Terminology
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| A | |
| absolute magnitude | the apparent brightness an object would have if it were 10 parsecs (32.6 light-years) from Earth |
| albedo | the percentage of light that an object reflects |
| altitude | 1. the height above sea level 2. the angle between an object’s position on the celestial sphere and the horizon |
| angular size | the apparent width of an object as seen by an observer, usually expressed in degrees, arcminutes, or arcseconds |
| annular eclipse | a solar eclipse in which the moon does not fully cover the disk of the sun, leaving a thin ring of sunlight visible |
| aphelion | the point farthest from the sun in an object’s orbit |
| apogee | the point in a satellite’s orbit when it is farthest from Earth |
| apparent magnitude | the measure of the brightness of an object as seen from Earth |
| arcminute | a unit of angular size equal to 1/60 of a degree |
| arcsecond | a unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute). |
| asterism | a small grouping of stars |
| astronomical unit | the average distance from Earth to the sun, equal to about 93,000,000 miles (150,000,000 km) |
| autumnal equinox | the time of year around September 23 when the sun crosses the celestial equator heading south. |
| averted vision | a technique that uses the more light-sensitive rods in the eye to better see a faint object by looking at it indirectly |
| azimuth | the angle along the horizon measured eastward from due north to the point on the horizon directly below an object |
| B | |
| bolide | a brilliant meteor or fireball that explodes in mid-air |
| C | |
| celestial pole | the imaginary projection of Earth’s rotational axis onto the celestial sphere |
| celestial sphere | the apparent sphere of the sky; an imaginary sphere of immense radius centered on Earth often used to plot the coordinates of objects in the sky |
| circumpolar stars | stars which don't drop below the horizon from a given observing point on Earth. At Earth's Geographical North Pole (90° north latitude), all stars in the sky are cirumpolar. On Earth's equator, no stars are circumpolar. |
| conjunction | a time when two or more bodies appear close together in the sky |
| constellation | one of the 88 patterns of stars in the sky, often named for a mythological god, hero, or animal |
| convection | the transfer of heat energy by moving currents of material |
| corona | the outer atmosphere of the sun or a star |
| crescent | the phase of a planet or moon during which less than half the surface is illuminated |
| D | |
| dark adaptation | the process by which the human eye becomes well adjusted to seeing dim objects in the dark. |
| declination | the angular distance of a celestial object above or below the celestial equator; the celestial sphere equivalent of latitude |
| deep sky objects | |
| 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 2. a unit of measure for temperature along a graded scale |
| diffraction | the spreading out of light as it passes the edge of an obstacle |
| E | |
| eclipse | an event in which one body passes in front of another, blocking it partially or completely from view, a specific type of occultation. |
| elongation | the apparent angular separation of an object from the sun |
| ephemerus | a table that gives the positions of astronomical objects at certain intervals of time. |
| 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. |
| evening star | the planet Venus when it appears in the evening sky |
| extragalactic | beyond the Milky Way Galaxy |
| F | |
| field of view | the area of sky visible in a telescope or binoculars |
| fireball | an extremely bright meteor; 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. |
| full moon | the phase of the moon when it is halfway around its orbit from new moon and opposite the sun in the sky; the full disk is illuminated |
| G | |
| galaxy | an enormous gravitationally bound assemblage of millions or billions of stars |
| gibbous | the phase of the moon between first quarter and last quarter, when the moon appears more than half illuminated |
| globular cluster | a roughly spherical congregation of hundreds of thousands of stars; most globular clusters consist of old stars and exist in a galaxy’s halo |
| H | |
| helical rising | the period of time when an object, such as a star, is briefly seen in the eastern sky before dawn and is no longer hidden from the glare of the sun |
| I | |
| inclination | the angle between a planet’s orbit and the ecliptic plane; or the angle between a satellite’s orbit and its host planet’s rotational plane |
| inferior conjunction | the configuration of an inferior planet when it lies between the sun and Earth |
| inferior planet | a planet that orbits the sun inside of Earth’s orbit; includes Mercury and Venus |
| International Space Station | a global cooperative program between the United States, Russia, Canada, Japan, and Europe, for the joint development, operation, and utilization of a permanently habitated space station in low-Earth orbit |
| interplanetary | the space between the planets |
