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AS101 Midterm.docx

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Patrick Mc Graw

AS101 Midterm 2/12/2013 9:53:00 AM Astronomical unit (AU): the average distance from the earth to the sun Solar system: the sun and its asteroids, planets, comets and so on Planet: a non luminous body in orbit around a star; large enough to be spherical and to have cleared its orbital zone of other objects Star: a globe of gas held together by its own gravity and supported by the internal pressure of its hot gases, which generate energy by nuclear fusion Light year (ly) unit of distance equal to the distance light travels in one year Lecture 2  The nearest star system is Alpha Centauri – 44 trillion km Lecture 3  From largest to smallest: o universe o Walls o Voids o Filaments o Superclusters o Clusters o Galaxy o Solar system o Star o Planet  The local group: a group of 40 galaxies  Belongs to the Local Supercluster (aka Virgo Supercluster)  Jupiter is 10 times bigger in diameter than the earth  Size of the Milky way: about 80 000 ly  Distance to a nearby galaxy: 2 600 000 ly  The sun is a medium sized star  Constellations: groups of stars o Aren’t necessarily close together  Asterism: any recognizable group or pattern of stars, more like a landmark ie the big dipper  The celestial sphere: imagine the stars as being on the surface of an imaginary sphere that surrounds the earth Lecture 4  A star can be part of more than one asterism  We measure angular size of an object and the angular distance between two objects  The angular size looks smaller if the object is further away  Angular size= physical size X 360 degrees 2(pi)x distance  Zenith: directly overhead  Horizon: all points 90 deg. away from the zenith  Meridian: a line passing through the zenith and connecting the N and S points on the horizon (the sun crosses the meridian around noon)  Altitude: angular distance from the horizon (if negative, that means something is below the horizon)  Azimuth: direction, expressed as an angle (usually from south)  Celestial Sphere: imaginary sphere surrounding the earth containing the stars, planets, sun etc  Celestial Pole: a point on the celestial pole directly above one of the earth’s poles  Celestial Equator: a circle around the celestial sphere directly above earth’s equator  The Earth rotates from West to East, so for someone standing on Earth’s surface, the celestial sphere appears to rotate from east to west  Latitude and longitude are to earth, as declination and right ascension are to the celestial sphere o Declination: angular distance from the celestial equator, positive if North, negative if South; from -90 to +90  Lines go around horizontally o Right ascension: an angle measured eastward from the vernal equinox. Usually measured in hours from 0-24  Lines go around vertically o The full circle is 360 deg or 24 hours o If you are at a latitude of 40 deg north:  Zenith= 40 degree declination  Celestial north pole= 90-40=50 degrees away from zenith  40 degrees above the horizon, to the North  * whatever your latitude is on earth, that is also the angular distance of the north celestial pole above the horizon Lecture 5  Great circle: on the surface of any sphere, a circle whose center is the same as the center of the sphere (ie, the equator is a great circle, but other latitude lines are not)  Hour circle: any great circle passing through both celestial poles— or, a line of right ascension, equivalent to longitude lines on Earth  Circumpolar star: a star that never sets, but always stays above the horizon o Depends on your latitude o At the north pole, all visible stars are circumpolar o At the equator, none are circumpolar o Suppose your latitude is 40 deg N  Celestial pole is 40 deg above the horizon  Any star less than 40 deg away from the NCP can never be below the horizon
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