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UT - PHYS 154 - Class Notes - Week 2

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UT - PHYS 154 - Class Notes - Week 2

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background image Astronomy: Chapter 1 8/22­31/16 Earth’s Motions­ General 
­Motions in the night sky as observed from the earth 
­Daily, or Diurnal, motions ­Rise in the east, set in the west ­Because of the earth’s rotation about its spin axis  ­Yearly, or annual, motion ­What stars/constellations are viewable at night changes from month to month  over the course of a year and then repeats  Because of the earth’s revolution, or orbital motion, around the sun ­The term sidereal means in reference to the background stars and so corresponds to a  rotation of revolution of 360 ­The stars are so very far away that they appear to not change their relative  positions (fixed background of stars) ­However, what stars are up when depend on the earth’s position in its orbit  around the sun.  Earth’s rotation: diurnal motions
­Celestial objects rise in the east and set in the west
­Due to earth’s rotation ­Earth’s rotation also defines a day  Earth’s rotation: defining days ­Daily or diurnal, noon to noon defines the solar day (24 hours) 
­Single 360 rotation is slightly shorter 
­Defined as the Sidereal Day (23 hours 56 mins) 
­Reason: Earth is not only rotating by revolving 
background image Earth’s Motions ­The result of this motion is a slow 12 month progression through the 
constellations/night sky objects, such that the winter sky is entirely different from the 
summer sky Understanding the Night Sky
­From any vantage point on the earth, you can see roughly 3,000 stars
­Only half of the Celestial Sphere, indicating that about 6,000 stars are visible  from earth with the naked eye ­The constellations that are visible depend on  ­Your location on earth (latitude) ­The time of year ­For the Northern hemisphere observer the depicted Southern sky stars are forver  below the horizon  The Celestial Sphere
­Stars seem to be held at a fixed distance on the inner surface of the sphere (Hung in 
the sky) 
­Projects Earth’s coordinate system (Poles, equator, latitude, and longitude)
­North and south celestial poles and the celestial equator  ­Right ascension is analogous to longitude ­For the curious: an angular measurement, given as if it was a measurement of  the hour hand around the 24 hour clock ­Declination is analogous to latitude  ­For the curious: traditional angular measurement, but with additional sub­degree unit  View from Earth
background image ­Zenith: The point directly overhead
­Meridian: Line going through south, zenith, and north
­Altitude­Azimuth  ­Altitude: angular measure of how far above horizon ­Azimuth: direction in number of degrees away from North  ­Celestial coordinates use right ascension and declination  ­Fixed coordinate system, such that every star and deep space object has a  specific RA and Dec.  ­Does not change ­Horizontal coordinates use altitude and azimuth  ­Convenient to point out a point of the local sky
­Because everything is moving in the sky, objects Alt.­Az. coordinates are 
constantly changing Earth’s Motions­ Axial Tilt 
­Definition: The Ecliptic is the path of the sun on the celestial sphere over the course of 
a year
­Because of Earth’s axial tilt of 23.5 degrees the ecliptic is also inclined 23.5 degrees 
with respect to the sun­earth orbital plane (defines the plane of the solar system) The reason for the seasons
­Includes two main effects
­Sunlight is more/less concentrated in summer/winter, respectively 
­Days are longer/shorter in summer/winter respectively 
­Solstice: means “Sun Stands”
­Ecliptic at northern most point in the sky ­ summer solstice
­Sunlight most direct in the N. Hemisphere  ­Ecliptic at Southern most point in the sky ­ winter solstice  ­Sunlight most direct in southern hemisphere 

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School: University of Tennessee - Knoxville
Department: Physics
Course: Stars/Galax/Cosmology Lecture
Professor: Sean Lindsay
Term: Fall 2016
Tags: Science, Physics, and astronomy
Name: Lectures 3-7
Description: This covers half of week 2, week 3, and week 4
Uploaded: 09/19/2016
10 Pages 16 Views 12 Unlocks
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