Martin Crow Crayford Manor House Astronomical Society The Solar System

THE EARTH – MOON SYSTEM

Martin Crow Crayford Manor House Astronomical Society

The Solar System


The Solar System : Earth Moon system

Last time:

The formation of the Sun (and Solar system).

About the Sun

The Sun’s effect on the Earth.


The Solar System : Earth Moon system

This week:

The formation of the Earth and Moon

Consequences of a gravitationally bound system.

The local gravitational environment

Phases of the Moon and its orbital characteristics.

Solar and Lunar eclipses.


Some Physical data

Earth

Mean Earth Sun distance = 149.6 x 10⁶ km (1 A.U.)

Inclination to the Sun’s equator = 7.16°

Mean diameter = 12,742 km

Mass = 5.97 x 10²⁴ kg

Mean density = 5.52 x 10³ kg/m³ (water = 1 x 10³ kg/m³)

Axial tilt = 23° 26’ 21”

Albedo = 0.3 (Bond)


Moon

Mean Earth Moon distance = 384.4 x 10³ km

Sidereal month = 27.321582 days (27 d 7 h 43.1 min) relative to fixed frameof reference.

Mean diameter = 3,474 km

Mass = 7.35 × 1022 kg

Mean density = 3.35 x 10³ kg/m³

Synodic period = 29.530589 d (29 d 12 h 44 min 2.9 s) New Moon to new Moon andis commonly called a Lunar month.

Albedo = 0.12 (Bond)

Draconic month = 27.2122 days (the nodes precess over a period of 18.6 years)

Anomalistic month = 27.5546 days (the line of Apsides connecting Perigee and Apogee precesses over a period of 8.85 years.


Formation of the Earth – Moon system

The Earth formed 4.567 x 10⁹ years ago from the disk of material orbiting the newly forming Sun.


The Moon is now thought to have formed during an impact with a Mars sized object the so called ‘giant impact hypothesis’. This would have been within the first 50 x 10⁶ years of the formation of the solar system.


One possibility is that Theia formedin the Lagrangian point L4 or L5.

Its growing mass and gravitationalPerturbations caused it to be destabilisedand collide with the Earth.



Computer simulations are consistent with observations such as:

The measured angular momentum of the Earth- Moon system.

The small size of the Moon’s core.

The composition of the Moon.

Also it is now thought that an additional smaller object formed at the same time and had a slow impact with the Moon accounting for the difference between the near side and the far side topography.


The Lunar surface

Near side Far side



A. Initial Accretion of the Moon, probably from debris launched into Earth orbit by a mega-impact.

B. In the last stages of accretion, so much heat accumulates that the outermost 100 km of the lunar crust melts to form a magma ocean.

C. Late impacts excavate giant basins.

D. Mare Nectaris and other basins form.

E. Mare Imbrium forms.

F. Mare Orientale forms

G. Mare basalts erupt and flood many of the impact basins.

H. Since 3000 Ma, only a few large rayed craters like Tycho and Copernicus have formed.




Structure

Earth

Moon


Life on Earth


Consequences of a gravitationally bound system.

Due to tidal drag the Moon’s rotation has become locked into its rotationaround the Earth.

The effect of this is that the Moon rotates only once for every orbit. Whichis why we only see one face.

The Moon raises tides on Earth with the effect that the Earth spin is slowed.This loss of energy is transferred to the Moon thereby increasing its speed. This causes the Moon to move away from the Earth by 38mm every year.

Although the Earth’s spin rate reduces by only 2.3 ms per year (not constantover time and depends on the configuration of the continents) this adds up over time. During the Devonian period 400 x 10⁶ years ago the year was 400days long with each day approx. 21.9 hrs.


The local gravitational environment.

The Lagrangian points.


The Phases of the Moon

Syzygy – When the Earth, Moon and Sun are aligned.

Quadrature – When the Moon’s elongation is either 90° or 180°

Orbital period = 27.321582 days Synodic period = 29.530589 d


The Moons orbit

The line of Apsides joins the points of Perigee and Apogee and precesses over8.85 years.

The Moons orbit is an ellipse.

The Moon crosses the same node every27.2122 days (the Draconic month)The Nodes precess in a retrogrademotion over a period of 18.6 years.

The Moons orbit is inclined at an angleof 5° 8´ to the Ecliptic.

Lunar and Solar eclipses can only occurwhen the line of the nodes point towards the Sun, roughly every 5.4 months.The type of eclipse will depend on the Moon’s orbital circumstance.


Images showing the apparent size difference due to the Moon’s non circular orbit and alsoLibration.


Eclipses

How does a solar eclipse occur.


Solar eclipses occur in cycles called Saros cycles. The Saros cycle is based on the recognition that 223 synodic months approximately equal to 242 draconic months and 239 anomalistic months.

At this time there are 41 different Saros series in progress.

Each Saros series is given a number. Odd numbers are used for solar eclipses occurring at the ascending node and evens for the descending node. The opposite is true for Lunar eclipses.

Each series lasts for between 1226 and 1550 years depending on the geometry.

During the life time of a series of eclipses the path for odd numbered Saros serieswill travel from south to north and visa versa.




Total eclipse showing the solar corona

Annular eclipse

The totally eclipsed Sun is safe to look directly at. Bright stars and any visible planets will be seen in the sky.

A Solar eclipse will only last a few minutes and depends on the geometry of the Earth, Sun, Moon alignment.


The 1919 Solar eclipse observed by Sir Arthur Eddington gave observationalevidence that proved the Einstein’s theory of general relativity was correct.

Martin Crow Crayford Manor House Astronomical SocietyCornwall 1999 Aug 11 MVCrow



The 2006 Turkey eclipse as seen from the ISS.


