellipse, a point (x,y) is on the ellipse if it satisfies
the following equation:

sqrt(x^2 + y^2) + sqrt[(x - xf)^2 + (y - yf)^2] = 2a      (1)

There are 3 unknowns in the equation: (xf,yf,a). To find the unknowns, we
need three (x,y) points to give us a system of three equations to solve.
For clarity, I will use "a" in quotes where needed.

But first, let's rearrange the equation by moving the first radical to the
right hand side, and then square both sides.

(x - xf)^2 + (y - yf)^2 = [2a - sqrt(x^2 + y^2)]^2        (2)

Do you see that we can think of this equation as a circle centered at
(x,y) with "a" constant? Each possible (xf,yf) is a point on the circle.

Now substitute (x,y) in (2) with (x1,y1), (x2,y2), and (x3,y3) to create
three circles that form the system of equations to solve. Let's call these
equations 2.1, 2.2, and 2.3. For the correct value of "a," the three
circles will intersect at a common point.

Now do this:

1. Expand the squares on both sides of 2.1 and 2.2.
2. Subtract each side of 2.2 from 2.1.
3. Simplify.

The result should look like this:

(x2 - x1)xf + (y2 - y1)yf = 2a[sqrt(x1^2 + y1^2)
- sqrt(x2^2 + y2^2)]         (3.1)

If we do the same with 2.1 and 2.3, we get the following:

(x3 - x1)xf + (y3 - y1)yf = 2a[sqrt(x1^2 + y1^2)
- sqrt(x3^2 + y3^2)]         (3.2)

Equations 3.1 and 3.2 are a linear system that can be solved for xf and yf
in terms of "a." The intersections of the first two circles must be on
3.1, and the intersections of the first and third circles must be on 3.2.
The intersection of these lines will be the intersection of all three
circles.

Think of the result like this, using c1, c2, c3 and c4 as constants:

xf = c1 + c2*a                                       (4.1)
yf = c3 + c4*a                                       (4.2)

Now substitute these expressions for xf and yf back into the expanded form
of 2.1. The result is an equation that can be solved to find "a." (I
expect that the equation will be quadratic, and the root of interest will
be positive.) Geometrically, we are finding the value of "a" so that the 3
circles share a common point.

Once we have the value of "a," we get (xf,yf) from 4.1 and 4.2. We now
know everything about the ellipse. The rotation angle will be found from

tan(angle) = yf/xf

The only remaining issue is how else you may need to express the
information in your application.

Does this make sense? Let me know where I can be of more help.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/12/2011 at 04:51:01
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

Thank you very much for your time.

In my opinion, what you show makes perfect sense. No more inputs and
outputs are needed.

What I would like to ask -- if in reason -- is if it's possible for you to
provide the C code. That would decrease the chance of errors.

I've read your reply 3 times, and the method seems perfect to me.

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/12/2011 at 15:50:12
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Dear Rui,

I write for Ask Dr. Math as a volunteer, so I have to squeeze things like
this in around my regular employment.

Having the code for you by the end of this week seems reasonable. If it is
urgent, please let me know.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/12/2011 at 15:58:02
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

What you are doing is already fantastic and extraordinarily generous. Take
the time you need; I don't dare to ask for more.

Thank you very much for your kindness. I would like to ask -- only if you
want -- your full name to place in the "thanks" page of Cosmos as an
important contributor.

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/13/2011 at 21:32:12
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Dear Rui,

There are two small additions to make to the problem analysis.

First, equations 4.1 and 4.2 can be written more simply.

xf = c1*a                                            (4.1)
yf = c2*a                                            (4.2)

Second, when we substitute these expression for xf and yf back into the
expanded form of 2.1, we do indeed get a quadratic equation in "a." The
form of the quadratic is

d1*a^2 + d2*a = 0

One of the roots is zero. This is a spurious root that was created by
squaring equation 2. The other root is the value that we want.

