In astronomy, what is the word applied to the alignment of three or more celestial bodies in the same gravitational system along a straight line?
Syzygy (astronomy) Syzygy (astronomy) In astronomy, a syzygy (; from the Ancient Greek ) is a (roughly) straight-line configuration of three or more celestial bodies in a gravitational system. The word is often used in reference to the Sun, Earth, and either the Moon or a planet, where the latter is in conjunction or opposition. Solar and lunar eclipses occur at times of syzygy, as do transits and occultations. The term is often applied when the Sun and Moon are in conjunction (new moon) or opposition (full moon). The word "syzygy" is often used to describe interesting configurations of astronomical objects in
Celestial coordinate system Celestial coordinate system In astronomy, a celestial coordinate system (or celestial reference system) is a system for specifying positions of celestial objects: satellites, planets, stars, galaxies, and so on. Coordinate systems can specify an object's position in three-dimensional space or plot merely its direction on a celestial sphere, if the object's distance is unknown or trivial. The coordinate systems are implemented in either spherical or rectangular coordinates. Spherical coordinates, projected on the celestial sphere, are analogous to the geographic coordinate system used on the surface of Earth. These differ in their choice of fundamental plane, which divides the celestial sphere