The Earth – Moon System
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Transcript of The Earth – Moon System
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THE EARTH – MOON SYSTEM
Martin Crow Crayford Manor House Astronomical Society
The Solar System
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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.
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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.
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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)
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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.
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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.
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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.
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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.
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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.
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The Lunar surface
Near side Far side
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Structure
Earth
Moon
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Life on Earth
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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.
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The local gravitational environment.
The Lagrangian points.
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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
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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.
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Images showing the apparent size difference due to the Moon’s non circular orbit and alsoLibration.
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Eclipses
How does a solar eclipse occur.
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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.
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Total eclipse showing the solar coronaAnnular 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.
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The 1919 Solar eclipse observed by Sir Arthur Eddington gave observationalevidence that proved the Einstein’s theory of general relativity was correct.
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Martin Crow Crayford Manor House Astronomical SocietyCornwall 1999 Aug 11 MVCrow
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The 2006 Turkey eclipse as seen from the ISS.
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Diamond ring
China 2008 Aug 01 MV Crow
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Eclipses
How does a lunar eclipse occur.
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The Planets
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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.
Albedo = 0.068 (Bond)
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Venus
Average distance from Sun = 108.9 x 10⁶ km
Diameter = 12,102 km
Albedo = 0.9 (Bond)Takes 224.7 days to complete one orbit.(Sidereal period)The synodic period is 583.9 days.
Very dense atmosphere of CO₂ with a surfacepressure of 93 bar (Earth = 1 bar)
Surface temperature of 460° C
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Mars
Average distance from Sun = 227.9 x 10⁶ km
Diameter = 6,792 km
Albedo = 0.25 (Bond)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.
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Jupiter
Average distance from Sun = 778.6 x 10⁶ km
Diameter = 142,984 km at equator
Albedo = 0.34 (Bond)Takes 11.9 years to complete one orbit.(Sidereal period)The synodic period is 399 days.
Atmosphere of Hydrogen with a rocky core overlaid by a deep layer of metallic hydrogen.
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Moons of Jupiter – the Galilean moons.
Io Europa
Ganymede Callisto
Jupiter has in total 65 confirmed moons.
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Saturn
Average distance from Sun = 1,422 x 10⁶ km
Diameter = 120,536 km at equator
Albedo = 0.34 (Bond)Takes 29.5 years to complete one orbit.(Sidereal period)The synodic period is 378 days.
Atmosphere of Hydrogen with a small rocky core.
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Moons of Saturn
Saturn has at least 62 moons.
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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.
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Uranus
Average distance from Sun = 2,876 x 10⁶ km
Diameter = 51,118 km at equator
Albedo = 0.3 (Bond)Takes 84.3 years to complete one orbit.(Sidereal period)The synodic period is 370 days.
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.
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Moons of Uranus
Uranus has 27 known moons and a ring system.
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Neptune
Average distance from Sun = 4,503 x 10⁶ km
Diameter = 49,528 km at equator
Albedo = 0.29 (Bond)Takes 164.8 years to complete one orbit.(Sidereal period)The synodic period is 367.5 days.
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.
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Moons of Neptune
Neptune imaged in Methane light andShowing Proteus, Larissa, Galatea andDespina.
Neptune also possesses a ring.
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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).
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