Reference Frames & Epochs

The older standard epoch, defined by the beginning of the Besselian year 1950, to which many historical star catalogues and coordinates are referred. Converting between B1950 and J2000 positions is a routine but necessary step when using older data.

The reference system centred on the barycentre — the balance point — of the Solar System, used to describe the motion of the planets and the propagation of light across the Solar System. It is the frame in which planetary ephemerides are computed.

The plane of the Earth's orbit around the Sun, and the equinox where it crosses the celestial equator. For millennia the equinox was the origin of celestial coordinates — but because it slowly precesses, modern frames are fixed to the stars instead.

The reference system centred on the Earth, aligned with the barycentric frame but following the Earth in its orbit. It is the natural frame for the motion of satellites and for observations made from the ground.

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.

The standard reference epoch of modern astronomy — noon on 1 January 2000, in Terrestrial Time. Because the sky's apparent frame drifts with precession, coordinates and orbital elements are stated for a fixed epoch, and J2000.0 is the one in near-universal use.