The Solar System
THE EARTH – MOON SYSTEM
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
Some Physical data
Earth
Mean Earth Sun distance = 149.6 x 10⁶ km (1 A.U.)
Inclination to the Sun’s equator = 7.16°
Axial tilt = 23° 26’ 21”
Albedo = 0.3 (Bond)
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)
Martin Crow
Crayford Manor House Astronomical Society
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 frame
of reference.
Draconic month = 27.2122 days (the nodes precess over a period of 18.6 years)
Synodic period = 29.530589 d (29 d 12 h 44 min 2.9 s) New Moon to new Moon and
is commonly called a Lunar month.
Anomalistic month = 27.5546 days (the line of Apsides connecting Perigee and
Apogee precesses over a period of 8.85 years.
Albedo = 0.12 (Bond)
Mean diameter = 3,474 km
Mass = 7.35 × 1022 kg
Mean density = 3.35 x 10³ kg/m³
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
One possibility is that Theia formed
in the Lagrangian point L4 or L5.
Its growing mass and gravitational
Perturbations caused it to be destabilised
and collide with the Earth.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
The Lunar surface
Near side
Martin Crow
Far side
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Structure
Earth
Moon
Martin Crow
Crayford Manor House Astronomical Society
Life on Earth
Martin Crow
Crayford Manor House Astronomical Society
Consequences of a gravitationally bound system.
Due to tidal drag the Moon’s rotation has become locked into its rotation
around the Earth.
The effect of this is that the Moon rotates only once for every orbit. Which
is 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 constant
over 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 400
days long with each day approx. 21.9 hrs.
Martin Crow
Crayford Manor House Astronomical Society
The local gravitational environment.
The Lagrangian points.
Martin Crow
Crayford Manor House Astronomical Society
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
Martin Crow
Synodic period = 29.530589 d
Crayford Manor House Astronomical Society
The Moons orbit
The Moons orbit is an ellipse.
The Moons orbit is inclined at an angle
of 5° 8´ to the Ecliptic.
The line of Apsides joins the points of
Perigee and Apogee and precesses over
8.85 years.
The Moon crosses the same node every
27.2122 days (the Draconic month)
The Nodes precess in a retrograde
motion over a period of 18.6 years.
Lunar and Solar eclipses can only occur
when 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.
Martin Crow
Crayford Manor House Astronomical Society
Images showing the apparent size difference due to the Moon’s non circular orbit and also
Libration.
Martin Crow
Crayford Manor House Astronomical Society
Eclipses
How does a solar eclipse occur.
Martin Crow
Crayford Manor House Astronomical Society
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.
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.
At this time there are 41 different Saros series in progress.
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 series
will travel from south to north and visa versa.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Annular eclipse
Total eclipse showing the solar corona
A Solar eclipse will only last a few minutes and depends on the geometry of the Earth,
Sun, Moon alignment.
The totally eclipsed Sun is safe to look directly at. Bright stars and any visible planets
will be seen in the sky.
Martin Crow
Crayford Manor House Astronomical Society
The 1919 Solar eclipse observed by Sir Arthur Eddington gave observational
evidence that proved the Einstein’s theory of general relativity was correct.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Cornwall 1999 Aug 11
Crayford Manor House Astronomical Society
MVCrow
Martin Crow
Crayford Manor House Astronomical Society
The 2006 Turkey eclipse as seen from the ISS.
Martin Crow
Crayford Manor House Astronomical Society
Diamond ring
China 2008 Aug 01
Martin Crow
MV Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Eclipses
How does a lunar eclipse occur.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
The Planets etc.
Martin Crow
Crayford Manor House Astronomical Society
The Planets
Martin Crow
Crayford Manor House Astronomical Society
Mercury
Average distance from Sun = 57.9 x 10⁶ km
Diameter = 4,878 km
Albedo = 0.068 (Bond)
Has quite an eccentric orbit
Has a 3:2 resonance so it rotates three
times for every two orbits.
Takes 88 days to complete one orbit.
(Sidereal period)
The synodic period is 115.9 days.
Martin Crow
Crayford Manor House Astronomical Society
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 surface
pressure of 93 bar (Earth = 1 bar)
Surface temperature of 460° C
Martin Crow
Crayford Manor House Astronomical Society
Surface contour map of Venus
Martin Crow
Crayford Manor House Astronomical Society
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 surface
pressure 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.
Martin Crow
Crayford Manor House Astronomical Society
Surface contour map of Mars
Martin Crow
Crayford Manor House Astronomical Society
The Asteroids
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.
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.
NASA Galileo probe
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
Asteroid groups
Martin Crow
Crayford Manor House Astronomical Society
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)
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
Moons of Jupiter – the Galilean moons.
Io
Europa
Ganymede
Jupiter has in total 65 confirmed moons.
Martin Crow
Crayford Manor House Astronomical Society
Callisto
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.
Martin Crow
Crayford Manor House Astronomical Society
Moons of Saturn
Saturn has at least 62 moons.
Martin Crow
Crayford Manor House Astronomical Society
Iaputus
Titan
Hyperion
Martin Crow
Enceladus
Mimas
Crayford Manor House Astronomical Society
Dione
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.
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
Moons of Uranus
Uranus has 27 known moons and a ring system.
Martin Crow
Crayford Manor House Astronomical Society
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.
Martin Crow
Crayford Manor House Astronomical Society
Moons of Neptune
Neptune imaged in Methane light and
Showing Proteus, Larissa, Galatea and
Despina.
Neptune also possesses a ring.
Martin Crow
Crayford Manor House Astronomical Society
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 named
after 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).
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Comets
Comets were historically seen as a omen (good or bad)
We now know them to be ‘dirty snow balls’ consisting
of 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 dust
with 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 dust
tail 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 km
long.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
Comet P1 McNaught
Martin Crow
Crayford Manor House Astronomical Society
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
13 Dec
120
2.6
3200 Phaethon
Geminids (GEM) 7 – 17 Dec
Martin Crow
Crayford Manor House Astronomical Society
Comets are thought to
originate in the Oort cloud.
Martin Crow
Crayford Manor House Astronomical Society