Where & When in the Sky
Pointing at a star precisely takes more than a name. A path through the astrometric foundation — the coordinate systems that give a position, the reference frames and epochs that anchor it, the time scales that say when, and the corrections, from precession to refraction, for every way the measured place differs from the true one.
Related entities
- Coordinate system
The Equatorial Coordinate System
The standard astronomical coordinate system, projecting the Earth's equator and poles onto the sky and fixing positions by right ascension and declination. Because it is tied to the slowly precessing equator and equinox, an equatorial position must be qualified by a reference frame and epoch, such as ICRS or J2000.
- Reference frame
The International Celestial Reference Frame (ICRF3)
The practical realisation of the International Celestial Reference System — a catalogue of precise positions for several thousand extragalactic radio sources, mostly quasars, measured by very-long-baseline interferometry. Its third realisation, ICRF3, was adopted in 2018; because quasars are effectively fixed, it provides the quasi-inertial grid to which optical frames such as Gaia's are aligned.
- Astrometric effect
Precession
The slow conical wobble of the Earth's rotation axis, driven by the gravitational pull of the Sun and Moon on the equatorial bulge, which carries the celestial poles and the equinoxes around the sky once in about 25,772 years. Precession is why equatorial coordinates drift with time and must be referred to a stated epoch, and why Polaris is only temporarily the pole star.
- Reference frame
The International Celestial Reference System
The modern, fixed reference frame for the sky, defined by the positions of hundreds of distant quasars whose motion is undetectable. It replaced frames tied to the slowly-shifting equinox with one anchored to some of the most distant objects known — the standard to which all precise astronomical positions are now referred.