Diamond ring

China 2008 Aug 01 MV Crow



Eclipses

How does a lunar eclipse occur.




The Planets etc.


The Planets


Mercury

Average distance from Sun = 57.9 x 10⁶ km

Diameter = 4,878 km

Has quite an eccentric orbit

Has a 3:2 resonance so it rotates threetimes for every two orbits.

Takes 88 days to complete one orbit.(Sidereal period)

The synodic period is 115.9 days.



Venus


Diameter = 12,102 km

Albedo = 0.9 (Bond)

Takes 224.7 days to complete one orbit.(Sidereal period)


Very dense atmosphere of CO₂ with a surfacepressure of 93 bar (Earth = 1 bar)

Surface temperature of 460° C


Surface contour map of Venus


Mars


Diameter = 6,792 km


Takes 687 days to complete one orbit.(Sidereal period)

The synodic period is 780 days.

Very thin atmosphere of CO₂ with a surfacepressure of 0.006 bar (Earth = 1 bar)

Surface temperature of -87° C to 20°C.

Mars has two moons – Phobos and Deimos both probably captured asteroids.

Surface contour map of Mars


NASA Galileo probe


Ceres was the first one discovered and is named 1 Ceres and is a main belt Asteroid.

With a diameter of 952 km Ceres is approx. ¼ the size of our Moon.

The Asteroids

It is now estimated that there are at least 750,000 asteroids larger than 1km and millions of smaller ones.

Ceres contains approx. 1/3 of the mass all of the asteroids.


There are three basic groups:

C Types – dark carbonaceous, make up approx. 75% of the total.

S Types – stony (silicaceous), relatively bright make up approx. 17%

M Types – Nickel / Iron – relatively bright objects make up most of the rest.

There are now a whole raft of sub groups as new subtle variations are recognised.

A great deal of uncertainty still surrounds the exact make up of certain objects.

It is now known that a lot of objects are just rubble piles.


It is thought that the asteroids formed at the same time as the rest of the Solar System 4.5 billion years ago. As Jupiter neared its current mass it disrupted this area of space ejecting a large percentage of the objects in it and halting the formation of any large body there.

Simulations indicate that bodies larger than 120km formed prior to the Jovian disruption and that those smaller than this are the result of subsequent collisions.


Asteroid groups


Near Earth Asteroids

Amors – orbit between Earth and Mars

(1221 Amor – discovered 1932)

Apollos – orbit > 1.0 a.u.

(1862 Apollo – discovered 1932)

Atens – orbit < 1.0 a.u. (2062 Aten – discovered 1976)


Jupiter


Diameter = 142,984 km at equator


Takes 11.9 years to complete one orbit.(Sidereal period)


Atmosphere of Hydrogen with a rocky core overlaid by a deep layer of metallic hydrogen.


Moons of Jupiter – the Galilean moons.

Io Europa

Ganymede Callisto

Jupiter has in total 65 confirmed moons.


Saturn

Average distance from Sun = 1,422 x 10⁶ km





Atmosphere of Hydrogen with a small rocky core.


Moons of Saturn

Saturn has at least 62 moons.

DioneMimas


Titan Enceladus

Iaputus

Hyperion


Saturn’s rings

The rings are 93% water ice and very thin – 20 m !!!

They are composed of objects ranging in size from mm to meters.


Uranus



Albedo = 0.3 (Bond)



Atmosphere of Hydrogen with a small rocky core.

Rotates on its side relative to the plane of the solar system.

Discovered 1781 March 13 by William Herschel.


Moons of Uranus

Uranus has 27 known moons and a ring system.


Neptune






Atmosphere of Hydrogen, Helium and Methane with a small rocky core.

Discovered 1846 September 23 by Urbain Le Verrier, John Couch Adams and Johann Galle.


Moons of Neptune

Neptune imaged in Methane light andShowing Proteus, Larissa, Galatea andDespina.

Neptune also possesses a ring.


Trans-Neptunian objects

A trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are namedafter Pluto, which follows an orbit trapped in the same resonance

Plutinos form the inner part of the Kuiper belt and represent about a quarter of the known Kuiper belt objects (KBOs).



Comets

Comets were historically seen as a omen (good or bad)

We now know them to be ‘dirty snow balls’ consistingof rock and ice.

Comets are made visible to us by the production of a ‘tail’. This is produced by the heating of the comet as it approaches the Sun causing the ice to sublimate taking dustwith it forming a coma. The tail always points away from the Sun.

Comets generally have two tails. One a dust tail and the other an ion tail. The dusttail is pushed back by solar radiation pressure and curves away from the comet.The ion tail is driven by charged particles in the solar wind.

Comet tails are the largest structures in the solar system and can be 100,000,000 kmlong.



Comet P1 McNaught


It is now known that most meteor showers are associated with particular comets.

Name Dates Date of Max

Max ZHR

r Comet

Lyrids (LYR) 16-25 Apr 22 Apr 20 2.1 C/1861 G1 Thatcher

Eta Aquarids (ETA)

19 Apr –28 May

6 May 60 2.4 1P/Halley

Perseids (PER) 17 Jul – 24 Aug

12 Aug 0-100 2.6 109P/Swift-Tuttle

Orionids (ORI) 2 Oct – 7 Nov

21 Oct 25 2.5 1P/Halley

Leonids (LEO) 14 – 21 Nov 17 and 18 Nov

20+ 2.5 55P/Tempel-Tuttle

Geminids (GEM) 7 – 17 Dec 13 Dec 120 2.6 3200 Phaethon


Comets are thought tooriginate in the Oort cloud.

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Martin Crow Crayford Manor House Astronomical Society The Solar System