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Astronomy Study Guide for Exam 1

by: Becca Petersen

Astronomy Study Guide for Exam 1 AST2002-16Fall 0001

Marketplace > University of Central Florida > Science > AST2002-16Fall 0001 > Astronomy Study Guide for Exam 1
Becca Petersen
University of Central Florida

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This is the study guide which includes lecture notes as well as a chapter outline from chapter 2 - since it was the most difficult chapter. It should cover everything done in class as well as the ...
Dr. James Cooney
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This 20 page Study Guide was uploaded by Becca Petersen on Thursday September 15, 2016. The Study Guide belongs to AST2002-16Fall 0001 at University of Central Florida taught by Dr. James Cooney in Fall 2016. Since its upload, it has received 174 views. For similar materials see Astronomy in Science at University of Central Florida.

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Date Created: 09/15/16
Astronomy Chapters 1 -3 (book/class notes) Picture  of  the  day:     Density  of  “stuff”    (gas  and  dust)  is  actually  very  thin     Hydrogen  and  helium  –  almost  all  of  the  universe  is  hydrogen  and  the  parts  that   aren’t  hydrogen  are  helium     Scattered  blue  light  –  is  light  reflectant  and  scattered  off  of  the  new  stars   Red  light  –  is  omitted  light  from  the  gases       10  billion  is  10^10   1  million  is  10^6     10^11+10^14=10^25  (just  know  how  that  works)       Earth  –  solar  system  –  galaxy-­‐  local  group  –  super  cluster  –  universe     Dimensions and units:   Length-­‐     SI  (shorthand  for  the  metric  system)  –  meter  –   distance  light  travels   in  1/299792458  s  –  meter  becomes  too  cumbersome  to  deal  with  when   distances  become  too  large.    So  in  order  to  not  have  to  carry  around  a  bunch  of   powers  of  tens  we  us  the  following  .  .  .       Astronomical  Unit    (AU)  –  150  million  km  –   average  distance  from   the  earth  to  the  sun  –  an  average  because  we  don’t  orbit  in  a  perfect  circle   around  the  sun  (standard  unit  of  distance  for  anything  within  our  solar   system)     Light  Year  –  9.5  trillion  km  –   the  distance  that  light  can  travel  in  a   vacuum  in  one  year  –  it  is  not  time  it  is  a  distance  –  good  for  measuring  our   neighborhood  as  far  as  stars  are  concerned     Parsec  –   How  far  away  a  star  would  have  to  be  to  appear  to  shift   by  one  arc  second  when  the  earth  moves  by  one  AU .    30  trillion  km   (definition  is  a  little  complicated  but  it  is  essentially  three  light  years)       2  concepts  that  go  into  its  definition:         Concept  1:    Parallax  –  it  is  something  you  experience  in   your  everyday  life  –  it  is  the  apparent  motion  of  something  not  because  it’s   actually  moving  but  because  the  observation  point  has  changed.    We  can  use   that  to  measure  the  distance  to  stars.    The  closer  the  object  is,  the  more  it  will   shift  (when  we  switch  eyes/observation  points)       Ex:    hold  out  your  finger  at  extended  arms  length  and  close  one  eye.     Then  switch  and  close  the  other.    The  point  of  your  finger  will  shift.     Parallax  only  works  with  nearby  stars  but  the  more  the  stars  shift  the   closer  it  is.           Concept  2: Arc  minutes  -­‐  You  are  really  measuring  an   angle  when  you’re  measuring  how  something  shifts.    In  astronomy  you   measure  angles  by  arc  minutes.       There  are  360  degrees  in  a  circle.    1  degree  =  60  arc  minutes      One  arc  minute  can  be  divided  up  into  60  pieces  to  be  called  an  arc   second.         Si  second  duration  of  9129631770  periods  of  radiation  given  off  by   hyperfine  transition  cesium  133     Mass  –  SI  kilogram  –  standard  kg  located  in  France     Solar  mass  =  mass  of  the  sun       Chapter  2:  The  Night  Sky       General  patterns  in  the  sky…  (Chapter  preview)   There  are  patterns  of  motions  in  the  sky       The  sunrises  and  sets  (so  does  everything  else…moon,  stars,   planets,  etc.)     