Source code for gala.coordinates.poincarepolar

# Third-party
import numpy as np

__all__ = ["cartesian_to_poincare_polar"]


def cartesian_to_poincare_polar(w):
    r"""
    Convert an array of 6D Cartesian positions to Poincaré
    symplectic polar coordinates. These are similar to cylindrical
    coordinates.

    Parameters
    ----------
    w : array_like
        Input array of 6D Cartesian phase-space positions. Should have
        shape ``(norbits, 6)``.

    Returns
    -------
    new_w : :class:`~numpy.ndarray`
        Points represented in 6D Poincaré polar coordinates.

    """

    R = np.sqrt(w[..., 0] ** 2 + w[..., 1] ** 2)
    # phi = np.arctan2(w[..., 1], w[..., 0])
    phi = np.arctan2(w[..., 0], w[..., 1])

    vR = (w[..., 0] * w[..., 0 + 3] + w[..., 1] * w[..., 1 + 3]) / R
    vPhi = w[..., 0] * w[..., 1 + 3] - w[..., 1] * w[..., 0 + 3]

    # pg. 437, Papaphillipou & Laskar (1996)
    sqrt_2THETA = np.sqrt(np.abs(2 * vPhi))
    pp_phi = sqrt_2THETA * np.cos(phi)
    pp_phidot = sqrt_2THETA * np.sin(phi)

    z = w[..., 2]
    zdot = w[..., 2 + 3]

    new_w = np.vstack((R.T, pp_phi.T, z.T, vR.T, pp_phidot.T, zdot.T)).T
    return new_w