PERIODIC EVENTS

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PERIODIC EVENTS
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Warm Up

• What causes the seasons?
• What is a solstice?
• When do they occur?
• What is an equinox?
• When do they occur?
• What are the eight phases of the moon?
• What time does the full moon rise?
• What time does the full moon set?
• What time is the full moon straight overhead?
• How much is the Earths rotational orbit tilted?

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Periodic Events Index

• Seasons Solar Eclipses
• Precession Lunar Eclipses
• Lunar Cycles
• Because of the clock-like order of our Universe,
• certain astronomical phenomenon repeat
  themselves.
  

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Seasons

• Many people believe that the seasons are caused
  by the Earth traveling closer to and farther away
  from the Sun.

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Seasons

• At its closest (aphelion), the sun is 91.4
  million miles away from us. At its farthest
  (perihelion), the sun is 94.5 million miles away.
  That might sound like a lot, but it isnt. The
  Earths temperature barely changes from it. The
  real reason for the seasons is..

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Seasons

• the tilt of the Earth on its axis!!!
• As the Earth orbits the Sun, the orientation of
  its axis remains fixed.

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Seasons

• During June, the northern hemisphere tilts
  toward the Sun. The more direct sunlight heats
  the northern hemisphere more than the southern
  hemisphere, where it is Winter.

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Seasons

• During December, the northern hemisphere tilts
  away from the Sun. The more direct sunlight in
  the southern hemisphere heats it up more than the
  northern hemisphere, where it is now Winter.

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Seasons

• During Autumn and Spring, the Sun shines on both
  hemispheres of the Earth equally which results in
  the mild weather associated with those seasons.

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Precession
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Precession

• The Earth's rotation axis is not fixed in space.
  Like a rotating toy top, the direction of the
  rotation axis executes a slow precession with a
  period of 26,000 years

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Precession

• Thus, Polaris will not always be the Pole Star
  or North Star. The Earth's rotation axis happens
  to be pointing almost exactly at Polaris now, but
  in 13,000 years the precession of the rotation
  axis will mean that the bright star Vega in the
  constellation Lyra will be approximately at the
  North Celestial Pole (which is perpendicular to
  the celestial equator), while in 26,000 more
  years Polaris will once again be the Pole Star.

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Precession

• Since the rotation axis is precessing in space,
  the orientation of the Celestial Equator (which
  lies in the same plane as the terrestrial
  equator) also precesses with the same period.
  This means that the position of the equinoxes is
  changing slowly with respect to the background
  stars. This precession of the equinoxes means
  that the right ascension and declination of
  objects changes very slowly over a 26,000 year
  period. This effect is negligibly small for
  casual observing, but is an important correction
  for precise observations.

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Precession

• Because of the precession of the equinoxes, the
  vernal equinox moves through all the
  constellations of the Zodiac over the 26,000 year
  precession period. Presently the vernal equinox
  is in the constellation Pisces and is slowly
  approaching Aquarius.

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Precession

• Around the year 130 BC, Hipparchus compared
  ancient observations to his own and concluded
  that in the preceding 169 years heavenly bodies
  had moved by 2 degrees. How could Hipparchus know
  the position of the Sun among the stars so
  exactly, when stars are not visible in the
  daytime? By using not the Sun but the shadow cast
  by the Earth on the moon, during an eclipse of
  the Moon! During an eclipse, Sun, Earth and Moon
  form a straight line, and therefore the center of
  the Earth's shadow is at the point on the
  celestial sphere, which is exactly opposite that
  of the Sun.

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Precession

• Hipparchus concluded that the intersection
  marking the equinox slowly crept forward along
  the ecliptic (the plain defined by the Earths
  orbit around the Sun), and called that motion
  "the precession of the equinoxes. " The rate is
  about one full circle in 26 000 years. In ancient
  times the intersection marking the spring equinox
  was in the constellation of Aries, the ram, and
  for that reason the intersection (wherever it
  might be) is still sometimes called "the first
  point in Aries.

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Precession

• Around the year 1 it moved into the
  constellation of Pisces and currently it is again
  in transition, to the constellation of Aquarius,
  the water carrier. If you ever heard the song
  "The dawning of the age of Aquarius" from the
  musical "Hair," that is what it is all about. To
  believers of astrology, the "dawning of the age
  of Aquarius" is a great portent marking the
  beginning of a completely new era.

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Precession
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Lunar Cycles
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Lunar Cycles

• The moon goes through the following phases every
  30 days. These phase changes always occur from
  right to left.

                                              
                                                  
                                                  
          
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Lunar Cycles

• The initial phase is called the new moon. The
  new moon is completely dark.
  
• As light begins to cover more and more of the
  moons surface, the moon is said to be waxing.
  
• As darkness begins to cover more and more of the
  full moons surface, the moon is said to be
  waning.
  

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Lunar Cycles

• The phase of the Moon can tell you the time of
  day. For example, because a full Moon is seen
  when the Moon is on the opposite side of the
  Earth from the Sun, an observer on the Earth will
  see the Moon rise just as the Sun sets. The next
  day, the Moon will rise approximately one hour
  later (because the Moon will have moved farther
  along on its orbit). This means that a waning
  half Moon will rise at midnight, a new moon at
  dawn, etc...

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Warm Up

• List and diagram the eight phases of the moon.
• Define waxing and waning.
• You see the full moon just rising above the
  horizon. What time is it?
  
• Do we ever see the new moon? Why or why not?
• You see the first quarter moon setting. What
  time is it? What time will it rise?
  
• You see the third quarter moon just rising above
  the horizon. What time is it? What time will it
  set?
  

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Warm Up

• The new moon is today!
• What is the current phase of the moon?
• What time does it rise?
• What time does it set?
• What will the moon phase be on 12/25/06?
• What time will it rise?
• What time will it set?
• What will the moon phase be on 1/1/07?
• What time will it rise?
• What time will it set?

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Eclipseshttp//sunearth.gsfc.nasa.gov/eclipse/OH/
OH2006.html
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The Two Types of Eclipses

• Solar eclipses- When the Sun grows dark when
  covered by the moon and
  

• Lunar eclipses- When
• the moon grows dark
• when covered by the shadow of the Earth.

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Lunar eclipses
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Solar eclipses

• There are three kinds of solar eclipses
• Total eclipse
• Partial eclipse and
• Annular eclipse

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Solar eclipses
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Total Eclipse
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Total Eclipse

• During a total eclipse, the moon completely
  covers the Sun and all you can see is the Suns
  corona. This only occurs when the moon is a
  certain distance from the Earth.
• The moon castes two kinds of shadows on the
  Earths surface
  
• The umbra The dark, internal portion of the
  shadow and
  
• The penumbra The lighter shadow formed outside
  the umbra.
  

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Partial Eclipse
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Partial Eclipse

• During partial eclipses, the sky never darkens
  and the moon never completely covers the Sun.
  These photographs were taken during a partial
  eclipse and show some of the strange effects to
  shadows caused by the eclipse.

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Annular Eclipse
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Annular Eclipse

• When the moon is farther away during a total
  eclipse alignment, an annular eclipse occurs.
  During an annular eclipse, the moon passed in
  front of the Sun, but because it is further away
  from the Earth, it casts a smaller shadow.
  During an annular eclipse the Sun is not
  completely obscured by the moon and the fringes
  of the Sun may be seen.

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The Earths Moon

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• The End