Download The Moon

Review for Exam #1
Tuesday, September 29
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Observations of the sky and how things move in it
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led to Newton and Laws of Motion and Gravity
Reviewed some aspects of Light, Radiation, Matter, and
Forces
Noticed patterns in objects in Solar System (composition,
location, age, etc.)
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led to theories about how Solar System formed
The Celestial Sphere
The sky as seen from Earth is divided into 88 constellations.
It is convenient to pretend the stars are attached to a celestial
sphere.
The celestial sphere appears to rotate about the celestial
poles (1 day).
The Sun appears to move west to east relative to stars
year).
(1
The Moon appears to move west to east relative to stars (1
month).
Celestial Sphere: A large imaginary sphere centered on Earth
Season & Calendars
The cause of the seasons is the tilt of the Earth’s
rotation axis relative to its orbit around the Sun.
The day is based on the time between one noon and
the next.
The year is based on the time between one vernal
equinox and the next.
The moon (month) is based on the time between one
new moon and the next.
Moon Phases & Eclipses: Key Concepts
Lunar phases change as we see more or less of the Moon’s
sunlit half.
The Moon rotates about its axis as it revolves around the
Earth.
The sidereal month=27.3 days; the synodic month=29.5
days.
A lunar eclipse occurs when the Moon passes through the
Earth’s shadow.
A solar eclipse occurs when the Earth passes through the
Moon’s shadow.
Solar eclipses occur when Moon
is between Sun and Earth.
Solar eclipses occur at NEW MOON.
A lunar eclipse occurs when the Moon passes
through the Earth’s shadow.
Lunar eclipses occur when Earth is between Sun
and Moon.
Lunar eclipses occur at FULL MOON.
From Ptolemy to Copernicus: Key Concepts
Aristotle (4th cent BC) showed that the Earth is round.
Greek astronomers developed a geocentric model for the
universe.
Ptolemy (2nd cent) used epicycles to explain retrograde
motion of planers.
Copernicus (16th cent) proposed a heliocentric model for the
universe.
In the model of Copernicus, retrograde motion is easily
explained.
The combination of small and
large circles produces “loopthe-loop” motion.
Tycho, Kepler, & Galileo: Key Concepts
Tycho Brahe made accurate measurements of planetary
motion.
Planetary orbits are ellipses with the Sun at one focus.
A line between planet & Sun sweeps out equal areas in equal
times.
The square of a planet’s orbital period is proportional to the
cube of its average distance from the Sun.
Galileo made telescopic observations supporting the
heliocentric model.
Kepler’s First Law
of planetary motion
The orbits of planets around the Sun are
ellipses with the Sun at one focus.
Kepler’s Second Law
of planetary motion
A line
to a planet
from the Sun
sweeps out equal
areas in equal time intervals.
Kepler’s Third Law
of planetary motion
The square of a planet’s orbital period is
proportional to the cube of its average
distance from the Sun*:
*A planet’s average distance from the Sun is equal to the
semimajor axis of its orbit.
Newton’s Laws
Three Laws of Motion:
(1)An object remains at rest, or moves in a straight line at
constant speed, unless acted on by an outside force.
(2) The acceleration of an object is directly proportional to
force, and inversely proportional to mass.
(3) For every action, there is an equal and opposite reaction.
Law of Gravity:
The gravitational force between masses M and m, separated
by distance r, is proportional to the product of the masses
divided by the square of the separation
Applying Newton’s Laws
Newton modified and expanded Kepler’s Laws of
Planetary Motion.
Kepler described how planets move; Newton
explained why they move.
Tides are caused by the difference between the
Moon’s gravitational force on different sides of the
Earth.
Tidal forces are slowing the Earth’s rotation &
enlarging the Moon’s orbit.
Newton’s First Law of Motion:
An object remains at rest, or moves in a
straight line at constant speed, unless acted
on by an outside force.
Precise mathematical laws require precise definitions of
terms:
SPEED = rate at which an object changes its position.
Example: 65 miles/hour.
VELOCITY = speed plus direction of travel.
Example: 65 miles/hour to the north.
Newton’s Second Law of Motion:
The acceleration of an object is directly
proportional to the force acting on it, and
inversely proportional to its mass.