| interstellar | the space between the stars |
| J | |
| Jovian planet | a planet with characteristics similar to Jupiter |
| K | |
| Kelvin | a unit of temperature equal to one degree on the Celsius scale and 1.8 degrees on the Fahrenheit scale; also the absolute temperature scale defined so that 0 kelvin is absolute zero |
| L | |
| last quarter | the phase of the moon three-quarters of the way around its orbit from new moon; the western half is illuminated |
| latitude | the angular distance north or south from the equator to a point on Earth's surface, measured on the meridian of the point |
| libration | the small oscillations in the moon’s motion that allow Earth-based observers to see slightly more than half the moon’s surface |
| light pollution | light, typically from artificial sources, that reaches the night sky, obscuring the view of faint astronomical objects |
| light-year | the distance light travels in one year, equivalent to approximately 5.9 trillion miles (9.5 trillion km) |
| limb | the apparent edge of a celestial object |
| limiting magnitude | the apparent magnitude of the faintest objects that can be seen given the local observing conditions and any telescope, film, or other detector you may be using |
| longitude | the angular distance of a particular place on Earth as measured east or west from the prime meridian running through Greenwich, England |
| luminosity | the total amount of light that an object radiates |
| lunar eclipse | a phenomenon caused by the Earth passing between the sun and moon |
| lunar month | the period of one complete revolution of the moon around Earth, 29.5 days |
| lunation | the time between two successive new moons; approximately 29.5 days |
| M | |
| magnetosphere | the dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind |
| magnitude | the measurement of an object's brightness; the lower the number, the brighter the object |
| main sequence | the band of stars on a Hertzsprung-Russell diagram stretching from the upper left to the lower right; stars spend most of their lives in the main sequence phase, in which they are fusing hydrogen into helium in their cores |
| mare | a dark and relatively smooth area on the surface of the moon or a planet. |
| megaparsec | one million parsecs; equal to 3.26 million light-years |
| meridian | an imaginary circle on the celestial sphere that connects the zenith to the north (or south) celestial |
| Messier objects | the 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s |
| meteor | a flash of light that occurs when a meteoroid burns up in Earth’s atmosphere; also popularly known as a shooting star |
| meteor shower | a period of enhanced meteor activity that occurs when Earth collides with a swarm of meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point |
| meteorite | a rock from space that survives passage through Earth’s atmosphere and falls to the ground |
| meteroid | a small rock that orbits the sun |
| Milky Way | the band of light that encircles the entire sky and results from the combined light of billions of stars in our galaxy’s disk |
| Milky Way Galaxy | the spiral galaxy to which Earth belongs |
| moon | a smaller body orbiting a larger body; often refers to Earth’s moon |
| morning star | the planet Venus when it appears in the morning sky |
| N | |
| nebula | a cloud of interstellar gas and dust; some nebulae represent stellar nurseries, others represent stellar graveyards |
| new moon | the phase in which the moon is in the same direction as the sun in Earth’s sky, so it is unilluminated and invisible |
| O | |
| obliquity | the angle between a planet’s equator and the plane of its orbit |
| occultation | the passage of one object in front of a smaller one, temporarily obscuring all or part of the background object from view |
| open cluster | a system containing a few dozen to a few thousand stars that formed from the same stellar nursery. |
| opposition | the moment when a planet farther from the sun than Earth appears opposite the sun in the sky; it is the best time to observe a planet |
| orbit | the path an object follows around a more massive object or common center of mass; usually elliptical in shape |
| orital period | he length of time it takes one body to orbit another |
| P | |
| parallax | the apparent shift of a relatively nearby object against a fixed background due to the motion of the observer; astronomers observe the parallax of stars to measure their distances |
| parsec | the apparent shift of a relatively nearby object against a fixed background due to the motion of the observer; astronomers observe the parallax of stars to measure their distances |
| penumbra | the region of a shadow from which part of the light source remains visible |
| perigee | the point in a satellite’s orbit when it is closest to Eart |
| perihelion | the point in an object’s orbit when it’s closest to the sun |
| period | the time interval for a regular event to take place |
| phase | the regular cycle of changes in the appearance of a moon or planet |
| planet | a large rocky or gaseous body that orbits a star |