The  circling  sky  is  a  result  of  the  rotation  of  the  earth  on  its  access   The  orbit  of  the  earth  around  the  sun  combined  with  the  earth’s  access   tilt  is  also  the  reason  for  seasons.   The  moons  orbit  around  the  earth  causes  it  to  go  through  different   phases.    The  ancient  mystery  of  the  Planets  –  the  various  orbits  around  the   sun.       Day  –  earth’s  access   Week -­‐  planets’  orbit  –  Mercury,  Venus,  Mars,  Jupiter,  moon,  sun.  The  7-­‐ day  week  was  created  named  after  those  objects^     Month -­‐  the  moon’s  orbit  around  the  earth   Year -­‐  the  earths  orbit  around  the  sun       Constellations:   A  region  of  the  sky,  within  official  borders  set  in  1928  by  the  IAU   Often  recognizable  by  a  pattern  or  grouping  of  stars     - Constellations,  while  interesting,  are  not  really  of  that  much  use  in   astronomy.    Astronomy  is  more  concerned  with  borders  of   constellations.    So  stars  that  do  not  constitute  the  stick  figure  of  Orion  are   still  considered  part  of  that  constellation  if  they  fall  within  the  set  borders.         To  us  the  sky  looks  like  a  hemispherical  screen.    We  can  only  understand  and   see  a  2  dimensional  view  of  a  3d  universe.    So  when  you  look  up  you  perceive   stars  and  planets  to  be  close  together  but  that’s  not  necessarily  the  case.       There  are  coordinates  in  the  sky  just  like  there  are  coordinates  on  the  earth     Coordinates on the earth:   Latitude  is  where  you  are  north  and  south  referencing  the  equator   Longitude  –  prime  meridian  is  in  Greenwich  England.    (We  are  west  of  the   prime  meridian)     Sphere  of  the  sky  is  called  the  celestial  sphere  –  obviously  in  real  life  the   universe  is  not  a  sphere  –  but  that’s  what  they  used  to  believe.    So  even  though   it’s  not  a  physical  reality  it’s  useful  to  describe  what  we  see.    And  we  can  apply   the  same  latitude  longitude  concepts  to  describe  the  night  sky  (picture  earth   sitting  smack  in  the  middle  of  another  sphere)     Reference  slide  for  photo     Take  earth’s  equator  and  move  it  out  that  becomes  the  celestial  equator.   Same  thing  for  the  celestial  poles   Ecliptic  –  a  word  we  use  for  the  path  that  the  sun  takes  around  the  celestial   sphere     Daily Motion:   The  sun  rises  and  sets  at  an  angle.    And  that  angle  depends  where  on  earth   that  you  are.     Going  off  of  that  idea  for  that  same  model:   If  you  are  at  either  of  the  poles  everything  moves  parallel  to  the  horizon.     And  if  you  are  at  the  equator  everything  goes  straight  up  and  straight  down   rises  and  sets  90  degrees  to  the  horizon.         ***Reference  the  photo  in  the  slide     Some  stars  never  set  (circumpolar)  and  others  never  rise   Many  stars  (and  sun  and  moon  and  planets)  rise  in  the  east  and  set  in  the  west   Angle  at  which  they  rise  and  set  is  determined  by  your  latitude     Zenith-­‐whatever  is  above  you  if  you  look  straight  up  (wherever  you  are,  it   does  not  matter)     The Day   Sidereal  day  –     How  long  it  takes  to  rotate  once   23  hours,  56  minutes,  and  4.1  seconds     Solar  day-­‐     Noon  to  noon   24  hours     Annual  Motion     As  earth  orbits  the  sun,  the  sun  appears  to  move  eastward  with  respect   to  the  stars       The  sun  circles  the  celestial  sphere  once  every  year.         