# Problems modelling astrometric orbits

## Problems modelling astrometric orbits

I've been working on a way to model astrometric orbits, essentially representing 3D orbits on a 2D plane for the purposes of this conversation. I've been having a hard time figuring out the math behind it. I've found what I thought was the proper formulation but it is producing results that are not particularly intuitive.

I will post some screenshots.

A simple, face-on configuration.

As expected, changing LongPeri for an inclined circular orbit does not produce a physical change.

But adding eccentricity seems to tilt the orbit.

Only changing e produces an unexpected result.

Here's my math.

I used my same formulation for the eccentric anomaly as used for these diagrams so I do not think it is a problem there.

Also, for eccentric orbits, the x and y coordinates of the orbit are not mapping them to positions that place the barycentre in the image centre.

I'll have to hack at it when I get home from work, but for now I can't figure out what's wrong. Any assistance would be appreciated.

I will post some screenshots.

A simple, face-on configuration.

As expected, changing LongPeri for an inclined circular orbit does not produce a physical change.

But adding eccentricity seems to tilt the orbit.

Only changing e produces an unexpected result.

Here's my math.

- Code:
`//Theile-Innes constants.`

double A = a * (Math.Cos(omega) * Math.Cos(Node) - Math.Sin(omega) * Math.Sin(Node) * Math.Cos(inc));

double B = a * (Math.Cos(omega) * Math.Sin(Node) + Math.Sin(omega) * Math.Cos(Node) * Math.Cos(inc));

double F = a * (-1 * Math.Sin(omega) * Math.Cos(Node) - Math.Cos(omega) * Math.Sin(Node) * Math.Cos(inc));

double G = a * (-1 * Math.Sin(omega) * Math.Sin(Node) + Math.Cos(omega) * Math.Cos(Node) * Math.Cos(inc));

- Code:
`double x_coord = A * (Math.Cos(E - ecc)) + F * Math.Sqrt(1 - (ecc * ecc)) * Math.Sin(E);`

double y_coord = B * (Math.Cos(E - ecc)) + G * Math.Sqrt(1 - (ecc * ecc)) * Math.Sin(E);

I used my same formulation for the eccentric anomaly as used for these diagrams so I do not think it is a problem there.

Also, for eccentric orbits, the x and y coordinates of the orbit are not mapping them to positions that place the barycentre in the image centre.

I'll have to hack at it when I get home from work, but for now I can't figure out what's wrong. Any assistance would be appreciated.

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**Sirius_Alpha**- Admin
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Location : Earth

Registration date : 2008-04-06

## Re: Problems modelling astrometric orbits

Follow-up. I've managed to get it working right. The orbit diagrams now match observations for known binary stars, etc.

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**Sirius_Alpha**- Admin
- Number of posts : 3403

Location : Earth

Registration date : 2008-04-06

## Re: Problems modelling astrometric orbits

Checked out Upsilon Andromedae yet? When I tried to do this a few years back I was unable to match all the data/figures in McArthur et al. (2010) simultaneously.

**Lazarus**- dG star
- Number of posts : 2772

Registration date : 2008-06-12

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