I will be sending you code that computes the solution in another message.

I am glad that this is helpful to you.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/14/2011 at 06:14:12
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

Thank you very much for your help.

I would like to ask, if possible, one last question: if the points are in
a 3 dimensional plane and the focus is at (0,0,0), what should I do?
Project only the x and y values into the x-y plane? That would get the
inclination of the major axis regarding the x-y plane. Or must I project
one by one according to its z-value through the x-axis?

The plane intercepts the x-y plane at the line y = mx, which I've already
calculated.

Kind regards,

Rui Miguel Fernandes
```

```

Date: 01/14/2011 at 09:52:32
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

I think I might have made a terrible mistake in not generalizing my first
question to three dimensions.

The transformations in my last note cause big problems with the

And I really hope that the solution isn't the 3D generalization --
although I'm convinced that it is; in this case, I'm very sorry.

The 2D solution is used in most of the cases; but in this case, I'm afraid
that the 3D might be needed. I'll wait for your advice.

Kind regards,

Rui Miguel Fernandes
```

```

Date: 01/14/2011 at 10:26:38
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Dear Rui,

There is a potential complication in 3D. If there is measurement error,
then the 3 points and the origin may not be in a common plane. If that is
the case, then a best fit plane may be needed. But let's ignore this issue
for now.

The projection of the major axis into the x-y plane is not in general the
major axis of the projected ellipse. To see this, take an ellipse in
standard position and tip it close to 90 degrees about the minor axis. The
major axis of the projected ellipse is the minor axis of the original
ellipse.

I am assuming that what you need is the projection of the major axis into
the x-y plane.

A good solution is to set up a secondary u-v-w coordinate system with the
origin still at (0,0,0) such that the u-v plane contains the ellipse. The
choice of u is not unique. Here is one way to define the new coordinate
system, with X representing the cross product:

u = (x1,y1,z1) - (0,0,0) / sqrt(x1*x1 + y1*y1 + z1*z1)
v = (w X u) / |w X u|
w = unit normal vector of the plane in x-y-z coordinates

Each point P from the x-y-z system has these coordinates in the u-v
system.

(P.u, P.v)

(To avoid confusion, let's switch to using . rather than * for the dot
product.)

Now find the other focus in the u-v plane as in the 2D problem. Let's call
it (uf,vf). We get that focus in x-y-z coordinates, F, as follows:

F = uf*u + vf*v

To get the projection into the x-y plane, just ignore the z coordinate.

Finally, get the x-y inclination angle from the projection of F.

Is this what you are looking for?

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/14/2011 at 10:57:38
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

Ultimately, I think that is what I'm looking for. But to be sincere, I
don't understand how to do it -- I'm very sorry. And I've lost the value
of "a."

The main thing is to know the projection of the second focus in the x-y
plane -- that's for sure. Is this a better way than the 3D generalization?

I'm terribly sorry to bother you again.

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/14/2011 at 16:10:45
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Dear Rui,

The 3D ellipse and the projected ellipse will in general have different
major semi-axis lengths. I think that is what you are saying. But we can
compute them both.

I will send you code to find the focus and major semi-axis length for 3D
data.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/17/2011 at 07:12:58
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

Thank you very much for your trouble.

I'm very sorry to have bother you again, but I have a question regarding
the code. How do you compute (wx, wy, wz)? Are they the components of the
plane Ax + By + Cz = 0 of the ellipse determined by the following
equations?

wx = A/sqrt(A*A + B*B + C*C)
wy = B/sqrt(A*A + B*B + C*C)
wz = C/sqrt(A*A + B*B + C*C)

Kind regards,

Rui Miguel Fernandes
```

```

Date: 01/18/2011 at 08:19:00
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Dear Rui,

That is correct. The vector should be normalized as you have done.

Write again if you need more help.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/20/2011 at 12:09:07
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

I would like to ask you one more clarification, if I may.