A  year  is  a  little  bit  complicated  but  it  is  about  365.25  days     It’s  not  an  even  number  –  thus  the  need  for  a  “leap”  year  every  four   years     Seasons      –  Has  to  do  with  the  angle  that  sun  is  shining  on  the  earth     The  earth’s  axis  is  tilted  23.5  degrees  from  being  perpendicular  to  the  elliptic   plane     Therefore,  the  celestial  equator  is  tilted  23.5  to  the  ecliptic   As  seen  from  earth,  the  sun  spends  6  months  north  of  the  celestial  equator   and  6  months  south  of  the  equator     Seasons  are  caused  by  the  earth’s  axis  tilt  –   Not  the  distance  from   the  earth  to  the  sun!!!!     Ecliptic  –  the  apparent  path  of  the  sun  through  the  sky   Equinox-­‐  where  the  ecliptic  intersects  the  celestial  equator   Solstice-­‐  where  the  ecliptic  is  the  farthest  from  the  celestial  equator   Zodiac-­‐     The  sun  is  never  actually  directly  overhead  at  noon  in  central  Florida  –  the  sun   is  most  directly  overhead  at  the  summer  solstice  (June  21)     In  order  for  the  sun  to  at  some  point  be  over  your  head  (at  some  point  during   the  year)  you  need  to  live  within  the  tropics  (cancer  and  Capricorn)  23.5   degrees  north  or  south       Example  test  questions:     Describe  the  motion  of  the  sun  in  the  sky  in  the  summer  and  in  the  winter…    The  sun  will  stay  at  23.5  degrees.    If  you  live  at  the  North  Pole  you’re  just   sitting  there  most  of  the  day.    The  sun  won’t  reach  your  horizon  until  the   equinox.       December  21  –  the  sun  is  23.5  degrees  below  your  horizon  so  it’s  dark  for  6   months       Be  able  to  describe  locations  of  the  sun  depending  on  your  location  on  earth     How  long  is  a  day  in  the  summer?       Winter?   When  will  the  sun  be  directly  overhead?     Special  lines  of  latitude  on  earth  –  tropic  of  cancer,  Capricorn  where  at  some   times  of  the  year  the  sun  will  be  directly  over  your  head.     Seasons  –  although  the  solstice,  which  occurs  around  June  21,  is  considered   the  first  day  of  summer         It  takes  time  or  more  direct  sunlight  to  heat  up  the  land  and  water     Therefore,  July  and  August  are  typically  hotter  than  June     It  takes  a  while  for  things  to  heat  up  or  things  to  cool  down     Why distance doesn’t matter… Small  variation  for  earth  –  about  3%  but  distance  does  matter  for  some  other   planets,  notably,  mars  and  Pluto     Surprisingly,  seasons  are  more  extreme  in  N  hemisphere  even  though  earth  is   closer  to  the  Sun  in  the  Southern  hemisphere  summer  and  farther  in  the   southern  hemisphere  winter.    Because  of  the  land/ocean  distribution.     Precession  of  the  equinoxes   The  earth’s  axis  processes  wobbles  like  a  top  once  every  26,000  years.     Precession  changes  the  positions  in  the  sky  of  the  celestial  poles  and  the   equinoxes     Polaris  wont  always  be  the  North  Star     The  spring  equinox  seen  by  ancient  Greeks  in  Aries  moves  westward  and  is   now  in  Pisces.         The moon   Lunar  moon:   Phases  of  the  moon’s  29.5  day  cycle  –  it  takes  28  days  for  the  moon  to  actually   orbit  the  earth     Waxing:   New  –  can’t  see  any  of  it   Crescent  –  can  see  less  than  half  of  the  face   First  quarter  –  once  you  can  see  half  (called  a  quarter  because  it’s  a   quarter  of  the  way  through  the  cycle)   Gibbous  –  more  than  half  of  the  face       Full  –  entire     Waning:   Gibbous   Last  quarter   Crescent       The  cycle  of  the  moon’s  phases  has  to  do  with  the  relative  position  of  he  sun,   earth,  and  the  moon     Why  do  we  see  the  phases?     Half  of  the  moon  is  illuminated  by  sun  and  half  dark   We  see  combination  of  the  bright  and  dark  faces       Perfectly  aligned  sun  and  moon  –  creates  a  shadow  on  the  earth  which  we  call   a  solar  eclipse     They  are  usually  not  perfectly  aligned       Lunar  month  –  new  moon  to  new  moon   Sidereal  month  –  the  time  it  takes  for  the  moon  to  go  around  the  sun     The  moon  rises  at  6am  –  (sunrise)   New  moons  always  rise  around  sunrise  and  set  around  sunset     Most  directly  over  your  head  at  midnight     Synchronous  rotation   –  how  long  it  takes  to  spin  once  –  rotation  period  =   orbital  period       Moon  is  more  than  250k  miles  away     Ancient  Greeks  didn’t  have  telescopes  and  couldn’t  see  parallax  (especially   with  the  stars)  so  in  their  eyes,  the  earth  was  at  rest.   