"Makhno" wrote in message ...
Hi Gents,
I'm trying to write something to go from kepler parameters to a position and
velocity vector.
This article was a great help
http://www.ucalgary.ca/~sneeuw/lectu...423/kepler.pdf

But I can only get orbits that look real (ie: the Earth is in the correct
place) if I change the sign of the rotations, specifically, change the line
r = R3(-Om)R1(-I)R3(-w)q

into

r = R3(Om)R1(I)R3(i)q

and same for velocity. Then all my planets end up in the right 'place'.

When you say they end up not in the right place, are
you referring to a screen plot of the position? Make
sure that things aren't getting confused by the
orientation of the screen axes. For example, in BASIC,
the Y-axis runs from the top-left corner to the
bottom-left corner, increasing Y-values are further
down the screen.


When going back from velocity to position, I again need the sign change, but
I'm having unrelated problems with totally circular orbits.
Obtaining a (semi-major azis) and e (eccentricity) from the position and
velocity vector is easy enough, but if e is zero, then I do not know how to
calculate the eccentric anomaly (E).
An equation is given for distance a body at a distance r
Cos E = (a-r)/(a*e)
As you can see, for circular orbits this is undefined.

I realise that in the circular orbit case the eccentric anomaly is equal to
the mean anomaly, but the above paper calculates the mean anomaly from the
eccentric anomaly! Simply obtaining the angle in the q-plane doesn't work
either, because one of the rotations to create the q-plane is created from
the Eccentric anomaly!
But I waffle on...How can I calculate the eccentric anomaly for circular
orbits?

Exactly which parameters are you given to start with
for your circular orbit? The classical orbit elements
can be tricky to work with in specific cases. For example,
for an equatorial orbit Om, the longitude of the ascending
node is undefined. For a circular orbit, the argument of
periapsis and the true anomaly (along with the mean and
eccentric anomaly) are undefined, since there is no
periapsis. In these cases you have to work from the
longitude of the ascending node. Perhaps the argument of
latitude at epoch will be given (angle between ascending
node and the position vector at epoch)?