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Astro 1050 Jan. 11, 2006
  • Extra Credit (once per semester):
    • Astronomy Articles on Fridays
    • Planetarium shows Friday nights (tickets/info from Physics and Astronomy Department office)


  • Some more from chapter 1:  Scientific Notation, Units, and the Scale of the Universe


  • Start Chapter 2 on the Sky
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Questionnaire Results
  • Some math strong, some weak, as normal.  We’ll try to hit a balance.  May add some options to lab.
  • Science backgrounds weakish in general, again normal.
  • Popular topics include black holes and constellations.  Constellations are not of great interest scientifically (we’ll see why), but we’ll make sure some constellations are covered in lab for certain.
  • Mixed on issues of coverage, depth.  OK.
  • Science fiction, in moderate doses OK, and many have a genuine interest in astronomy.
  • Some of your classmates play guitar, rugby, dance, and one admits that s/he likes “getting wasted.”  One of you will apparently “throw up” if called on in class – so I’ll usually ask for volunteers first!  Please volunteer when asked, so we can avoid this potential timebomb.
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Scientific Notation
  • 101 =       10
  • 102 =       100
  • 103 =     1,000  (one thousand)
  • 106 = 1,000,000  (one million)


  • You can think of this as raising 10 to some power –
    or just think of it as moving decimal place over some given number of steps.  Think of computer speeds and disk space.


  • 100 =         1
  • 10-1 =         0.1 = 1 / 10
  • 10-2 =         0.01 = 1 / 100
  • 10-3 =         0.001 = 1 / 1,000
  • 10-6 =         0.000001 = 1 / 1,000,000


  • How to write numbers which are not powers of 10:
    1 A.U. = 149,597,900 km = 1.496 ´ 108  km
                            = mantissa     ´ 10exponent
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Arithmetic and Scientific Notation
  • Multiplication:  Multiply the mantissa
            Add the exponents


  • 20 AU = (2 ´ 101 ) ´ (1.496 ´108 km)
     
          = (2     ´ 1.496) ´  (101 x 108) km
          =   2.9992        ´      109       km


  • Division:        Divide the mantissa
          Subtract the exponents
  • 1 AU / 500 = (1.496 ´108 km) / (5 ´ 102)
               = (1.496 / 5 )     ´  (108 / 102) km

  •            =  0.2992      ´      106     km
               =  2.992           ´      105     km


  • Be careful when adding or subtracting:
  •   (2.0´106) + (2.0´103) = 2,002,000 = 2.002´106
                                  not 4.       ´106
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Scientific notation and the metric system
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What is a light-year, mathematically?
  • 1 light-year  =     c ´ t    (where c is the speed of light and t is one year)

    = 3.0     ´ 108 m/s    ´   365 days
    = 1.1     ´ 1011    (m ´ days /s)
  • but we know light-years is a distance and must have “dimensions” of distance.  We should have units of just meters.  The fact that we have this extra (days/s) means we have left something out.
  • If we multiply by  (24 hr/day ´ 60 min/hr ´  60 sec/min) the units will work out right and so will the numerical answer
  •        = 1.1 ´ 1011 ´ 24 ´ 60 ´ 60  m
           = 9.5     ´ 1015    m
           = 9.5     ´ 1012 km
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A final word from the Guide…
  • “The simple truth is that interstellar distances will not fit into the human imagination.”  -- Douglas Adams
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Chapter 2:
The Sky
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Constellations
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The Magnitude Scale
  • First introduced by Hipparchus
  • (160 - 127 B.C.):


  • Brightest stars: ~1st magnitude
  • Faintest stars (unaided eye): 6th magnitude


  • More quantitative:


  • 1st mag. stars appear 100 times brighter than 6th mag. stars
  • 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude = fainter object!)
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Example:
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Formula for Intensity vs. m:
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More Examples:
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More Examples:
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More Examples:
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The Celestial Sphere
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The Celestial Sphere (II)
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Example:
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The Celestial Sphere (III)
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Apparent Motion of the Celestial Sphere
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Apparent Motion of the Celestial Sphere II
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Precession (I)
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Precession (II)