Claudius Ptolemy (AD 100-170)   He  made  the  first  scientific  intellectual  model  of  the  universe.  (A  model   that  makes  predictions  on  what  will  happen  and  where  things  will  be  a  week   from  now)     His  model  puts  earth  in  the  center  of  the  solar  system  /  universe  (we  call  this   a  geocentric  (earth  centered)  model     How  do  you  explain  how  sometimes  the  planets  switch  and  travel  the  wrong   way?  (Retrograde  movement)     He  said  the  planets  move  in  little  epicycles  (their  own  circles)  around  the   earth   One  thing  spinning  around  in  a  circle  and  simultaneously   traveling  in  a  bigger  circle     The  reason  for  this  was  that  he  thought  that  in  the  midst  of   traveling  around  in  those  little  epicycles,  that  it  would   momentarily  travel  the  opposite  direction  –  reference-­‐mastering   astronomy  online  models  to  better  understand  this  movement     In  his  model  the  celestial  sphere  was  outside  our  solar  system       As  time  went  on  and  they  found  flaws  in  his  theory,  he  and  his  followers  just   adjusted  their  hypothesis  adding  more  and  more  random,  arbitrary  epicycles   and  circles  that  didn’t  really  make  sense     Copernicus  –  came  up  with  a  sun  centered,  heliocentric  model/hypothesis  –   much  simpler     Occam’s  Razor     The  idea  that  if  you  have  two  different  ideas  that  correctly  explain  the  same   thing,  the  simpler  one  is  probably  more  accurate.     With  that  idea  in  mind,  we  adopted  the  Copernicus  model  to  replace   Ptolamine’s     Copernicus –    Polish  scientist  and  founder  of  our  modern  view  of  the  solar  system     His  uncle  was  a  bishop  and  got  him  a  position  as  a  canon  of  frauenburg   cathedral  which  he  held  for  the  rest  of  his  life     In  that  day  and  age  when  he  was  working  on  this  model  is  was  dangerous  to   suggest  that  the  sun  was  at  the  center  –  the  religious  community  found  his   theory  to  be  troublesome  to  say  the  least     He  wrote  a  book  of  his  hypotheses,  which  he  did  not  publish  until  the  year  he   died     The  Catholic  Church  actually  banned  this  book     There  is  still  something  wrong  with  the  Copernicus  system     Tycho Brahe   Super  rich  Danish  guy     He  wanted  to  build  the  greatest  astronomical  abservatory  in  the  world   –  he   built  these  great  instruments  –  not  telescopes  because  those  didn’t  exist  –  but   they  had  instruments  to  measure  things     He  was  the  greatest  observational  astronomer  of  his  day   His  assistant  was  Kepler     He  collected  all  this  data  about  things  like  how  venus  moves  day  to  day     Johannes Kepler     Kepler  found  flaws  with  the  Copernicus  theory  -­‐  he  realized  Tycho   Brahe  had  all  the  best  data  –  he  needed  Brahe’s  data  but  he  wouldn’t  give  it  to   him,  even  though  he  was  his  ‘assistant’  …  Tycho  Brahe  mysteriously  died  from   mercury  poisoning…     After  his  death  he  got  ahold  of  all  of  his  data  to  test  his  own  ideas  of  what  was   happening  but  after  years  of  sort  of  playing  around  with  Brahe’s  data,  he   finally  found  a  hypothesis  to  match  the  data     He  was  also  an  astrologer  as  well  as  an  astronomer       He  came  up  with  a   set of three laws   to  be  tacked  on  to  the  Copernicus   theory  that  made  it  work  perfectly     1. He  recognized  that  one  of  the  principle  problems  with  the  Copernicus   theory,  as  so  many  thought  before  him,  was  that  everything  in  heaven   moves  in  perfect  circles   Each  planet’s  orbit  around  the  sun  is  in  an  ellipse  with  the  sun  at   one  focus.    