Revised geometric estimates of the North Galactic Pole and the Sun's height above the Galactic mid-plane

Astronomers are entering an era of μas-level astrometry utilizing the 5-decade-old IAU Galactic coordinate system which was only originally defined to ∼0.1° accuracy, and where the dynamical centre of the Galaxy (Sgr A*) is located ∼0.07° from the origin. We calculate new independent estimates of the North Galactic Pole (NGP) using recent catalogues of Galactic disc tracer objects such as embedded and open clusters, infrared bubbles, dark clouds and young massive stars. Using these catalogues, we provide two new estimates of the NGP. Solution 1 is an `unconstrained' NGP determined by the Galactic tracer sources, which does not take into account the location of Sgr A*, and which lies 90.120° ± 0.029° from Sgr A*, and Solution 2 is a `constrained' NGP which lies exactly 90° from Sgr A*. The `unconstrained' NGP has International Celestial Reference System (ICRS) position: α<sub>NGP</sub> = 192.729° ± 0.035°, δ<sub>NGP</sub> = 27.084° ± 0.023° and θ = 122.928° ± 0.016°. The `constrained' NGP which lies exactly 90° away from Sgr A* has ICRS position: α_NGP = 192.728° ± 0.010°, δ_NGP = 26.863° ± 0.019° and θ = 122.928° ± 0.016°. The difference between the solutions is likely due to the Sun lying above the Galactic mid-plane. Considering the angular separation between Sgr A* and our unconstrained NGP, and if one adopts the recent estimate of the Galactocentric distance for the Sun of R₀ = 8.2 ± 0.1 kpc, then we estimate that the Sun lies z_⊙ ≃ 17 ± 5 pc above the Galactic mid-plane. Our value of z_⊙ is consistent with the true median of 55 previous estimates published over the past century of the Sun's height above the Galactic mid-plane (z_⊙ ≃ 17 ± 2 pc).