Chapter 19

Chapter 19:

 

III.Positions in the sky 

   A.Stars appear to be fixed on a spherical shell that surrounds Earth

   B.Equatorial system of location

      1.A coordinate system that divides the celestial sphere

      2.Similar to the latitude- longitude system that is used on Earths surface 

      3. Two locational components 

         a.Declination- the angular distance north or south of the celestial equator 

         b.Right ascension- the angular distance measured eastward along the celestial equator from the position of the vernal  equinox

IV. Earths motions 

   A.two primary motions

      1.Rotations

         a.Turning or spinning of a body on its axis

         b.Two measurements for rotation 

            1.Mean solar day- the time interval form one noon to the next about 24 hours

            2.Sidereal day- the time it takes for earth to make one complete rotation with respect to  a star other than the sun 

      2.Revolution

         a.The motion of a body such as a planet or moon along a path around some point in space

         b.Earths orbit is elliptical 

            1.Earth is closest to the sun in Jan.

            2.Earth is farthest from the sun in July

         c.The plane of the ecliptic is an imaginary plane that connects Earth's orbit with the celestial sphere

   B.Other Earth motions

      1.Precession  

         a.Very slow Earth movement

         b.Direction in which Earth's axis points continually changes 

      2.Movement with the solar system in the direction of the star Vega

      3.Revolution with the sun around the galaxy 

      4.Movement with the galaxy within the universe 

V. Motions of the Earth-moon system

   A. Phases of the moon

      1.When viewed from above the north pole the moon orbits Earth in a counterclockwise (eastward direction) 

      2.The relative positions of the sun, Earth and moon constantly change

      3.Lunar phases are a consequence of the motion of the moon and the sunlight that is reflected from its surface

   B. Lunar motions

      1.Earth-moon

         a.Synodic month

            1.Cycle of the phases

            2.Takes 29 1/2 days 

         b.Sidereal moth

            1.True period of the moon's revolution around earth

            2.Takes 27 1/3 days

         c.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

      2.Moons period of rotation about its axis and its revolution about its axis and its revolution around Earth are the same 27 1/3 days 

         a. Causes the same lunar hemisphere to always face Earth

         b.Causes high surface temperature on the day side of the moon 

   C. Eclipses 

      1.Simply shadow effects that were first understood by the early Greeks

      2.Two types of eclipse

         a.Solar eclipse

            1.Moon moves in a line directly between Earth and the sun 

            2.Can only occur during the new moon phase

         b.Lunar Eclipse 

            1. Moon moves within the shadow of Earth

            2.Only occurs during the full moon phase

      3. For any eclipse to take place, the moon must be in the plane of the ecliptic at the time of new or full moon

      4.Because the moon's orbit is inclined about 5 degrees to the plane of the ecliptic, during most of the times of new and full moon the moon is above or below the plane, and no eclipse can occur

      5. The usual number of eclipses is four per year