Physics of Astronomy
Winter Week 1 - Thus. - Astro Ch.1
Yesterday: WebX and Seminar teams
Astro-A: Astronomy Ch.1 (Kaufmann & Freedman)
Everyone take weekly online Astronomy quizzes
Looking ahead
SEMINAR 2:15-3:30
4:00 Math-B: Calculus pre-tests
Astro-B: start Astrophysics Ch.1 (Carroll & Ostlie)
Astronomy pre-test
Astronomy Ch.1: Distances and angles
Even if we don’t know the distance to an object, angles are easy
to measure. By total coincidence, the Sun and Moon happen to
subtend nearly the same angle from Earth at the moment.
Therefore, we can have eclipses.
Small-angle formula
D = a d where the angle a is in radians. 360o=2p radians
D and d have the same units (e.g. both in meters).
D(m)  a (rad ) d (m) but it ' s easier to measure a in arc sec
D(m)  ___?__ a (arc sec) d (m)
radians are much bigger than arc sec : Is _?_ great or small ?
2p rad
1o
1 arc min
rad
 _______
o
360 60 arc min 60 arc sec
arc sec
Angles and distances
2p radians = 360 degrees
1 degree = 60 arcminutes
1 arcminute = 60 arcseconds
1 AU = 93 million miles = 150 million km
1 parsec ~ 3 light years
Ly = distance light travels in one year
Ly = speed * time ~ 3x108 m/s*3x107 s
Ly =_______________________
Peer instruction: Astronomy Ch.1 One problem setup per team (you need not calculate the solution)
Starry Night preview
Use the install disk in back of your text
Observing Exercise 1.45: Which planets are
visible now?
Looking ahead
Intro to Modern Astrophysics
Carroll and Ostlie = CO
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Basic astronomy
Gravity + orbits
Light + spectra
Modern physics + QM
Electromagnetism
Sun and Stars
Thermal + radiation
Cosmology
Ch.1: The Celestial Sphere
(Figures from Freedman and Kaufmann, Universe)
1.1: The Greek Tradition (Team 1): Celestial Sphere
1.2 The Copernican Revolution (Team 2): Periods; prob.1.3
Next week: 1.3 Positions on the Cel.Sph.
Team 1: Altitude+ Azimuth (p.10-13), prob. 1.5
Team 2: Right Ascension and Declination (p.13-15), prob.1.4
Team 3: Precession and motion of the stars (p.15-19), prob.1.6
1.4 Physics and Astronomy
1.1 The Greek Tradition
The Celestial Sphere
Geocentric Ptolemaic System
Epicycles and Deferents
Angles measure the sky:
2p radians = 360 degrees = 360 °
1° = 60 arcminutes = 60’
1’ = 60 arcseconds = 60 ’’
Example: Moon subtends 1/2°
1.2 The Copernican Revolution
Geocentric model actually uses
MORE epicycles and deferents,
but it is conceptually simpler
Synodic period S = how long we
see a planet take to return to the
same place on the sky (e.g. near
the same star)
Sidereal period P = how long the
planet takes to orbit Sun
 1 1
1
   
S
 P P 
Freedman and Kaufmann #1.30: The average distance to the Moon is 384,000
km, and the Moon subtends an angle of 1/2°. Use this information to
calculate the diameter of the Moon in km.
arclength D = d a when a is in radians