An  ellipse  is  sort  of  just  an  elongated  circle.             The  one  “focus”  is  the  sun  and  the  other  focus  is  just  empty–   reference  pic  on  slides           Semi  major  axis  –  is  essentially  just  the  radius  of  an  ellipse       Perihelion  –  the  point  on  the  ellipse  when  the  object  is   closest  to  the  sun     Eccentricity  of  an  ellipse  –  spreading  the  focus  points  out   will  increase  the  eccentricity       2. As  a  planet  is  orbiting  the  sun  it  doesn’t  always  move  at  the  same  speed.     It  moves  faster  when  its  closer  to  the  sun  and  slower  whenever  it  is   farther  away.  (qualitative  explanation)     Text  book  definition  -­‐  “A  planet  moves  along  its  orbit  with  a  speed   that  changes  in  such  a  way  that  a  lone  from  the  planet  to  the  sun   sweeps  out  equal  areas  in  equal  intervals  of  time.”     That  is  to  say  that  in  that  picture,  all  of  the  intervals  will  have   exactly  the  same  area       3. a^3  =  P^2     When  comparing  two  planets  –  lets  say  Mars  to  Jupiter…   The  planets  that  area  farther  out  (like  Jupiter)  orbit  more  slowly   than  the  planets  (like  Mars)  that  are  closer  to  the  sun  in  our  solar   system         The  ratio  of  the  cube  of  a  planet’s  average  distance  from  the  sun  to   the  square  of  its  orbital  period  is  the  same  for  each  planet     P  is  the  period  of  the  orbit  –  so  for  example,  earth’s  orbital  period   is  one  year     Jupiter  has  a  semi-­‐major  axis  of  about  5  AU  (remember  1  au  is  the   average  distance  from  the  earth  to  the  sun)     Galileo –   Galileo  popularized  the  Copernicus  model  and  Kepler’s  laws  –  up  until  then  they   were  not  very  accessible  –  so  he  went  around  giving  tours,  and  lectures  and   writing  books.    He  eventually  got  into  trouble  for  that   with  the  church  and  was   placed  under  house  arrest  and  forced  to  recant     He  is  most  famous  for  being  the  first  astronomer  to  look  through  a  telescope  –  it   was  made  for  military  purposes  so  no  one  else  was  really  using  it  for   astronomy  so  he  made  all  kinds  of  discoveries  with  that.     Almagest     Star  catalogue   Instruments     Some basic physics –   Kepler’s  laws  seem  kind  of  arbitrary  –  there  are  basic  laws  of  nature  and   physics  that  underlie  Kepler’s  laws     Motion   Position     Requires  a  coordinate  system  (ex:  it  lies  3  meters  above  …..)     Displacement  –  change  in  the  position  of  things       Velocity     Rate  of  change  of  position  –  describes  how  quickly  that  displacement   takes  place     A  vector     Not  only  the  speed  but  the  direction**  (ex:  60  miles  per  hour,  north)     Acceleration   Rate  of  change  of  velocity  (ex  –  if  you  go  from  one  meter  per  second  to   to  meters  per  second  then  your  acceleration  was  2  meters  per  second-­‐ per  second)     Also  a  vector  –  has  a  direction     Understand  that….     You  can  accelerate  without  changing  your  speed  –  going  around  in  a  circle   where  your  speed  is  the  same  but  your  direction  changes     Moving  in  a  straight  line  in  uniform  speed   –  you  have  a  velocity  but  no   acceleration     You  can  have  acceleration  not  equal  to  zero  but  velocity  equal  to  zero  –  if  you   toss  something  up  in  the  air  –  its  direction  of  its  velocity  is  going  up  but  the   acceleration  is  going  down  –  it  slows  down  as  it  travels  higher  in  the  air  –  and   that  statement  is  the  case  at  its  very  peak       Issac Newton   Between  1665-­‐1667  (he  was  22  in  65)   Known  for…   *He  sort  of  co-­‐invented  calculus   *Newton’s  laws   *Basis  of  all  physics  until  Einstein     *Law  of  universal  gravitation     He  wanted  to  understand  the  nature  of  the  universe  and  why  Kepler's  laws   worked  the  way  they  did.    