Glossary of astronomy

This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields.

Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth.

The field of astronomy features an extensive vocabulary and a significant amount of jargon.

Also visual brightness (V).Also argument of perifocus or argument of pericenter.Also the north node.Also exobiology.Also planetary geology.Also celestial body.Also spelled astronomical catalog.Also celestial object.Also obliquity.Also critical velocity or critical rotation.Also spelled circumstellar disk.Also compact object.Also space dust.Also cosmic microwave background radiation (CMBR).Also break-up velocity.Also meridian transit.Also the south node.Also distant detached object and extended scattered disc object.Also ecliptic plane or plane of the ecliptic.Also elliptic orbit.Also exoplanet.Also the Cusp of Aries.Also background stars.Also galactic core or galactic center.Also galactic year or cosmic year.Also group of galaxies (GrG).Also geosynchronous equatorial orbit (GEO).Also the Hill radius.Also Laplace's invariable plane or the Laplace plane.Also Keplerian orbit.Also Edgeworth–Kuiper belt.Also Lagrange point, libration point, or L-point.Also the Lenakaeia Supercluster, Local Supercluster, or Local SCI.Also Moon phase.Also the Northward equinox.Also shooting star or falling star.Also normalized polar moment of inertia.Also minor moon or minor natural satellite.Also MK classification.Also rise width.Also stellar association.Also bare eye or unaided eye.Also moon.Also arc length.Also the Öpik–Oort cloud.Also orbital plot.Also revolution period.Also simply called space.Also pericenter.Also reference plane.Also planetary object.Also sometimes called planetology.Also planemo or planetary body.Also gravitational primary, primary body, or central body.Also direct motion.Also quasi-stellar radio sourceAlso interstellar planet, nomad planet, orphan planet, and starless planet.Also twinkling.Also major semi-axis.Also southward equinox.Also positional astronomy.Also standard acceleration due to gravity.Also spelled star catalog.Also stellar system.Also stellar envelope.Also spectral classification.Also simply stellar model.Also substar.Also synodic rotation period.Also tidal acceleration.Also Tisserand parameter.Also the Johnson system or Johnson–Morgan system.Also the Local Supercluster (LSC or LC).An acronym of X-ray bright optically normal galaxy.

Syrtis Major (center) is a prominent dark albedo feature on Mars .
Two bodies of similar mass orbit a common barycenter external to both, as usual in binary star systems
A diagram of the relationships between the Earth's axis of rotation , its celestial equator , and the plane of its orbit around the Sun, known as the ecliptic . Note that the Earth's rotational axis is not perpendicular to the ecliptic but rather is tilted ; this means that the path of the Sun, as viewed from Earth, appears to move both above and below the celestial equator during the course of the year.
Circumstellar debris disks as detected by the Hubble Space Telescope , and artist's impressions of the disks' orientations around their host stars
The plane of the ecliptic (grey) is defined by the Earth's orbit around the Sun, and is distinct from the plane of the celestial sphere 's equator (green), which is permanently tilted 23.4 degrees with respect to the ecliptic. To an observer on Earth, this means that the path the Sun appears to follow upon the celestial sphere is not a straight line (i.e. the ecliptic does not appear "flat"). Twice during each complete orbit, at the two equinoxes , the Sun's path appears to intersect the celestial equator, though the two planes are never in fact coplanar.
A relatively small body (such as a planet) orbiting a larger one (such as a star) in an elliptical orbit , with the larger body located at one of the focal points of the ellipse
Sample evolutionary tracks for stars of different mass
A luminous red galaxy (LRG) acting as a gravitational lens , distorting the light from a much more distant blue galaxy into an Einstein ring
The phases of the Moon are caused by the visible side of the Moon being alternately illuminated by sunlight and immersed in shadow during its orbit around the Earth.
The bright center of the Milky Way Galaxy is visible in dark skies on clear nights, in the direction of the constellation Sagittarius .
A diagram showing four of the six canonical orbital elements . The orbital plane (yellow) intersects a reference plane (grey).
A planetary system showing a 1:2
orbital resonance between the orbital periods of two planets (small bodies), both of which are orbiting a large central star. The inner planet completes two revolutions in the time it takes the outer planet to complete one.
The parallax shift of a star at a distance of one parsec as seen from the Earth (not to scale)
A moon or planet is said to be in quadrature when its position as viewed from Earth is at a right angle to the direction of the Sun. In such cases the moon or planet appears to be in its quarter phase (or nearly so), where half of the body is illuminated by the Sun and the other half is in shadow.
In a retrograde orbit, a satellite (red) orbits in the direction opposite the rotation of its primary (blue/black).
The Earth's atmosphere permits certain wavelengths of electromagnetic energy to pass through but reflects or absorbs others, making it difficult or impossible to detect them from the surface. As a result, spectroscopic instruments are often placed in orbit, above the atmosphere, where detection of all parts of the spectrum is uninhibited.
A total solar eclipse as seen from Earth