AST101 Vocab

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Terms in this set (66)
Star
A large, glowing ball of as that generates heat and light through nuclear fusion in its core. Our Sun is a star.
Planet
A moderately large object that orbits a star and shines primarily by reflecting light from its star. According to the current definition, an object can be considered a planet only if it 1) orbits a star, 2) is large enough for its own gravity to make it round, and 3) has cleared most other objects from its orbital path. An object that meets the first two criteria but has not cleared its orbital path, like Pluto, is designated as a dwarf planet.
Moon (or satellite)
An object that orbits a planet. The term satellite is also used more generally to refer to any object orbiting another object.
Comet
A relatively small and ice-rich object that orbits a star.
Small Solar System Body
An asteroid, comet, or other object that orbits a star, but is too small to qualify as a planet or dwarf planet.
Solar System
The Sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies. Although the term 'solar system' technically refers only to our own star system ('solar' means 'Of the Sun'), it is often applied to other star systems as well.
Star System
A star (sometimes more than one star) and any planets and other materials that orbit it.
Galaxy
A great island of stars in space, all held together by gravity and orbiting a common center, with a total mass equivalent to that of millions, billions, or even trillions of stars.
Cluster (or group) of Galaxies
A collection of galaxies bound together by gravity. Small collections of galaxies are generally called groups, while larger collections are called clusters.
Supercluster
A gigantic region of space in which many groups and clusters of galaxies and packed more closely together than elsewhere in the universe. Consists of many clusters of galaxies, groups of galaxies, and individual galaxies.
Universe (or Cosmos)The sum total of all matter and energy - that is, all galaxies and everything between them.Observable UniverseThe portion of the entire universe that can be seen from Earth, at least in principle. The observable universe is probably only a tiny portion of the entire universe.Astronomical Unit (AU)The average distance between Earth and the Sun, which is about 150 million kilometers. More technically, 1 AU is the length of the semimajor axis of Earth's orbit.Light-Year (LY)The distance that light can travel in 1 year, which is about 10 trillion kilometers (more precisely, 9.46 trillion kilometers).RotationThe spinning of an object around its axis. For example, Earth rotates once each day around its axis, which is an imaginary line connecting the North and South Poles.Orbit (revolution)The orbital motion of one object around another due to gravity. For example, Earth orbits the Sun once each year. An orbit may be 'bound' (elliptical) or 'unbound' (parabolic or hyperbolic).Expansion (of the universe)The increase in the average distance between galaxies as time progresses.Milky Way GalaxyUsed both as the name of our galaxy and to refer to the band of light we see in the sky when we look into the plane of our galaxy.Local GroupThe group of galaxies to which the Milky Way Galaxy belongs. The Local Group has at least 70 members, most relatively small compared to the Milky Way.Nuclear FusionThe process in which two (or more) smaller nuclei slam together and make one larger nucleus.Ecliptic PlaneThe plane of Earth's orbit around the Sun.Axis TiltThe amount by which a planet's axis is tilted with respect to a line perpendicular to the ecliptic plane.Dark EnergyName sometimes given to energy that could be causing the expansion of the universe to accelerate.ConstellationA region of the sky; 88 official constellations cover the celestial sphereCelestial SphereThe imaginary sphere on which objects in the sky appear to reside when observed from Earth.North Celestial PoleThe point on the celestial sphere directly above Earth's North Pole.South Celestial PoleThe point on the celestial sphere directly above Earth's South Pole.Celestial EquatorThe extension of Earth's equator onto the celestial plane.EclipticThe Sun's apparent annual path among the constellations.Local SkyThe sky as viewed from a particular location on Earth (or another solid objects). Objects in the local sky are pinpointed by the coordinates of 'altitude' and 'direction' (or azimuth).HorizonA boundary that divides what we can see from what we cannot see.ZenithThe point directly overhead, which has an altitude of 90°.MeridianA half-circle extending from your horizon (altitude 90°) due south, through