Moon _ Tides

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					Moon & Tides

         By
 Diana L. Duckworth
Rustburg High School
Campbell County, VA
             Cause of Tides
• Gravitational attraction of Sun & Moon for
  Earth & its water
• Sun is larger but is farther away than Moon
• F= Gm1m2/r2 indicates that distance has a
  greater impact on gravity
• Moon affects tides to greater extent than Sun
        Equilibrium tides on water
              covered Earth
• Mass of Moon pulls water
  toward moon & pulls                     Low tide
  Earth away from water on
  opposite side
                                                        Moon
• Creates two tidal bulges                 NP

  – Direct high tide under
    moon
  – Indirect high tide away   Indirect               Direct
                              high tide              high tide
    from moon
                                        Low tide
• Low tides are in between      Earth rotates through 2 high
                                and 2 low tides a day!
            Timing of Tides
• Moon is at highest point 50 minutes later
  each day
• High tides come when moon is overhead &
  12 hours & 25 minutes later
• Tides come 50 minutes later each day
  – 24 hrs &50 minutes between successive direct
    tides
  – 12 hrs & 25 minutes between high tide & next
    high tide
  – 6 hrs &12.5 minutes from high tide to low tide
                                         Tidal Cycle
            Water Level March 21, 1998
                                Chesapeake Bay-Bri dge T unnel
Water Level (Meters +/- MSL)

                               0.9
                               0.8
                               0.7
                               0.6
                               0.5
                               0.4
                               0.3
                               0.2
                                     0    5      10     15     20     25
                                         Hours (0 = m i dni ght EST )
                                          Another cycle
                           Water Level Sept. 20 1998
                                Chesapeake Bay-Bri dge T unnel
Water Level (Meters +/- MSL)

                               0.8
                               0.6
                               0.4
                               0.2
                                 0
                               -0.2
                                      0      5     10     15     20    25
                                           Hours (0 = m i dni ght EST )
              Kinds of tides
• On a water covered earth two high & two
  low tides would occur each day -
  semidiurnal tides as on U.S. East Coast
• Continents get in way, so two other kinds of
  tides
  – Diurnal - one high & one low tide each day
     • Gulf of Mexico
  – Mixed - mostly semidiurnal but diurnal
    occasionally as on U.S. West Coast
 Tides Correlated with Moon Phases
• Spring tides - 2 weeks apart
• Largest tidal range of month
                                                         Sun
• Occur when Sun & Moon              Earth
                                              New Moon
  both pull on the high tides
  – Largest range during New
    Moon
  – Next largest range during Full
    Moon                                     Earth       Sun
                            Full Moon
    Tides Correlate with Moon
             Phases
• Neap Tides
                                    First Quarter
• Two weeks apart
                             High
• Low tidal ranges
• Sun pulls on low tides                              Sun
                       Low             Low
• Moon pulls on high
  tides
• Occur during 1st &         High

  Last Quarter Moons                   Last Quarter
    Other factors affecting tides
• Because distance is factor in gravity,
• height of tides affected by apogee & perigee
  of moon
• also by aphelion & perihelion of Earth
• Greatest tidal range would occur when
  Earth is in perihelion & Moon is at perigee
  during New Moon Phase!

				
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posted:5/5/2010
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