Tau Ceti - Motion

Motion

The proper motion of a star is its amount of movement across the celestial sphere, determined by comparing its position relative to more distant background objects. Tau Ceti is considered to be a high-proper-motion star, although it only has an annual traverse of just under two arc seconds. It will require about two thousand years before the location of this star shifts by more than a degree. A high proper motion is an indicator of closeness to the Sun. Nearby stars can traverse an angle of arc across the sky more rapidly than the distant background stars and are good candidates for parallax studies. In the case of Tau Ceti, the parallax measurements indicate a distance of 11.9 ly. This makes it one of the closest star systems to the Sun, and the next-closest spectral class-G star after Alpha Centauri A.

The radial velocity of a star is its motion toward or away from the Sun. Unlike proper motion, a star's radial velocity cannot be directly observed, but must be determined through measurement of the spectrum. Due to the Doppler shift, the absorption lines in the spectrum of a star will be shifted slightly toward the red (or longer wavelengths) if the star is moving away from the observer, or toward blue (or shorter wavelengths) when it moves toward the observer. In the case of Tau Ceti, the radial velocity is about −17 km/s, with the negative value indicating that it is moving toward the Sun.

The distance to Tau Ceti, along with its proper motion and radial velocity, allow the motion of the star through space to be calculated. The space velocity relative to the Sun is about 37 km/s. This result can then be used to compute an orbital path of Tau Ceti through the Milky Way galaxy. It has a mean galacto-centric distance of 9.7 kiloparsec (32,000 ly) and an orbital eccentricity of 0.22.