So  he  basically  found  the  fundamental  laws  of  nature     Came  up  with  the  universal  law  of  gravitation       Newton’s Laws of Motion 1. A  body  remains  at  rest  or  moves  with  constant  velocity  unless  acted   upon  by  an  outside  force  –     I.e.  whatever  state  of  motion  they  are  in,  they  are  going  to   keep  that  exact  velocity  until  you  do  something  to  it.    There   are  things  in  our  every  day  lives  (friction,  gravity,  air   resistance)  that  will  act  on  objects  to  change  their  velocity   or  stop  them.         Being  at  rest  is  essentially  the  same  thing  as  being  in  perfect   constant  motion  (think  about  being  on  an  airplane-­‐  do  you   know  you’re  moving?  –  the  only  way  you  know  you’re   moving  is  by  gaging  it  on  other  things.    So  motion  is  very   relative.       2.    The  change  in  a  body’s  velocity  due  to  an  applied  force  is  in  the  same   direction  as  the  force  and  proportional  to  it.    But  is  inversely  proportional  to   the  body’s  mass.    F=ma       Force  =  mass  times  acceleration     Mass  is  a  measure  of  how  much  something  does  not  want  its   motion  changed.    High  mass  thing  will  have  a  very  low   acceleration  and  vise  versa.    So  pushing  a  brick  will  move   more  slowly  than  pushing  a  stack  of  sticky  notes.         Think  of  the  example  from  class  –  if  you  out  a  lead  brick  over  your  hand  (lets   say  its  30  pounds)  it  has  a  higher  mass  and  doesn’t  want  to  move  so  you  can   hit  it  really  hard  with  a  hammer  and  feel  no  pain.    And  vice  versa  with  a  block   of  wood.    It  has  less  mass  and  is  much  easier  to  move  because  of  the  less   resistance  so  you  will  experience  much  more  pain     3.For  every  applied  force,  a  force  of  equal  size  but  opposite  direction  arises.     Object  A  acts  on  object  B  and  object  B  acts  on  object  A   So  the  earth  is  pulling  you  down  by  160lbs  and  you  are  pulling  the   earth  up  by  160lbs.             Astronomy   Chapter  2  Book  Notes     Key:   Blue  –  vocab  term   Yellow  –  definition   Italics  –  important  info   Bold  –  big  umbrella  vocab  terms     Constellations-­‐     A  region  of  the  sky  with  well-­‐defined  borders     Patterns  of  stars  (ex  the  little  dipper)  just  help  us  locate  these  regions  but  are   not  actually  the  constellations  in  the  eyes  of  the  International  Astronomical   Union  who  chooses  official  constellations.     -­‐88  total   -­‐every  point  in  the  sky  belongs  to  some  constellation     Celestial  Sphere-­‐   An  illusion  that  the  Greeks  mistook  for  a  reality.    Now  we  just  use  it  as  a  visual  to   better  understand  a  map  of  the  sky  as  seen  from  earth.         -­‐Earth  sits  in  the  center     -­‐Earth’s  equator  matches  up  with  the  celestial  equator        “projection  of  earth’s  equator  into  space”    -­‐  makes  a  complete  circle     North  Celestial  Pole-­‐     The  point  directly  over  earth’s  north  pole   South  Celestial  Pole-­‐     The  point  directly  over  earth’s  south  pole     Ecliptic-­‐   The  path  the  sun  follows  as  it  appears  to  circle  around  the  celestial  sphere  once  a   year   It  crosses  the  celestial  equator  at  231/2  degrees  because  that  is  the  tilt  of  the   earth’s  axis.         Milky  Way-­‐   The  band  of  light  that  circles  all  the  way  around  the  celestial  sphere  *Note  that  this  is   different  than  the  Milky  Way  Galaxy       Relationship  between  the  Milky  Way  and  the  Milky  Way  Galaxy  –  It  traces  our   galaxy’s  disk  of  stars  –  the  galactic  plane  –  as  it  appears  from  our  location   within  the  galaxy.         -­‐Our  galaxy  is  shaped  like  a  thin  pancake  with  a  bulge  in  the  middle   -­‐Stars  and  interstellar  clouds  make  up  the  milky  way  we  see  in  the  night  sky  –  Which  is  the   band  of  light  that  makes  a  full  circle  around  our  sky       -­‐The  dark  parts  are  the  parts  with  the  densest  clouds   They  prevented  us  from  seeing  more  than  a  few  thousand  lightyears  into  our   galaxy’s  disk  until  recent  advancements  in  technology.     