Lunar Phases Eclipses

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Lunar PhasesEclipses
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Apparent motion The Moons orbitThe phases of
the Moon

• Every object in the solar system is always light
  on one side and dark on the other
• If we sometimes see the bright side of an object,
  and sometimes see the dark side, then the object
  will appear to go through phases
• This is what causes the phases of the Moon
• The Moon takes about 27.3 days to go once around
  the Earth - siderial period
• Because of Earths orbit the Moon takes 29.5 days
  to get back to the same place relative to the Sun
  - synodic period

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Siderial vs. synodic periods
New
New
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The Moon

• Moon phases cycle over a period of 29.5 days
• Always has the same face to Earth, which means
  it MUST rotate on its own axis!
• Moon phases - result of the Moon, Sun, and Earth
  positions relative to each other
• Moon rises later and later as the month goes on
  (50 min/day)
• Plane of Moons orbit almost the same as Earths
  orbit (5o tilt)

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Lunar Phases

• New Moon - The Moon is on the same part of the
  sky as the Sun and rises and sets with the Sun
• Waxing Moon Increasing from day to day
• Waning Moon Decreasing from day to day
• Full Moon - The Moon is in the opposite side of
  the sky as the Sun and rises when the Sun sets
  and sets when the Sun rises
• Crescent concave/convex edges, ending in a
  point
• Gibbous Rounded edges

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Names/shapes of Moon phases(as seen from
Northern Hemisphere)
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Phases of the Moon
1st Q.
Wax. Gib.
Wax. Cr.
New M.
Full M.
Wan. Gib.
Wan. Cr.
3rd Q.
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Moonrise/Moonset

• The time the Moon rises and sets is correlated
  to its phase

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Naming the Moon
Blue Moon Definition One The second of
two full moons in a single month. It is uncommon
since the cycle of phases of the moon is 29.5
days (synodic month) which is almost a month.
Definition Two The third of four full moons
occurring in a season. Again this is uncommon
since the cycle of phases of the moon is 29.5
days and seasons are 91 days (365/491.25).
http//www.enchantedlearning.com/subjects/astronom
y/moon/Phases.shtml
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Naming the Moon
Harvest Moon The full moon that occurs
closest to the autumnal equinox. This results
in the near full gibbous moons and the full moon
lighting the fields for harvest for a longer
period.
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Eclipses

• Eclipses occur when the Sun, the Earth and the
  Moon all lie along a straight line
• They must line up in all 3 dimensions time
• the Moons orbit is tilted 5o with respect to the
  ecliptic, so there are only two times a year when
  the paths overlap

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Apparent motion The Moons orbitEclipses

• When the Moon is in its New Moon phase, it might
  pass directly between us and the Sun
• When this happens it will at least partially
  block the Sun from our view
• This is known as a solar eclipse
• Three kinds of solar eclipse
• Total eclipse Total coverage
• Partial eclipse - Moon not covering all Sun
• Annular eclipse Moon too far from Earth

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Frequency of Solar Eclipses

• Similar to a lunar eclipse, the Sun, Moon, and
  Earth, must all be aligned in the same plane
• Much less frequent than lunar eclipses because
  the Moon is much smaller in size, and thus casts
  a much smaller shadow on Earth
• Because the moon is on a slightly elliptical
  orbit, it may be too far to cast a shadow on the
  Earth

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Total Solar Eclipse of 21 June 2001 from Zimbabwe
See Richard Monks webpage on eclipses www.willia
ms.edu/ astronomy/IAU_eclipses/
Baileys Beads
Solar Corona
Diamond Ring
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Future Solar Eclipses

• If a solar eclipse occurs on a given day,
  another will occur in exactly 18 years and 11 1/3
  days saros cycle
• But the eclipse may change type and location
  during that cycle

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Apparent motion The Moons orbitEclipses

• When the Moon is in its Full Moon phase, it might
  pass through Earths shadow
• When this happens it will be at least partially
  blocked from our view
• This is known as a lunar eclipse
• Three kinds of lunar eclipse (of interest to us)
• Penumbral eclipse Moon in Earths Penumbra
• Partial eclipse - Moon in part of Earths Umbra
• Total eclipse Moon totally in Earths Umbra

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Shadows

• Umbra
• dark cone of complete shadow
• Penumbra
• lighter area of partial shadow

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Frequency of Lunar Eclipses

• One might expect a lunar eclipse to occur every
  month during full moon
• But the moons orbit is titled 5º to the Earths
• For a lunar eclipse, the moon phase must be full
  and the moon must be aligned with Earths orbit
  (at a lunar node) every 5-6 months
• Partial eclipse moon in penumbra where only
  some sunlight is blocked
• Total eclipse moon is located in the umbra
  where all of the sunlight is blocked

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Eclipses

• Lunar Eclipse
• Earth positioned between the Sun and the Moon and
  casts a shadow onto the Moon
• Much more common than a solar eclipse
• Solar Eclipse
• Moon positioned between the Earth and the Sun,
  casting a shadow onto the Earth
• During a total eclipse the Suns Corona, normally
  invisible is visible

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Upcoming Lunar and Solar Eclipses
Solar Eclipses 8 Apr 2005 (annular) South
Pacific and Northern South America 3
Oct 2005 (annular) Spain Africa 29 Mar 2006
(total) Africa Asia 22 Sep 2006 (annular)
South Atlantic
Lunar Eclipses 24 Apr 2005 (penumbral) 17 Oct
2005 (partial) 14 Mar 2006 (penumbral) 7 Sep
2006 (partial)
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Motion of the Sun Solar vs. Sidereal period

• Since the Earth moves in its orbit each day, the
  motion of (nearby) solar system objects is
  slightly different from stars.
• The Earth needs to rotate an additional 1 degree
  daily for the Sun to repeat its position (see
  diagram at right).
• This takes an additional 4 min
• This means that the Suns position repeats every
  24 hrs. This is called the solar period.

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Length of the Year

• It takes the Earth 365.242199 days to go from one
  vernal equinox to the next
• NOT an integer number
• But the extra is close to ¼ 0.25
• So every 4 years (leap year) we add an extra day
  to the calendar (Feb. 29)
• But this is too much (weve added 0.25!)
• So every 100 years (on the century) we dont add
  the extra day (no leap year)
• But this isnt right either, so every 4th 100
  years, we do include the leap year
• This is why 2000 was a leap year

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www.hep.fsu.edu/tadams/courses/
spr04/ast1002-2/Lecture011404.ppt