| planisphere | a two-dimensional map of the sky with an adjustable overlay to show the part of the sky visible at any time of the night or year |
| position angle | the direction in the sky of one celestial object from another, measured eastward from due north. |
| precession | the slow, periodic change in the direction an object’s rotational axis caused by the gravitational influence of another body |
| prime meridian | the line of longitude that runs through Greenwich, England |
| proper motion | the apparent yearly motion of a star across the sky |
| R | |
| retrograde | objects that move or appear to move in the opposite direction of most solar system bodies; for example planets that appear to move east-to-west in the sky or objects that revolve or rotate clockwise as seen from north of the solar system |
| revolution | the orbital motion of one body around another body or a common center of mass |
| right ascension | the angular distance of a celestial object east of the vernal equinox; the celestial sphere equivalent of longitude |
| rotation | the spin of a galaxy, star, planet, moon, or asteroid about a central axis |
| rotation period | the length of time it takes a body to complete one rotation |
| S | |
| satellite | a small body that orbits a planet or asteroid |
| seeing | the quality of observing conditions induced by turbulence in Earth’s atmosphere, which blurs the images of astronomical objects |
| sidereal | relating to or measured with respect to the stars |
| sidereal year | the amount of time it takes one body to revolve about another with respect to the stars |
| solar eclipse | an eclipse of the sun caused by the moon passing between Earth and the sun |
| solar filter | a filter used to block almost all of the sun’s light so our star can be viewed safely and comfortably. |
| solar system | the system containing the sun and all the smaller bodies in orbit around it |
| solar wind | the stream of charged subatomic particles emanating from the sun |
| solstice | either of the two points on the celestial sphere where the sun is farthest north or south of the celestial equator; when the sun is at a solstice, the amount of daylight hours is greatest for summer and least for winter |
| South Celestial Pole | the point in the sky to which Earth's Geographical South Pole points |
| spectral class | the designation of a star based on its spectrum, which is determined by its surface temperature |
| spectral type | the designation of a star based on its spectrum, which is determined by its surface temperature |
| spectroscopy | the study of spectra from astronomical objects |
| spectrum | 1. the energy emitted by a radiant source 2. the entire range of electromagnetic radiation (light) |
| speed of light | the fastest possible speed in a vacuum, equivalent to 186,000 miles per second (300,000 km per second). |
| star | a self-luminous sphere of hot gas held together by gravity; ordinary stars generate energy by nuclear fusion in their cores |
| star atlas | an collection of maps that marks the positions of stars, nebulae, galaxies, and other astronomical objects on a coordinate system |
| star hopping | the technique of using recognizable patterns of stars to “hop” from one part of the sky to another; useful in observing both with the naked eye and a telescope |
| star party | a gathering of people to observe the night sky |
| sunspot | a dark, temporary, relatively cool spot on the surface of the sun |
| sunspot cycle | a cycle averaging 11 years in which the number of sunspots increases and decreases. |
| superior conjunction | the configuration of an inferior planet when it lies on the far side of the sun |
| superior planet | a planet farther from the sun than Earth; includes Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto |
| T | |
| telescope | a tubed instrument used to brighten and magnify the view of astronomical objects (telescopes gather more light than the eye) |
| terminator | the boundary on a planet or moon separating the illuminated side from the unilluminated. |
| terrestrial | of or relating to Earth |
| terrestrial planet | a small, rocky planet such as Mercury, Venus, Earth, and Mars |
| transit | the passage of a smaller body in front of a larger body; also, the passage of a celestial body across an observer’s meridian. |
| transparency | the clarity of the sky. |
| tropical year | the time it takes Earth to revolve around the sun with respect to the vernal equinox |
| U | |
| umbra | the dark, central region of a shadow from which none of the light source can be seen |
| universsal time | the local time of day on a line of longitude centered on Greenwich, England (also known as Greenwich Mean Time); it forms the basis for all civil timekeeping |
| V | |
| variable star | a star that varies in luminosity |
| visible light | the portion of the electromagnetic spectrum visible to the human eye |
| W | |
| waning | the period between full moon and new moon |
| waxing | the period between new moon and full moon |
| Page Created 07/17/01 Page Last Updated 02/21/02 |
Copyright © 2001
C. Gino cgino@astrophys-assist.com |