A measure of the distance to a visual binary star based on estimates of the masses of the stars deduced from their spectra and the observed properties of their orbits around each other.

The concept of time that is used as the variable in gravitational equations of motion. The dynamical time formerly used for the computation of ephemerides was ephemeris time ET, but this has been replaced by terrestrial dynamical time TDT and barycentric dynamical time TDB. TDT is essentially a continuation of ET, intended for use as the timescale in calculations of geocentric ephemerides. No assumption is made about the theory of gravity used. It is based on the SI second and measured by atomic clocks. It commenced in 1977, when the relationship between TDT and International Atomic Time TAI was defined to be: 1977 Jan 1, 0 hours TAI = 1977 Jan 1.000 372 5 TDT. TDB is intended for use in the equations of motion of planetary bodies referred to the barycentre of the solar system. It is not uniquely defined but depends on the theory of gravitation that is adopted. However, it is stipulated that it differs from TDT by only periodic discrepancies.

The study and theory of how and why objects move.

A Dyson sphere is a hypothetical megastructure first described in 1959 by the physicist Freeman Dyson in a short paper published in the journal Science entitled "Search for Artificial Stellar Sources of Infra-Red Radiation". It is an artificial hollow sphere of matter around a star designed to capture nearly all of the star's radiated energy for industrial use.

Although Dyson is credited with being the first to formalize and popularize the concept of the Dyson sphere, Dyson himself got the idea in 1945 from a science fiction novel titled Star Maker by Olaf Stapledon. The original proposal by Dyson did not go into much detail about how a Dyson sphere would be constructed, focusing instead on the more fundamental issue of how an advanced civilization could expand its energy production to the maximum possible for a given solar system. Such a civilization would be classified as a Type II civilization under the Kardashev classification scheme developed by the astronomer Nikolai Kardashev.

A star contained within a Dyson sphere would not be directly visible to the outside universe, but the Dyson sphere itself would radiate an equivalent amount of energy in the form of infrared light due to solar heating from within. In addition, since Dyson spheres are composed of solid matter instead of heated gas, the emission spectrum of the Dyson sphere would more closely resemble a black body spectrum than the typical emission spectrum of a star, which has absorption features introduced in the stellar atmosphere. Dyson proposed that astronomers search for such giant anomalous "stars" in order to detect advanced alien civilizations, but none have been recorded. Attempts to detect Dyson Spheres using the IRAS (Infrared Astronomical Satellite) sky survey data are currently underway.

However, if the creators of a Dyson sphere wanted to increase the temperature gradient (and thereby increase the efficiency of the energy gathering process), they could reflect the light from the inner surface of the sphere onto some areas on the surface of the sphere. The concentration of light would be restricted by the maximum allowable temperature of the materials involved in the energy conversion process. As a result the radiation toward the outside world would not have to be omnidirectional, so Dyson spheres may well be invisible.