The rotation around the x, y, or z axes is given by simple formulas. But
I've searched the Internet and found different formulas -- which led me
to different results.

For example:

z-axis rotation
x' = x*cos q - y*sin q
y' = x*sin q + y*cos q
z' = z

x-axis rotation
y' = y*cos q - z*sin q
z' = y*sin q + z*cos q
x' = x

y-axis rotation
z' = z*cos q - x*sin q
x' = z*sin q + x*cos q
y' = y

Can you please tell me the correct formulas, if possible? It's connected
with the same problem, which is almost solved.

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/21/2011 at 09:02:14
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Dear Rui,

There is no unique method for performing rotations. There are simply
different conventions for the order of rotations and which direction is
considered a positive angle.

Take a look at these articles.

http://mathworld.wolfram.com/EulerAngles.html
http://en.wikipedia.org/wiki/Euler_angles

In the test case that I sent, it only mattered that I used the same method
when rotating the two ellipse points, the focus, and the unit normal
vector. I could have used any rotation convention to produce a useful
test.

Now if you have input angles that have been defined with a particular
convention, then that convention must be applied to get the correct final
orientation.

Does that help? Please send additional details if you need more
explanation.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/21/2011 at 12:53:56
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

Yes, that helps. Thank you.

Actually the problem is this: the previous routine solved a major problem
that must be tested now. I don't know if you're familiar with the elements
of an elliptic orbit. But I've got the position of one point, the focus
(0,0,0), the semi-axis "a," the longitude of the ascending node, the
inclination of the orbit, and the longitude of the perigee measured within
the plane -- not the one measured in the x-y plane. What I need to find is
this last value, that is, the longitude of the perigee in the x-y plane.

If I add this longitude to the ascending node, I won't get a real value,
since the longitude of the perigee is in the orbit plane. The inclination
isn't necessarily perpendicular to the longitude of the ascending node
also.

Do you have any ideas?

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/21/2011 at 22:29:50
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

To simplify things, I just need to know how to rotate a plane
ax + bx + cx = 0 over the Z axis; rotating (a,b,c) as if it was a
point is the right thing? Or is this method the correct one?

a' = a cos(alpha) - b sin(alpha)
b' = a sin(alpha) + b cos(alpha)
c' = c

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/22/2011 at 05:41:49
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Dear Sir,

I've wrote a piece of JavaScript to test my values, but something is
wrong. Here's the code:

// argument of perigee in xy axis
aop = 246.9845807;

// argument of perigee
ai = 5.2855334;

// inclination of orbit
lan = 261.5156997;

// longitude of ascending node
aop = aop * DEG_TO_RAD;
ai = ai * DEG_TO_RAD;
lan = lan * DEG_TO_RAD;

// create a plane that passes through the point (x,y,z) and the node
// and the center (0,0,0), and rotate it by the lan
ax = x;
ay = y;
az = z;
bx = 400000 * Math.cos(lan);
by = 400000 * Math.sin(lan);
bz = 0;

if (az != 0){
a = ay*bz-az*by;
b = az*bx-ax*bz;
c = ax*by-ay*bx;
alpha = -lan;
al = a * Math.cos(alpha) - b * Math.sin(alpha);
bl = a * Math.sin(alpha) + b * Math.cos(alpha);
cl = c;
// calculate a point with the aop angle and rotate by the true inclination
x = Math.cos(aop);
y = Math.sin(aop);
z = -(al * x + bl * y) / cl;
inc = Math.atan(z/y);
q = inc;
yl = y*Math.cos(q) + z*Math.sin(q);
zl = z*Math.cos(q) - y*Math.sin(q);
xl = x;
angle = Math.atan2(xl,yl) * RAD_TO_DEG;
if (angle<0){ angle += 360.0;}
angle = angle + lan * RAD_TO_DEG;
if (angle>360){ angle -= 360.0;}
document.form.argumento_perielio_xy.value = Math.floor
(angle*1000000)/1000000;
} else {
document.form.argumento_perielio_xy.value = "---";
}

What do you think might be wrong? Sorry to bother you.