In mathematical form:
Or alternatively:
Newton’s Third Law of Motion:
For every action, there is
an equal and opposite reaction.
Whenever A exerts a force on B, B exerts a force
on A that’s equal in size and opposite in
direction.
All forces come in pairs.
Kepler’s
Third Law:
Light
Visible light is just one form of
electromagnetic radiation.
Light can be though of as a wave or as a
particle.
Light forms a spectrum from short to long
wavelengths.
A hot, opaque object produces a continuous
blackbody spectrum.
Light forms a spectrum from short to long
wavelength
Visible light has wavelengths from 400 to 700
nanometers. [1 nanometer (nm) = 10-9 meter]
Color is determined by wavelength:
Blue: 480 nm
Green: 530 nm
Red: 660 nm
Visible light
occupies only
a tiny sliver
of the full
spectrum.
Matter and Forces
Matter can come in various forms that are composed
of fundamental particles
An element is known by it number of protons
Isotopes of an element contain different number of
neutrons
Isotopes can be radioactive and spontaneously decay
There are four fundamental forces (Gravity,
Electromagnetism, Strong, and Weak)
Hydrogen
1 proton
1H
2H
3He
4He
Helium
2 protons
Lithium
3 protons
6Li
Proton:
7Li
Neutron:
3H
Spectra
A hot, transparent gas produces an emission
spectrum.
A cool, transparent gas produces an absorption
spectrum.
Every type of atom, ion, and molecule has a unique
spectrum.
The most abundant elements in the universe are
hydrogen and helium.
The radial velocity of an object is found from its
Doppler shift.
Continuum
Source
Cloud
Solar System Constituents
The terrestrial planets are made primarily of rock
and metal.
The Jovian planets are made primarily of hydrogen
and helium; also have large amounts of water,
methane, and ammonia
Moons (a.k.a. satellites) orbit the planets; some
moons are large.
The terrestrial planets are made primarily of
rock and metal.
Mercury, Venus, Earth, & Mars.
The terrestrial planets are:
low in mass (< Earth mass)
high in density (> 3900 kg/m3).
Water = 1000 kg/m3
Air = 1 kg/m3
Rock = 3000 kg/m3
The Earth
The study of seismic waves tells us about the Earth’s
interior.
The Earth is layered into crust, mantle, inner core,
and outer core.
The Earth is layered because it underwent
differentiation when molten.
The crust is broken into plates that move relative to
each other.
Seismic waves
radiating through
the Earth after an
earthquake:
Note: S waves do
not travel through
the outer core!
The Moon
The Moon’s surface has both smooth maria
and cratered highlands.
The surface was shaped by heavy
bombardment, followed by lava floods.
The Moon has a thick crust but a tiny iron-rich
core.
The Moon may have been ejected when a
protoplanet struck the Earth.
Computer simulation
of impact:
Mantle of the
colliding body was
ejected to form the
Moon.
Iron core of the
colliding body sank
to the Earth’s center.
Mercury
Mercury has a 3-to-2 spin-orbit coupling (not
synchronous rotation).
Mercury has no permanent atmosphere because
it is too hot.
Like the Moon, Mercury has cratered highlands
and smooth plains.
Mercury has an extremely large iron-rich core.
Radius of
Mercury =
2400 km.
Radius of iron
core =
1800 km.
Venus
The surface of Venus is hidden from us by
clouds of sulfuric acid.
The atmosphere of Venus is hot because of a
runaway greenhouse effect.
The surface of Venus shows volcanic activity
but no plate tectonics.
The interior of Venus is similar to that of the
Earth.
The interior of Venus is similar to that of
the Earth.
Uncompressed
density of Venus =
uncompressed
density of Earth =
4200 kg/m3.
Venus probably has a
metal core and
rocky mantle, like
the Earth.
Mars
Mars has a tenuous atmosphere, with little
water vapor and few clouds.
(Mars has large volcanoes and a huge rift
valley, but no plate tectonics.
Robotic “rovers” have given us a close-up look
at Mars.
Mars has two small irregular moons, Phobos
and Deimos.
Mars
interior:
Jupiter and Saturn
Jupiter and Saturn consist mainly of hydrogen
and helium.
Jupiter and Saturn have belts and zones of
clouds, plus circular storms.
Jupiter and Saturn have magnetic fields created
in metallic hydrogen.
