Eclipses W

7/28/2019 Eclipses W

1/26

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


2/26

LunarPhases

~29.5 daysbetween new

moons Reflected

light from

the Sun 3-D system!

Sphere, notdiscs.


3/26

Lunar Phases II


4/26

Sidereal and Synodic Months

Due to earths

motion aroundthe Sun: themoon must

travel slightlymore than one

full rotation toreturn to the

same phase.


5/26

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.


6/26

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.)


7/26

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!


8/26

Orbital Planes of the Earth and Moon

Moons orbitalplane isinclined at 5

to the plane ofthe ecliptic.

The two planesintersect alongtheLine of

Nodes.


9/26

Solar and Lunar Eclipses

Coincidentally, the angular sizes of theSun and Moon are very similar.

This enables the possibility for eclipses!


10/26

Conditions for Eclipses

Sun, Earth, and Moon need to beco-linear and nearlyco-axial with theLine of nodes for eclipses to occur.


11/26

Lunar Eclipse: Earth Shadows the Moon

Moon is reddish due to Earthsatmosphere scattering sunlight.


12/26

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


13/26

MoonIllusion

The moon appearsbiggernear the horizon.

Our eyes/brainsperceive it,

cameras dont!

Debate as totrue cause.

Must one have

an horizon tosee it?!


14/26

Eclipse Terms

Note: The Sun has a finite size andtherefore different degrees of shadowexist.

Umbra - Central region

Penumbra - Annular ring

Partial shadow/shade


15/26

Total, Penumbral and Partial

Lunar Eclipses

Darkness dependson the degree of

shadowing of the Sun.


16/26

Total Lunar Eclipse Sequence

The Moon moving through the Earths umbraover a 3 hour period in J anuary 2000.


17/26

Solar Eclipse (in 5 Minute Intervals)

Suns coronain at themoment of

total eclipse


18/26

View from

Mir (1999)

Dark spot is ~105 kmwide moving at3000km/hr!

AnnularSolar

Eclipse


19/26

Annular or Total Eclipse?

Depends on the precise Earth-Moon distance

at the time: Elliptical orbits .


20/26

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.


21/26

What If ??

If theMoon werelarger.annular eclipseswould be rarer and totalsmore common.

If theSun werelarger..never seetotal

eclipses, onlyannular!


22/26

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.


23/26

Regression of the Line of Nodes

The ~19 day difference from theTropical year is known as theRegression of the Line of Nodes


24/26

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


25/26

Eclipses are Predictable!

and have been since the Egyptians and Babylonians!

Windsor April 8 2024 SarosCycle


26/26

Windsor April 8 2024 SarosCycle

Documents

Eclipses W