Chapter 19

Chapter 19

1. Earth history of astronomy

   • Ancient Greeks
     • Use philosophical arguments to explain natural phenomena
     • Also used some observational data
     • Most ancient greeks held a geocentric (earth centered view of the universe)
       • Earth Centered view
         • Earth was a motionless sphere at the center of the universe
         • Stars were on the celestial sphere
           • Transparent, hollow sphere
             • Celestial sphere turns daily around Earth
               • Seven heavenly bodies
                 • Changed position in the sky
                   • The seven wanderers included the
                     • Sun
                       • Moon
                     • Mercury through Saturn
     • Aristarchus (312-230 B.C) was the first Greek to profess a sun-centered or heliocentric, universe
     • Planets exhibit an apparent westward drift
       • Called retrograde motion
       • Occurs as Earth with its faster orbital speed overtakes another planet
     • Ptolemaic system
       • A.D 141
       • Geocentric model
       • To explain retrograde motion, Ptolmy used two motions for the planets
         • Large orbital circles called deferents and
         • Small circles call epicycles
   • Birth of modern astronomy
     • five noted scientists
       • Nicolaus Copernicus
         • Concluded Earth was a planet
         • Constructed a model of the solar system that put the sun at the center but he used circular orbits for the planets
         • Ushered out old astronomy
       • Tycho Brahe
         • Precise observer
         • Tried to find stellar parallax
         • Did not believe in the Copernican system because he was unable to observe stellar parallax
       • Johannes Kepler
         • Ushered in new astronomy
         • Planets revolve around the sun
         • Three laws of planetary motion
           • Orbits of planets are elliptical
           • Planets revolve around the sun at varying speed
           • There is a proportional relation between a planet's orbital period and its distance to the sun (Aus) one AU averages about 150 million kilometers or 93 million miles.
       • Galileo Galilei
         • Supported copernican theory
         • Used experimental data
         • Constructed an astronomical telescope in 1609
         • Galileo's discoveries using the telescope
           • Four large moons of Jupiter
           • Planets appeared as disks
           • Phases of Venus
           • Features on the moon
           • Sunspots
         • Tried and convicted by the Inquisition
       • Sir Isaac Newton
         • Law of universal gravitation
         • Proved that the force of gravity combined with the tendency of a planet to remain in straight line motion, results in the elliptical orbits discovered by Kepler
     • 1500s and 1600s

2. Motions of the Earth-moon system

   • Phases of the moon
     • When viewed from above the north pole the moon orbits Earth in a counterclockwise (eastward direction)
     • The relative positions of the sun, Earth and moon constantly change
     • Lunar phases are a consequence of the motion of the moon and the sunlight that is reflected from its surface
   • Lunar motions
     • Earth-moon
       • Synodic month
         • Takes 29 1/2 days
       • Cycle of the phases
       • Sidereal moth
         • True period of the moon's revolution around earth
         • Takes 27 1/3 days
       • The difference of two days between the synodic and sidereal cycles is due to the Earth moon system also moving in an orbit around the sun

3. Constellations

   • Configurations of stars named in honor of mythological characters or great heros
   • Today 88 constellations are recognized
   • Constellations divide the sky into units, like state boundaries
   • The brightness stars in a constellation are identified in order of their brightness by the letter of the Greek Alphabet

4. Positions in the sky

   • Stars appear to be fixed on a spherical shell that surrounds Earth
   • Equatorial system of location
     • A coordinate system that divides the celestial sphere
     • Similar to the latitude- longitude system that is used on Earths surface
     • Two locational components
       • Declination- the angular distance north or south of the celestial equator
       • Right ascension- the angular distance measured eastward along the celestial equator from the position of the vernal equinox

5. Earths motions

   • two primary motions
     • Rotations
       • Turning or spinning of a body on its axis
       • Two measurements for rotation
         • Mean solar day- the time interval form one noon to the next about 24 hours
         • Sidereal day- the time it takes for earth to make one complete rotation with respect to a star other than the sun
     • Revolution
       • The motion of a body such as a planet or moon along a path around some point in space
       • Earths orbit is elliptical
         • Earth is closest to the sun in Jan.
         • Earth is farthest from the sun in July
           • The plane of the ecliptic is an imaginary plane that connects Earth's orbit with the celestial sphere
   • Other Earth motions
     • Precession
       • Very slow Earth movement
       • Direction in which Earth's axis points continually changes
     • Movement with the solar system in the direction of the star Vega
     • Revolution with the sun around the galaxy
       • Movement with the galaxy within the universe