The  celestial  sphere  is  a  useful  diagram  but  its  not  a  good  representation  of  what  we  actually   see  whenever  we  go  outside.         Local  Sky     -­‐The  sky  that  you  see  from  wherever  you  happen  to  be  standing       Appears  to  take  the  shape  of  a  hemisphere  or  dome     We  only  see  half  of  the  celestial  sphere  because  the  ground  blocks  the  other  half.      (Imagine   you  are  on  flat  ground  with  nothing  intervening  in  your  vision  of  the  horizon  that  would   surround  you.)     Horizon-­‐     The  boundary  between  earth  and  sky     Zenith-­‐     The  point  directly  overhead  from  wherever  you’re  standing  –  90  degrees     Meridian-­‐   An  imaginary  half-­‐circle  stretching  from  the  horizon  due  south  through  the  zenith,   to  the  horizon  due  north     Direction-­‐   We  can  pinpoint  the  position  of  any  object  in  the  local  sky  by  stating  its  direction   along  the  horizon  (sometimes  referred  to  as  the  azimuth,  which  is  degrees  clockwise   from  due  north)             Altitude-­‐     This  one  is  pretty  straightforward  –  but  how  high  up  something  is     It  is  pretty  hard  to  know  off  the  bat  how  far  away  things  are  from  each  other.    Depth   perception  is  a  difficult  thing  to  gage  when  you’re  speaking  in  terms  of  light  years.    But  we   can  use  angles  to  describe  relative  locations.     Angular  Size-­‐   The  angular  size  of  an  object  is  the  angle  it  appears  to  span  in  your  field  of  view       The  farther  away  an  object  is,  the  smaller  its  angular  size  will  be   *View  diagram  on  page  28  to  better  understand       Angular  distance-­‐   Is  measured  between  a  pair  of  objects  in  the  sky.    It  is  the  angle  that  appears  to   separate  them.     Arc  minutes-­‐       Exist  as  a  subdivision  between  degrees  for  a  more  precise  measurement.       1  degree  is  broken  down  into  60  arc  minutes  (‘).    Each  arc  minute  is  broken  down  into  60   arc  seconds  (“)     Stars  “rising  and  setting”  –     Although  this  is  not  physically  the  case,  it  appears  that  the  celestial  sphere  rotates   around  earth.    –  every  object  in  the  celestial  sphere  appears  to  circle  around  earth  in   a  simple  daily  motion.         The  motion  looks  a  little  more  complex  from  the  local  sky  because  the  motion  is  cut   in  half  since  we  only  ever  see  half  of  the  celestial  sphere  at  any  given  time.       -­‐stars  near  the  north  celestial  pole  are  circumpolar,  meaning  that  the  remain   perpetually  above  the  horizon,  circling  (counterclockwise)  around  the  north   celestial  pole  each  day   -­‐ Stars  near  the  south  celestial  pole  never  rise  above  the  horizon  at  all.       -­‐ All  the  other  stars  have  daily  circles  that  are  partly  above  the  horizon  and  partly   below,  which  means  they  appear  to  rise  in  the  east  and  set  in  the  west.         Earth’s  west  to  east  rotation  makes  stars  appear  to  move  from  east  to  west  though  the  sky   as  they  circle  around  the  celestial  poles.    If  they  are  large  enough,  the  circles  cross  the   horizon,  so  that  the  stars  rise  in  the  east  and  set  in  the  west.     You  will  see  different  constellations  at  different  times  of  the  year.     Variation  with  latitude-­‐     Latitude-­‐     Measures  north-­‐south  position  on  earth     Defined  to  be  0  degrees  at  the  equator     Longitude-­‐     Measures  east-­‐west  positions   Defined  to  be  0  degrees  along  the  prime  meridian     Latitude  effects  the  constellations  that  we  see  because  it  affects  the  locations  of  the  horizon   and  zenith  relative  to  the  celestial  sphere     Although  the  sky  varies  with  latitude,  it  does  not  vary  with  longitude.     