The Earth and the Moon

The Moon is our nearest neighbour.

It crosses the sky at ~12 per day, or its own diameter (~30 arc minutes) in ~ 1 hour

Lunar Phases

~29.5 days between new moons

Reflected light from the Sun

3-D system! Sphere, not discs.

Lunar Phases II

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.

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.

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.)

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!

Orbital Planes of the Earth and Moon

Moons orbital plane is inclined at 5to the plane of the ecliptic.

The two planes intersect along the Line of Nodes.

Solar and Lunar Eclipses

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

This enables the possibility for eclipses!

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.

Lunar Eclipse: Earth Shadows the Moon

Moon is reddish due to Earths atmosphere scattering sunlight.

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

MoonIllusion

The moon appears biggernear the horizon.

Our eyes/brainsperceive it,cameras dont!

Debate as to true cause.

Must one have an horizon to

see it?!

Eclipse Terms

Note: The Sun has a finite size and therefore different degrees of shadow exist.

Umbra - Central regionPenumbra - Annular ring

Partial shadow/shade

Total, Penumbral and Partial Lunar Eclipses

Darkness depends on the degree of

shadowing of the Sun.

Total Lunar Eclipse Sequence

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

Solar Eclipse (in 5 Minute Intervals)

Suns corona in at the

moment of total eclipse

View from Mir (1999)

Dark spot is ~105 km wide moving at 3000km/hr!

Annular Solar

Eclipse

Annular or Total Eclipse?

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

Annular, Total and Partial Eclipses

Depends on:

(a) Earth-Moon distance.

(b) Your position!(c) New moon in

eclipse seasoni.e. depends on the line of nodes

Total Eclipses are relatively rare.

What If ??

If the Moon were larger.annular eclipses would be rarer and totals more common.

If the Sun were larger..never see totaleclipses, only annular!

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.

Regression of the Line of Nodes

The ~19 day difference from the Tropical year is known as the

Regression of the Line of Nodes

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

Eclipses are Predictable!

and have been since the Egyptians and Babylonians!

Windsor April 8 2024 Saros Cycle

The Earth and the MoonLunar PhasesLunar Phases IISidereal and Synodic MonthsMore Definitions and Observations IIMore Definitions and Observations IIThe Moons RotationOrbital Planes of the Earth and MoonSolar and Lunar EclipsesConditions for EclipsesLunar Eclipse: Earth Shadows the MoonIncidentallyMoon IllusionEclipse TermsTotal, Penumbral and Partial Lunar EclipsesTotal Lunar Eclipse SequenceSolar Eclipse (in 5 Minute Intervals)Annular Solar EclipseAnnular or Total Eclipse? Annular, Total and Partial EclipsesWhat If ??Eclipse CyclesRegression of the Line of NodesLong-Term Eclipse CyclesEclipses are Predictable!