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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
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
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)
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
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celestial pole: the imaginary projection of Earth’s rotational axis onto the celestial sphere
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
crescent: the phase of a planet or moon during which less than half the surface is illuminated
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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
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
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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.
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field of view: the area of sky visible in a telescope or binoculars
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
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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
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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
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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
interplanetary: the space between the planets
interstellar: the space between the stars
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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
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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
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magnitude: the measurement of an object's brightness; the lower the number, the brighter the object
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
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
moon: a smaller body orbiting a larger body; often refers to Earth’s moon
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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
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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
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: the length of time it takes one body to orbit another
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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
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
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
proper motion: the apparent yearly motion of a star across the sky
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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
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
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satellite: a small body that orbits a planet or asteroid
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 wind: the stream of charged subatomic particles emanating from the sun
spectral type: the designation of a star based on its spectrum, which is determined by its surface temperature
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 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
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
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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
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
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universal 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
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visible light: the portion of the electromagnetic spectrum visible to the human eye
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waning: the period between full moon and new moon
waxing: the period between new moon and full moon
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Autumnal Equinox: the time of year around September 23 when the sun crosses the celestial equator heading south
Apogee: the point in a satellite’s orbit when it is farthest from Earth
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.
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
Extragalactic: beyond the Milky Way Galaxy
Fireball: an extremely bright meteor; generally brighter than magnitude –4
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
Jovian Planet: a planet with characteristics similar to Jupiter
Luminosity: the total amount of light that an object radiates
Lunar Eclipse: a phenomenon caused by the Earth passing between the sun and moon
Magnetosphere: the dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind
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
Morning Star: the planet Venus when it appears in the morning sky
Open Cluster: a system containing a few dozen to a few thousand stars that formed from the same stellar nursery
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
Prime Meridian: the line of longitude that runs through Greenwich, England
Right Ascension: the angular distance of a celestial object east of the vernal equinox; the celestial sphere equivalent of longitude
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
Umbra: the dark, central region of a shadow from which none of the light source can be seen
Variable Star: a star that varies in luminosity
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