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/22/2011 at 09:02:30
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Hi,

I think the first correction is that:

x = Math.cos(aop);
y = Math.sin(aop) * Math.cos(ai);
//z = -(al * x + bl * y) / cl;
z = Math.sin(aop) * Math.sin(ai);

Something's wrong with the rotation of the angles and planes -- I think ...

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/22/2011 at 10:04:38
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Hi,

I think I've got it...

// argument of perigee in xy axis
aop = 246.9845807;
ai = 5.2855334;
lan = 261.5156997;

aop = aop * DEG_TO_RAD;
ai = ai * DEG_TO_RAD;
lan = lan * DEG_TO_RAD;

if (z != 0){
x = Math.cos(aop);
y = Math.sin(aop) * Math.cos(ai);
z = Math.sin(aop) * Math.sin(ai);
angle = Math.atan2(y,x) * RAD_TO_DEG;
if (angle<0){ angle += 360.0;}
angle = angle + lan * RAD_TO_DEG;
if (angle>360){ angle -= 360.0;}
document.form.argumento_perielio_xy.value = Math.floor
(angle*1000000)/1000000;
} else {
document.form.argumento_perielio_xy.value = "---";
}

I was trying to make useless rotations. And atan2 in Javascript works the
other way around.

What do you think? Sorry to bother you.

Kind regards,

Rui Miguel Fernandes

```

```
Date: 01/23/2011 at 15:18:14
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Hi Rui,

It sounds like you are making progress.

You are correct that some of this terminology is outside of my knowledge
base. I would need some precise definitions, and maybe some pictures to
translate the terms to pure math.

1. Your rotation about the z axis looks right.

2. I wrote my test code using separate rotations for each axis without
matrices, but using rotation matrices could make the code much more
compact.

3. Rotating a vector that has an endpoint at the origin is the same as
rotating a point.

4. There is variation among programs in the atan2 arguments, so you are
probably right on that one.

What is your status now? Do you think you have everything? If you need
more help, I would refer you to one of the other math doctors, who has
expertise in celestial geometry.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/24/2011 at 05:05:25
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Hi,

The results are consistent to others -- but our method has failed, as it's
an approximation. Maybe you could ask your friend if there's a way of
calculating the longitude of the perigee given 3 points and the focus
(0,0,0), knowing the ascending node and inclination of the orbit, but NOT
knowing the velocities?

Once again, thank you very much for your help and kindness. You're one of
the best contributors Cosmos has ever had -- and I hope you continue that
way, if you want.

Kind regards,

Rui

```

```
Date: 01/24/2011 at 08:30:56
From: Doctor George
Subject: Re: Ellipse given n minimum points and knowing one of the focus

Hi Rui,

The best way for you to get help from another math doctor would be to
frame your question as a new submission to Dr. Math. If you make the
subject "Celestial Geometry," I will look for it and mark it for a math
doctor who knows this subject. He is the busiest of the math doctors, but
I am hopeful that he can at least point you in the right direction.

I too have an interest in astronomy, but have not had the opportunity to
study it formally.

It has been fun to help.

- Doctor George, The Math Forum
http://mathforum.org/dr.math/

```

```
Date: 01/24/2011 at 13:30:59
From: Rui
Subject: Ellipse given n minimum points and knowing one of the focus

Hi,

Thank you very much for your help. I have the feeling we'll be in touch
again soon -- if you're available and if you want, of course. Any ideas of
math applications for our website are welcome!!!

I submitted a new question, as you suggested, named "Celestial Geometry."
I've already shared my ideas; I hope your friend can give me some of his.

Kind regards,

Rui Miguel Fernandes
```
Associated Topics:
College Conic Sections/Circles
College Coordinate Plane Geometry
College Higher-Dimensional Geometry

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