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The Earth and the Moon
The Moon is ournearest neighbour.
It crosses the skyat ~12per day, orits own diameter(~30 arc minutes)in ~1 hour
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LunarPhases
~29.5 daysbetween new
moons Reflected
light from
the Sun 3-D system!
Sphere, notdiscs.
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Lunar Phases II
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Sidereal and Synodic Months
Due to earths
motion aroundthe Sun: themoon must
travel slightlymore than one
full rotation toreturn to the
same phase.
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More Definitions and Observations IISidereal means with respect to the Stars.
Synodicmeans with respect to the Earth.
Thesidereal month is is about 27 1/3days.
(i.e. the time for the moon to return to itsoriginal position relative to the fixed stars.)
Thesynodic month (the time between
successive new moons) is about 29 1/2days.
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More Definitions and Observations IITwelvesynodicmonths 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, servesto 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 axisonce per orbital
cycle. We therefore see
the only oneface of the moon
The Dark Side of the Moon is a misnomer!
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Orbital Planes of the Earth and Moon
Moons orbitalplane isinclined at 5
to the plane ofthe ecliptic.
The two planesintersect alongtheLine of
Nodes.
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Solar and Lunar Eclipses
Coincidentally, the angular sizes of theSun and Moon are very similar.
This enables the possibility for eclipses!
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Conditions for Eclipses
Sun, Earth, and Moon need to beco-linear and nearlyco-axial with theLine of nodes for eclipses to occur.
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Lunar Eclipse: Earth Shadows the Moon
Moon is reddish due to Earthsatmosphere scattering sunlight.
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Incidentally
Full moons that occurwhen the moon is low inthe sky (near the
horizon) also appear redin colour.
Red colouration is anatmospheric effect
It appears larger too..
Harvest Moon the full moonnearest the
autumnalequinox
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MoonIllusion
The moon appearsbiggernear the horizon.
Our eyes/brainsperceive it,
cameras dont!
Debate as totrue cause.
Must one have
an horizon tosee it?!
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Eclipse Terms
Note: The Sun has a finite size andtherefore different degrees of shadowexist.
Umbra - Central region
Penumbra - Annular ring
Partial shadow/shade
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Total, Penumbral and Partial
Lunar Eclipses
Darkness dependson the degree of
shadowing of the Sun.
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Total Lunar Eclipse Sequence
The Moon moving through the Earths umbraover a 3 hour period in J anuary 2000.
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Solar Eclipse (in 5 Minute Intervals)
Suns coronain at themoment of
total eclipse
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View from
Mir (1999)
Dark spot is ~105 kmwide moving at3000km/hr!
AnnularSolar
Eclipse
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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:
(a) Earth-Moondistance.
(b) Your position!(c) New moon in
eclipse season
i.e. depends onthe line of nodes
Total Eclipses are relatively rare.
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What If ??
If theMoon werelarger.annular eclipseswould be rarer and totalsmore common.
If theSun werelarger..never seetotal
eclipses, onlyannular!
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Eclipse Cycles Sun-Moon gravitational interaction causes
the line of nodes to change orientationslowly with time.
1Eclipse 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 theTropical year is known as theRegression of the Line of Nodes
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Long-Term Eclipse Cycles
Combination of Synodic months (29.5days) and Eclipse years (346.6 days)
19 Eclipse years ~223 Lunar months= 6585 days (18 years 11.3 days)
This is the 18-year SarosCycle
...Well-known to ancient Astronomers
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Eclipses are Predictable!
and have been since the Egyptians and Babylonians!
Windsor April 8 2024 SarosCycle
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Windsor April 8 2024 SarosCycle