astrometry-basicslisted

## Overview Astrometry measures where things are and how they move: positions, proper motions, and parallaxes. The Gaia mission revolutionized the field with microarcsecond precision for billions of stars. This skill covers the astrometric observables, reference frames (ICRS), working with Gaia catalog data correctly, and deriving distances and kinematics. ## When to use - Querying Gaia for parallaxes, proper motions, and photometry of a sample - Converting parallax to distance (correctly — it's not 1/π for noisy data) - Computing space velocities (U, V, W) and Galactic orbits - Cross-matching catalogs with proper-motion propagation to a common epoch - Assessing astrometric quality: RUWE, excess noise, and binary contamination ## Core concepts - **The five parameters:** position (α, δ), proper motion (μ_α, μ_δ), parallax (ϖ) — plus radial velocity for full 6D phase space. Gaia solves these jointly from years of scanning. - **Parallax → distance:** d = 1/ϖ only for high-S/N parallaxes; for noisy or negative parallaxes use Bayesian inference with a sensible prior (Bailer-Jones) — naive inversion is biased (Lutz–Kelker). - **ICRS:** the International Celestial Reference System, realized by distant quasars (effectively inertial) — Gaia's frame; always state the reference frame and epoch (J2016.0 for Gaia DR3). - **Proper motion:** angular motion across the sky (mas/yr); transverse velocity v_t = 4.74·μ·d (km/s, with μ in mas/yr, d in pc) — needs the distance, inheriting its