Equatorial coordinate systemThe equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the Marchequinox, and a right-handed convention.
Astronomical objectAn astronomical object, celestial object, stellar object or heavenly object is a naturally occurring physical entity, association, or structure that exists within the universe. In astronomy, the terms object and body are often used interchangeably.
Celestial sphereIn astronomy and navigation, the celestial sphere is an abstractsphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemisphericalscreen over the observing location.
Celestial navigationCelestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the surface of the Earth without relying solely on estimated positional calculations, commonly known as dead reckoning.
Spherical trigonometryGeometry of figures on the surface of a sphereThe octant of a sphere is a spherical triangle with three right angles.Spherical trigonometry is the branch of spherical geometry and trigonometry that deals with the metrical relationships between the sides and angles of spherical triangles, traditionally expressed using trigonometric functions. On the sphere, geodesics are great circles.
EarthThird planet from the SunSurface equivalent doserate0.274 μSv/hAbsolute magnitude (H)−3.99AtmosphereSurface pressure101.325 kPa (at sea level)Composition by volume78.084% nitrogen (dry air)20.946% oxygen (dry air)≤1% water vapor (variable)0.9340% argon0.0430% carbon dioxide0.00182% neon0.00052% helium0.00017% methane0.00011% krypton0.00006% hydrogen Source:Earth is the third planet from the Sun and the only astronomical object known to harbor life.Astronomical coordinate systemsIn astronomy, coordinate systems are used for specifying positions of celestial objects (satellites, planets, stars, galaxies, etc.) relative to a given reference frame, based on physical reference points available to a situated observer (e.g. the true horizon and north to an observer on Earth's surface).
Observational astronomyObservational astronomy is a division of astronomy that is concerned with recording data about the observable universe, the practice and study of observingcelestial objects with the use of telescopes and other astronomical instruments. As a science, the study of astronomy does not rely on direct experiments with the objects in a distant universe. Rather astronomers have a vast number of visible examples of stellar phenomena that can be examined.
AstrometryAstrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.AstrologyAstrology is a range of divinatory practices, recognized as pseudoscientific since the 18th century, that propose that information about human affairs and terrestrial events may be discerned by studying the apparent positions of celestial objects.Ancient historyPeriod between prehistory and the medieval era This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these messages) This article may be too technical for most readers to understand. Please help improve it to make it understandable to non-experts, without removing the technical details. (July 2026) (Learn how and when to remove this message) This article may be confusing or unclear to readers.
TimeContinuous progression from past to futureNot to be confused with Thyme.Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future.*As an Amazon Associate I earn from qualifying purchases.