An angular measurement of an object's orientation along the horizon of the observer, relative to the direction of true north.
The solid, rocky body that orbits the Earth as its only natural satellite, completing a full orbit every 27.3 days
The motion of one orbiting body relative to another, as an ellipse, parabola, or hyperbola, which forms a two-dimensional orbital plane (or sometimes a straight line) in three-dimensional space
A measure of the abundance of elements other than hydrogen and helium within an astronomical object
A measure of the resistance of a medium to the radiative transmission of energy
A curve on the Hertzsprung–Russell diagram that represents the evolutionary positions of stars having the same age but differing masses
A large, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter, each of which orbits a center of mass
A property of two objects orbiting the same body whose orbital periods are in a rational proportion
The scientific study of celestial objects and phenomena, the origins of those objects and phenomena, and their evolution
A unit of length defined as the distance at which a star would show a parallax shift of exactly one arcsecond as observed from Earth's orbit
A mostly convex region formed when a plasma, such as the solar wind, interacts with the magnetic field of a body, such as a planet or star.
Any astronomical object of indistinct nebulosity. In modern usage, the term typically refers to an interstellar cloud of dust, hydrogen, helium, and other ionized gases
The point at which an orbiting body is furthest from its primary
The total amount of energy emitted per unit time by a star, galaxy, or other astronomical object
Any solid object (generally larger than 1 kilometre (0.62 mi) in diameter) that arises during the formation of a planet whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gas drag