Differences between Jupiter and Saturn are due
to Jupiter’s higher mass.
All Jovian planets have rings.
Jupiter and Saturn
are
differentiated.
Moons of Jupiter and Saturn
The Galilean Moons of Jupiter:
Callisto: heavily cratered
Ganymede: larger then Mercury
Europa: covered with smooth ice
Io: volcanically hyperactive
The Giant Moon of Saturn:
Titan: wrapped in an atmosphere
4 of the moons of Jupiter are large (> 3000 km across)
and spherical (like our Moon).
These are the four Galilean moons:
Io, Europa, Ganymede, Callisto
Titan: Saturn’s ATMOSPHERIC moon
Nearly the same size as
Ganymede: escape
speed is the same.
Twice as far from
the Sun as Ganymede:
temperature is lower.
Titan, alone among
moons, has a
substantial atmosphere.
Uranus and Neptune
Uranus and Neptune are nearly identical in
their internal structure.
The rotation axis of Uranus is tilted by about
90 degrees, causing extreme seasons.
Neptune has surprisingly strong storms, driven
by internal heat.
Triton, the giant moon of Neptune, is a cold
world with nitrogen geysers.
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Interiors of Uranus and Neptune
Gaseous atmosphere: hydrogen, helium, methane
Liquid outer layer: hydrogen, helium
Liquid or slushy mantle: water, ammonia
Solid core: rock, metal
Triton: Neptune’s Frosty Moon
Surface
temperature =
38 Kelvin.
Covered with
“frost” at poles:
frozen methane,
frozen nitrogen.
Pluto and its moon Charon are icy worlds that resemble Triton.
Eris, the troublemaker (Greek goddess of strife).
The Kuiper belt, beyond Neptune, contains small, icy, Pluto-like
objects.
The icy Kuiper Belt Objects are leftover planetesimals.
Comets are “dirty snowballs”: ice mixed with dust & carbon
compounds.
Most comets are in the Kuiper belt or the Oort cloud, far from
the Sun.
A comet or asteroid impact may have caused the extinction of
dinosaurs
Studies of the Outer Solar System continue.
Pluto and Charon have many properties in
common with Neptune’s moon Triton.
•  Cold surfaces (about 40 Kelvin)
•  Icy mantles and rocky cores (about
2000 kg/m3)
•  Pluto has a thin atmosphere (like Triton);
Charon has none.
Eris (“Xena”), the troublemaker.
Discovered in 2005 by Mike
Brown and collaborators.
It has a moon.
It is BIGGER than Pluto!
Led to re-definition of what
a Planet is
Created new class of object
called “Dwarf Planets
Most comets are in the Kuiper belt or the
Oort cloud, far from the Sun.
Comets with short orbital periods come from the
Kuiper belt,
30-50 A.U.
from the Sun.
We know the Kuiper
belt is full of icy
objects – we
have seen them!
Origin of the Solar System: Key Concepts
How the Solar System formed:
A cloud of gas & dust contracted to form a diskshaped solar nebula.
The solar nebula condensed to form small
planetesimals.
The planetesimals collided to form larger planets.
When the Solar System formed:
Radioactive age-dating indicates the Solar System is
4.56 billion years old.
The contraction of the solar nebula made it spin faster
and heat up. (Compressed gas gets hotter.)
Temperature of solar nebula:
> 2000 Kelvin near Sun; < 50 Kelvin far from Sun.
How does this “nebular theory” explain the
current state of the Solar System?
Solar System is disk-shaped:
It formed from a flat solar nebula.
Planets revolve in the same direction:
They formed from rotating nebula.
Terrestrial planets are rock and metal:
They formed in hot inner region.
Jovian planets include ice, H, He:
They formed in cool outer region.
Radioactive age-dating
Radioactive decay: Unstable atomic nuclei emit
elementary particles, forming a lighter, stable nucleus.
Example: Potassium-40 (19 protons + 21 neutrons = 40)
89% of the time, Potassium-40 decays to Calcium-40.
11% of the time, Potassium-40 decays to Argon-40.
Age of oldest Earth rocks = 4 billion years
Age of oldest Moon rocks = 4.5 billion years
Age of oldest meteorites (meteoroids that survive the
plunge to Earth) = 4.56 billion years
This is the age of the Solar System