Observing the Moon and Eclipses

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Observing the Moon and Eclipses

• Astronomy 311
• Professor Lee Carkner
• Lecture 4

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Moon Basics

• Moon orbits the Earth once every 30 days (1
  month)
• 1/2 of the moon is always lit by the Sun

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Moon Phases

• Crescent
• Gibbous
• Waxing
• Waning

• New --
• Quarter
• Full

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Phases of the Moon
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Sidereal and Synodic

• Sidereal period -- time for moon to return to
  initial position with respect to the stars (27.32
  days)
• Synodic period -- time for moon to return to
  initial position with respect to the sun (29.53
  days)

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Why is a Synodic Month Longer than a Sidereal
Month?
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Where is the Moon?

• The moon makes one orbit in about 30 days
• Moons orbit moves it west to east
• Moon rises about 45 minutes later each night

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Eclipses

• Solar Eclipse
• Lunar Eclipse
• Happens during Full moon

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When do Eclipses Happen?

• Why arent there eclipses every new and full
  moon?
• Only have eclipses when Sun falls on line of
  nodes (line where the orbital plane of the Earth
  and Moon intersect)

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Line of Nodes
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Shadows

• Umbra -- Darkest part of the shadow
• Penumbra -- Less dark part of shadow, region is
  still getting some sunlight
• Penumbral eclipses can be hard to notice

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Geometry of a Lunar Eclipse
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Types of Lunar Eclipses

• Total Eclipse -- Moon is completely covered
• Partial -- Moon is partially covered
• Penumbral -- only the penumbra covers the Moon
• You can still faintly see the Moon even during a
  total lunar eclipse because of scattered light
  (circular sunset)

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Lunar Eclipse
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Types of Solar Eclipses

• Total Eclipse -- Sun is completely covered
• Partial -- Sun is partially covered

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Total Solar Eclipse
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Diamond Ring Effect
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Solar Eclipse in Action
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Annular Eclipse

• Moon is about 400 times smaller than the Sun, but
  is also about 400 times closer
• When the Moon is the furthest from the Earth it
  does not completely cover the Sun
• Annular Eclipse --

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Annular Eclipse
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Solar Eclipse from Space
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Partial Solar Eclipse Christmas 2000
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Next Eclipses

• Eclipses visible from central U.S.
• Lunar eclipses are visible from the entire night
  side of the Earth, but the shadow of the Moon
  only covers a small fraction of the Earths
  surface

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Summary

• Orbit
• one complete orbit in one sidereal month
• same side always faces the Earth
• Phases
• complete set in one synodic month
• where the Moon is in sky at a particular time
  depends on the phase

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• Eclipses
• caused by Earth or Moon blocking out the Sun
• only occur when line of nodes points at Sun
• are darkest when in the umbra
• occur in cycles

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Next Time

• Read chapter 2.5-2.7 for next time