At  the  north  pole,  you  can  only  see  objects  that  lie  on  the  northern  half  of  the  celestial   sphere,  and  they  are  all  circumpolar.    The  sun  remains  above  the  horizon  for  6  months  at  the   north  pole  for  this  reason.    It  lies  north  of  the  celestial  equator  for  half  of  each  year.         The  Reason  for  Seasons:     The  earth’s  rotation  makes  the  sky  appear  to  circle  us.    –  the  combination  of  the  earth’s   rotation  and  orbit  leads  to  the  progression  of  seasons       The  tilt  of  the  earth’s  axis  causes  sunlight  to  fall  differently  on  earth  at  different   times  of  the  year     The  tilt  of  the  earth’s  axis  remains  pointed  in  the  same  direction  in  space  (towards  Polaris)   throughout  the  year.    The  orientation  relative  to  the  sun  changes  over  the  course  of  each   orbit:    The  Northern  hemisphere  is  tipped  toward  the  Sun  in  June  and  away  from  the  sun  in   December  while  the  reverse  is  true  for  the  Southern  Hemisphere  –  that  is  why  the  two   hemispheres  experience  opposite  seasons.     A  sidereal  day  –  is  how  long  it  takes  any  star  to  make  one  full  circuit  through  our  sky.       Solar  Day  –  our  24  hour  day  –  the  average  time  it  takes  for  the  sun  to  make  on  circuit   through  the  sky       The  earth  is  only  about  3%  farther  from  the  sun  at  its  farthest  point  than  at  its  nearest.    The   difference  in  the  strength  of  sunlight  due  to  this  small  change  in  distance  is  overwhelmed   by  the  effects  caused  by  the  axis  tilt.    –  Which  is  just  reiterating  that  the  distance  from  the   sun  DOES  NOT  cause  seasons     To  help  mark  the  changing  seasons,  we  define  four  positions  in  earth’s  orbit  throughout  the   year     1. June  Solstice  –  summer  solstice   2. December  Solstice  –  or  winter  solstice     3. March  equinox  –  or  spring  equinox   4. September  Equinox  –  fall  equinox     Precession  –  a  gradual  wobble  that  alters  the  orientation  of  earth’s  axis  in  space     Lunar  Phases  –  a  time  period  of  about  29  ½  days  where  the  moon  orbits  earth  and  returns   to  the  same  position  relative  to  the  sun  in  our  sky     To  understand  phases  understand  that…     Sunlight  essentially  comes  at  both  the  moon  and  the  earth  from  the  same  direction   2  basic  facts:   1.    Half  of  the  moon  always  faces  the  sun  and  therefore,  is  bright,  while  the  other  half  faces   away  from  the  sun  and  is  dark   2.    As  the  moon  orbit  its  just  different  combinations  of  its  bright  and  dark  faces.     The  moons  phase  is  directly  related  to  the  time  it  raises,  reaches  its  highest  point  in  the  sky,   and  sets.    For  example,  the  full  moon  must  rise  around  sunset  because  it  occurs  when  the   moon  is  opposite  the  sun  in  the  sky.         See  figure  2.21******  (discuses  waxing  and  waning)     Although  we  see  many  phases  of  the  moon,  we  do  not  see  many  faces.    From  earth  we   always  see  nearly  the  same  face  of  the  moon.    This  is  because  the  moon  rotates  on  its  axis  in   the  same  amount  of  time  it  takes  to  orbit  earth.    This  is  called  Synchronous  rotation.       Eclipse  –  when  the  sun,  moon,  and  earth  fall  in  a  straight  line   Lunar  eclipse  –  when  earth  lies  directly  between  the  sun  and  the  moon  so  earth’s  shadow   falls  on  the  moon   Solar  eclipse  –  occurs  when  the  moon  lays  directly  between  the  sun  and  the  earth,  so  the   moon’s  shadow  falls  on  earth.         Nodes-­‐  two  points  in  each  orbit  at  which  the  moon  crosses  the  surface  of  the  ecliptic  plane     Eclipses  can  only  occur  when  the  phase  of  the  moon  is  full  for  a  lunar  eclipse  or  new  for  a   solar  eclipse  and  the  new  or  full  moon  occurs  at  a  time  when  the  moon  is  very  close  to  a   node        


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