# How to find instantaneous positions of a central body of specified mass that results in an osculating orbit?

If I understand correctly, osculating orbital elements such as those posted in JPL's Horizons represent the mathematical osculating (tangential or "kissing") Keplerian orbit about a specified location in space, without regard to what mass that body would need to have in order for that orbit to happen.

Instead, I'd like to calculate the position of a body of specified mass such that an orbit about it would have a specified state vector; at a given position $$\mathbf{x}$$ would have the velocity vector $$\mathbf{v}$$.

The goal is to formulate an alternative, more numerical, graphic, and slightly creative answer to the question What point does Earth actually orbit?

I'd like to calculate where to put the Sun with 1 solar mass such that it's orbit would include a given state vector.

1. Is this mathematically possible
2. Will this produce a different set of orbital elements than the traditional osculating elements?

If I understand you correctly the state vector you would use would be defined in an arbitrary reference frame and you would like to find the position of the central body in that same arbitrary reference frame that would result in an orbit which includes that state vector.

Mathematically there are infinitely many solutions to this problem, any position you choose for this central body will result in a keplerian orbit. Of course ruling out singularities such as zero velocity or zero position (with respect to the central body).

Say you have some state vector with a small velocity. This may be the result of a circular orbit about the central body with a large semi-major axis and low orbital velocity. The same state vector may be the result of a highly elliptical orbit around a central body with the same mass, where the body is much closer than in the other case, but the spacecraft is near or at apogee.

The Wikipedia page on Kepler orbits has a section on this initial value problem. The choice of location of a central body will just change the coordinate transformation you have to apply to the given state vector in the reference frame in which you also define the location of the central body and will simply result in different state vectors in the reference frame centered at the central body which will be used to determine the kepler orbit corresponding to this state.

• Thank you for looking at the question! I had a suspicion that this might be under-constrained and so wouldn't work, but I couldn't think of any further physically reasonable constraints. After reading through your clear discussion I'm 100% convinced. I like to give answers a day or two to before clicking "accept" but I think we can call this one case closed. – uhoh May 16 '19 at 10:16
• Yikes! it's been much more than a day or two. Yes, this question was woefully under-constrained. Maybe this is what I was thinking of: How to find instantaneous mass of a central body that results in a given osculating orbit? – uhoh Jan 16 at 1:19

Here's a clever idea that doesn't work:

• Find the Earth's position relative to the solar system barycenter at several points in time.
• Find the best fit ellipse to those positions (similar to finding osculating elements, but not instantaneous ones).
• Look at the foci of the result to determine where the central object must be and it's mass.

Why it won't work:

• Small problem: this assumes the solar system barycenter is in a non-accelerating frame of reference. In reality, it is orbiting the center of the Milky Way. However, this acceleration is small.
• Bigger problem: even if your ellipse is non-accelerating, it can still have a constant velocity relative to the barycenter. Mathematically, I'm sure you could work out the best velocity for the two foci (identical for both) relative to the solar system barycenter that make the best fit. This is just an ugly optimization problem, and there's another issue: the result will give an ellipse that moves further and further away from the solar system barycenter, which we know is unrealistic.

Other stuff:

• It's a small difference, but do you want where the Earth orbits or where the Earth-Moon Barycenter (EMB) orbits?

• I tried working this out once but have no idea what happened-- despite this, my notes somewhere in https://github.com/barrycarter/bcapps/tree/master/ASTRO/ may or may not be helpful.

• Thanks for sharing ;-) I'll have a look at the Github. For the Other stuff's " I'm just asking for the algorithm or methodology. Given a state vector $\mathbf{r}, \mathbf{v}$ and a central body mass $m$, where would you put $m$ such that $\mathbf{r}, \mathbf{v}$ can be part of a Keplerian orbit around said mass. It's a one-body question (mass of orbiting body is unspecified therefore neglected. – uhoh Mar 27 '19 at 21:11
• You probably know this already, but R^3/T^2 (R = semimajor axis, T = orbital period) is constant based on the mass of the central body. That may or may not help. Also, given r and v, you can always find the two foci of the ellipse, and then maybe just tweak the pass of one focus until the numbers work out? – barrycarter Mar 28 '19 at 17:18