The Earth and the Moon

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The Earth and the Moon

• The Moon is our nearest neighbour.
• It crosses the sky at 12º per day, or its own
  diameter (30 arc minutes) in 1 hour

2
Lunar Phases

• 29.5 days between new moons
• Reflected light from the Sun
• 3-D system! Sphere, not discs.

3
Lunar Phases II
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Sidereal and Synodic Months

• Due to earths motion around the Sun the moon
  must travel slightly more than one full rotation
  to return to the same phase.

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More Definitions and Observations II

• Sidereal means with respect to the Stars.
• Synodic means with respect to the Earth.
• The sidereal month is is about 27 1/3 days.
• (i.e. the time for the moon to return to its
  original position relative to the fixed stars.)
• The synodic month (the time between successive
  new moons) is about 29 1/2 days.

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More Definitions and Observations II

• Twelve synodic months make about 360 days, or
  approximately one year.
• 235 synodic months is almost exactly equal to 19
  years (with an error of only two hours.)
• This coincidence, together with the apparent
  similarity in the size of the sun and moon,
  serves to create the illusion that there is some
  fundamental connection between their motions. (It
  also enables the possibility of eclipses.)

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The Moons Rotation

• The Moon rotates on its own axis once per orbital
  cycle.
• We therefore see the only one face of the moon

The Dark Side of the Moon is a misnomer!
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Orbital Planes of the Earth and Moon

• Moons orbital plane is inclined at 5º to the
  plane of the ecliptic.
• The two planes intersect along the Line of
  Nodes.

9
Solar and Lunar Eclipses

• Coincidentally, the angular sizes of the Sun and
  Moon are very similar.
• This enables the possibility for eclipses!

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Conditions for Eclipses

• Sun, Earth, and Moon need to be co-linear and
  nearly co-axial with the Line of nodes for
  eclipses to occur.

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Lunar Eclipse Earth Shadows the Moon

• Moon is reddish due to Earths atmosphere
  scattering sunlight.

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Incidentally

• Full moons that occur when the moon is low in the
  sky (near the horizon) also appear red in colour.
• Red colouration is an atmospheric effect
• It appears larger too..

Harvest Moon the full moon nearest the
autumnal equinox
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Moon Illusion
Debate as to true cause. Must one have an
horizon to see it?!

• The moon appears bigger near the horizon.
• Our eyes/brains
• perceive it,
• cameras dont!

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Eclipse Terms

• Note The Sun has a finite size and therefore
  different degrees of shadow exist.
• Umbra - Central region
• Penumbra - Annular ring
• Partial shadow/shade

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Total, Penumbral and Partial Lunar Eclipses
Darkness depends on the degree of shadowing of
the Sun.
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Total Lunar Eclipse Sequence

• The Moon moving through the Earths umbra over a
  3 hour period in January 2000.

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Solar Eclipse (in 5 Minute Intervals)

• Suns corona in at the moment of total eclipse

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Annular Solar Eclipse
View from Mir (1999)
Dark spot is 105 km wide moving at 3000km/hr!
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Annular or Total Eclipse?

• Depends on the precise Earth-Moon distance at the
  time Elliptical orbits .

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Annular, Total and Partial Eclipses

• Depends on
• Earth-Moon distance.
• Your position!
• New moon in eclipse season
• i.e. depends on the line of nodes

º
º
Total Eclipses are relatively rare.
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What If ??

• If the Moon were larger.annular eclipses would
  be rarer and totals more common.
• If the Sun were larger..never see total
  eclipses, only annular!

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Eclipse Cycles

• Sun-Moon gravitational interaction causes the
  line of nodes to change orientation slowly with
  time.
• 1 Eclipse year 346.6 days.
• Time between one orbital configuration (with the
  line of nodes pointing at the Sun) and the next
  with the Moon crossing the Ecliptic in the same
  manner.

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Regression of the Line of Nodes

• The 19 day difference from the Tropical year is
  known as the Regression of the Line of Nodes

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Long-Term Eclipse Cycles

• Combination of Synodic months (29.5 days) and
  Eclipse years (346.6 days)
• 19 Eclipse years 223 Lunar months 6585 days
  (18 years 11.3 days)
• This is the 18-year Saros Cycle
• ...Well-known to ancient Astronomers

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Eclipses are Predictable!

• and have been since the Egyptians and
  Babylonians!

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Windsor April 8 2024
Saros Cycle