normal_triad

pygaia.astrometry.vectorastrometry.normal_triad(phi, theta)

Calculate the so-called normal triad [p, q, r] associated with a spherical coordinate system. The three vectors are:

p - The unit tangent vector in the direction of increasing longitudinal angle phi. q - The unit tangent vector in the direction of increasing latitudinal angle theta. r - The unit vector toward the point (phi, theta).

Parameters:

• phi (float or array) – Longitude-like angle (e.g., right ascension, ecliptic longitude) in radians

• theta (float or array) – Latitide-like angle (e.g., declination, ecliptic latitude) in radians

Returns:

p, q, r – The normal triad as the vectors p, q, r as (N,3) arrays

Return type:

array