your zenith, to your horizon due north.Direction (in local sky)One of the two coordinates (the other is altitude) needed to pinpoint an object in the local sky. It is the direction, such as north south, east or west, in which you must face to see the object.Altitude (above horizon)The angular distance between the horizon and an object in the sky.Angular Size (or Angular Distance)A measure of the angle formed by extending imaginary lines outward from our eyes to span an object (or the space between two objects).Arcminutes (or Minute of Arc)1/60 of 1°.Circumpolar (star)A star that always remains above the horizon for a particular latitude.LatitudeThe angular north-south distance between earth's equator and a location on Earth's surface.LongitutdeThe angular east-west difference between the prime meridian (which passes through Greenwich, England) and a location on the Earth's surface.Prime MeridianThe meridian of longitude that passes through Greenwich, England; defined to be longitude 0°.Sidereal DayThe time of 23 hours 56 minutes 4.09 seconds between successive appearances of any particular star on the meridian; essentially, the true rotation period of Earth.Solar Day24 hours, which is the average time between appearances of the Sun on the meridian.June SolsticeBoth the point on the celestial sphere where the ecliptic is farthest north of the celestial equator, and the moment in time when the Sun appears at that point each year (around June 21st).December SolsticeBoth the point on the celestial sphere where the ecliptic is farthest south of the celestial equator, and the moment in time when the Sun appears at the point each year (around December 21st).March EquinoxBoth the point in Pisces on the celestial sphere where the ecliptic crosses the celestial equator, and the moment in time when the Sun appears at that point each year (around March 21st).September EquinoxBoth the point in Virgo on the celestial sphere where the ecliptic crosses the celestial equator, and the moment in time when the Sun appears at that point each year (around September 21st).PrecessionThe gradual wobble of the axis of a rotating object around a vertical line.Lunar PhasesThe state determined by the portion of the visible face of the Moon (or of a planet) that is illuminated by sunlight. For the Moon, the phases cycle through new, waxing crescent, first-quarter, waxing gibbous, full, waning gibbous, third-quarter, waning crescent, and back to new.Synchronous RotationThe rotation of an object that always shows the same face to an object that it is orbiting because its rotation period and orbital period are equal.EclipsesAn event in which one astronomical object casts a shadow on another or crosses our line of sight to the other object.Lunar EclipseAn event that occurs when the Moon passes through Earth's shadow, which can occur only at full moon. A lunar eclipse may be total, partial, or penumbral.Solar Eclipsean event that occurs when the Moon's shadow falls on Earth, which can occur only at a new moon A solar eclipse may be total, partial, or annular.Nodes (of Moon's orbit)The two points in the Moon's orbit where it crosses the ecliptic plane.UmbraThe dark central region of a shadow.PenumbraThe lighter, outlying regions of a shadow.Total Lunar EclipseA lunar eclipse in which the Moon becomes fully covered by Earth's umbral shadow.Partial Lunar EclipseA lunar eclipse during which the Moon becomes only partially covered by Earth's umbral shadow.Penumbral Lunar EclipseA lunar eclipse during which the Moon passes only within Earth's penumbral shadow and does not fall within the umbra.Totality (eclipse)The portion of a lunar eclipse during which the Moon is fully within Earth's umbral shadow, or a total solar eclipse during which the Sun's disk is fully blocked by the Moon.Total Solar EclipseA solar eclipse during which the Sun becomes fully blocked by the disk of the Moon.Annular Solar EclipseA solar eclipse during which the Moon is directly in front of the Sun but its angular size is not large enough to fully block the Sun; thus, a ring (or 'annulus') of sunlight is still visible around the Moon's disk.Partial Solar EclipseA solar eclipse during which the Sun becomes only partially blocked by the disk of the Moon.Saros CycleThe period over which the basic pattern of eclipses repeats, which is about 18 years 11 1/3 days.Apparent Retrograde MotionThe apparent motion of a planet, as viewed from Eath, during the period of a few weeks or months when it moves westward relative to the stars in our sky (moves backward).Stellar ParallaxThe apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in Earth's orbit of the Sun each year.