Glossary
Astronomy glossary
514 definitions · page 3 of 9.
- cosmos
- a synonym for universe
- crescent
- the phase of a planet or moon during which less than half the surface is illuminated
- critical density
- the density of the universe that provides just enough gravity to bring the expansion to a halt after an infinite time
- crust
- the thin, outermost geological layer of a planet, moon, or asteroid
- cryovolcanism
- the eruption of water and other liquid or vapor-phase volatiles, together with gas-driven solid fragments, onto the surface of a planet or moon due to internal heating
- Damocloid
- a rare type of asteroid with an elliptical, comet-like orbit; named for the first one discovered, asteroid 5335 Damocles
- dark adaptation
- the process by which the human eye becomes well adjusted to seeing dim objects in the dark.
- dark energy
- a type of "negative gravity" that seems to play a role in the acceeleration of universal expansion
- dark matter
- matter that exerts gravitational force but does not emit any detectable light or radiation; dark matter comprises most of the mass of the universe but its exact nature remains unknown
- dark nebula
- a cloud of dust grains that is thick enough to obscure the light from background stars
- declination
- the angular distance of a celestial object above or below the celestial equator; the celestial sphere equivalent of latitude
- deep-sky objects
- objects located beyond the solar system; consist of stars, nebulae, star clusters, and galaxies.
- degree
- 1. a unit of angular size equal to 1/360 the circumference of the celestial sphere; the sun and full moon both appear about half a degree wide
- Denison Olmsted
- Denison Olmsted (1791-1859) is credited with giving birth to meteor science after the 1833 Leonid Meteor Storm over North America spurred him to study this phenomenon. He subsequently demonstrated that meteors are not an atmospheric phenomenon, but cosmic in…
- density
- the amount of mass per unit volume of an object or region of space
- deuterium
- an isotope of hydrogen; its nucleus, consisting of one proton and one neutron, has double the mass of the nucleus of ordinary hydrogen
- diffraction
- the spreading out of light as it passes the edge of an obstacle
- Dobsonian telescope
- a telescope with a simple but stable altazimuth mount that rotates easily
- Doppler effect
- the change in wavelength of radiation coming from a source that’s moving toward or away from an observer; produces either a blueshift or redshift
- dust
- tiny particles floating in space
- dwarf galaxy
- a small galaxy containing a few million stars; the most common type of galaxy in the universe
- dwarf star
- a main-sequence or smaller star
- eccentric
- deviating from a circle (used to describe the shape of an orbit)
- eccentricity
- the extent to which a body’s elliptical orbit deviates from a circle
- eclipse
- an event in which one body passes in front of another, blocking it partially or completely from view; a specific type of occultation
- eclipsing binary
- a binary star with an orbital plane oriented so that one star passes in front of the other, thus completely or partially blocking the light from the other star during each orbital period
- ecliptic
- he plane of Earth’s orbit around the sun; all the planets except Mercury and Pluto have orbits in nearly the same plane
- Edwin Hubble
- Edwin Hubble was born in Marshfield, Missouri on November 20, 1889. He studied at the University of Chicago and at Oxford as a Rhodes Scholar. His interests turned to astronomy around 1914, but he delayed his entry into the field to enlist in the U.S. Army…
- ejecta
- material thrown about by an impact or volcano
- electromagnetic radiation
- the various forms of light; includes radio waves, infrared light, visible light, ultraviolet light, x rays, and gamma rays
- electromagnetic spectrum
- the spectrum encompassing the entire range of electromagnetic radiation (light)
- electron
- a subatomic particle with a negative electric charge; electrons surround the atomic nucleus and are much less massive than protons or neutrons
- electron volt
- a unit of energy equal to the energy gained by an electron that falls through a potential difference of one volt; 1.60 x 10^-19 joule
- element
- a fundamental unit of matter; consists of a fixed number of protons, although the number of neutrons and electrons can vary
- elliptical galaxy
- a gravitationally bound system of stars in a spherical or elliptical shape with no spiral structure
- elongation
- the apparent angular separation of an object from the sun
- emission
- the discharge of electromagnetic radiation from an object
- emission nebula
- a cloud of very hot gas that is being illuminated from within by the radiation of energetic, young stars
- ephemeris
- a table that gives the positions of astronomical objects at certain intervals of time
- equatorial mount
- a telescope mount in which one axis lies parallel to Earth’s rotational axis; the motion of the telescope about this axis can compensate for Earth’s rotation
- equinox
- the two times of year when the sun crosses the celestial equator, giving day and night an equal 12-hour length everywhere on Earth
- escape velocity
- the velocity an object or rocket needs to escape the gravitational clutch of a more massive object
- evening star
- the planet Venus when it appears in the evening sky.
- event horizon
- the boundary of a black hole from inside which light cannot escape
- exit pupil
- the image of the objective lens or primary mirror of a telescope formed on the eye side of the eyepiece
- exobiologists
- a person who studies the origin, development, and distribution of 'living' systems that may exist outside of Earth
- extragalactic
- beyond the Milky Way Galaxy
- extrasolar
- beyond the sun
- extraterrestrial
- beyond Earth
- eye relief
- the distance between the eyeball and the lens nearest the eye of an eyepiece at which an observer can clearly see the entire field of view
- eyepiece
- a magnifying lens used to view the image produced by a telescope’s primary lens or mirror.
- far ultraviolet
- ultraviolet radiation with the shortest wavelengths ("farthest" from visible light in the electromagnetic spectrum)
- field of view
- the area of sky visible in a telescope or binoculars
- filter
- a device that transmits light of only certain wavelengths; used by astronomers to observe specific wavelengths or to reduce the light of exceptionally bright objects
- finder scope
- a small, low-powered telescope attached to a larger telescope that helps the observer locate objects in the sky
- fireball
- an extremely bright meteor; generally brighter than magnitude –4
- fireballs
- extremely bright meteors; generally brighter than magnitude –4
- first quarter
- the phase of the moon a quarter of the way around its orbit from new moon; the eastern half is illuminated during this phase
- flare
- a sudden, violent outburst of energy from the surface of a star
- focal length
- the distance from a lens or mirror to the point where it brings light to a focus