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Astronomy 12 Test Review
Chapter 4 (Gravity)
1. Fill in the Blanks
____________________________ models of the universe, such as the Ptolemaic model, have the Sun, the
Moon, and all the other planets orbiting Earth. ____________________________ model of the solar system holds that
Earth, like all the other planets, orbits the Sun and was made popular by ____________________________. This model
naturally explains ____________________________ motion, which is the backward or westward motion of the planets as
viewed from Earth. Retrograde motion was explained using ____________________________ in the geocentric model.
____________________________ motion, which is the direct or eastward planetary motion.
____________________________ is the apparent shift in the position of a distant star, due to Earth’s revolution around
the Sun. Farther stars have a ____________________________ parallax than nearer ones. A
____________________________ is defined as the distance to a star, which has a parallax of one arcsecond. The
distance to a star (in parsecs) is equal to the reciprocal of the ____________________________.
____________________________ was the first experimental scientist. His telescopic observations of the Sun,
the Moon, and the planets provided experimental evidence against the heliocentric theory and supporting Copernicus’s
heliocentric model. ____________________________ constructed a set of three simple laws describing the motions of
the planets, explaining the mass of observational data accumulated by his benefactor, Tycho Brahe.
Kepler’s three laws of planetary motion state that (1) planetary orbits are ____________________________ having the
Sun as one focus, (2) a planet moves fastest at its orbital ____________________________ and slowest at its orbital
____________________________, and (3) the ratio of a planet’s orbital ____________________________ cubed to its
orbital ____________________________squared is the approximately same for all planets.
An object at rest tends to stay at rest or if in motion, tends to stay in motion. This statement is called Newton’s
____________________________. To make a body speed up, slow down and/or change direction, a
____________________________ must be applied. The rate of change of velocity, or acceleration, is equal to the applied
force divided by the body’s mass. This statement is called Newton’s ____________________________. Forces always
act in pairs that are equal in size but opposite in direction. This statement is called Newton’s
____________________________. ____________________________ attracts the planets to the Sun. Every object
having any mass exerts a gravitational force on all other objects, and the strength of this force
____________________________ with the square of the distance, according to an inverse-square law.
____________________________ are the raising of Earth’s oceans (and to a lesser degree, Earth’s land mass) due to
gravity. ____________________________ tides are that cause the greatest deformation of the Earth;
____________________________ tides are that cause the least deformation of the Earth.
Answers to fill in blanks (in order):
Geocentric
Perihelion
Heliocentric
Aphelion
Copernicus
Radius
Retrograde
Period
Epicycles
First Law of Motion
Prograde
Second Law of Motion
Parallax
Third Law of Motion
Smaller
Gravity
Parsec
Decreases
Parallax (in arcsec)
Tides
Galileo
Spring
Kepler
Neap
Elliptical
2. Sketch and label Ptolemy’s geocentric model for the planet Mars, highlighting the deferent and epicycle.
3. What discoveries of Galileo helped confirm the Copernican model.
 phases of Venus
 Jupiter’s moons
 Rings of Saturn
 Moon landscape (topography)
 Sunspots
4. Ceres is an asteroid that revolves around the Sun in 4.6 years. What is the orbital radius of Ceres? (Remember, the
orbital radius of Earth is 1 A.U. and orbital period is 1 year)
 P2 = a3  a = cube root [(4.6 years)2] = 2.77 AU.
5. Pallas is an asteroid that revolves around the Sun with an orbital period of 4.62 years and an average orbital radius of
2.77 A.U. Vesta is another asteroid with an orbital radius of 2.36 A.U. What is the orbital period of Vesta?
 P2 = a3  P = square root [(2.36 AU)3] = 3.63 years
6. List the revisions made by Newton to Kepler’s laws of planetary motion.
 1st law: Newton agreed with Kepler and added that orbits of object revolving around the Sun may be
elliptical, circular, hyperbolic or parabolic (conic sections)
 2nd law: Newton agreed that a planet travels the fastest at the perihelion and slowest at the aphelion due
to gravity
 3rd law: Newton expanded P2 = a3, showing that P2/ a3 = G(Msun + Mplanet) using the law of universal
gravitation. This revision allowed him to calculate the mass of the Sun.
7. Jimmy, a 60 kg weakling, accelerates in a car at 4 m/s2. What is size of the force accelerating Jimmy?
 F = m*a = (60 kg)( 4 m/s2) = 240 N
8. A 50 kg rock is pushed with a force of 200 N. What is the acceleration of the rock?
 a = F/m = (200 N)/(50 kg) = 4 m/s2
9. The Sun exerts a gravitational force on the Earth, with a particular size (or magnitude) towards the Sun. What is the
size and direction of the gravitational force Earth exerts on the Sun? Explain.
 Use FG = G (Msun*Mearth)/R2 = 3.98 x 10^13 N towards the Earth (Msun = 1.98 x 10^30; Mearth = 5.98 x
10^24; G = 6.67 x 10^-11)
10. Jupiter is 5 A.U. from the Sun. An unknown planet, Q, of equal mass to Jupiter is 40 A.U. from the Sun. Compare the
size of the gravitational force on Jupiter by the Sun with that of planet Q.
 If the radius increases by a factor of 8 (40/5), then the size of the gravitational force will decrease by a
factor of 82 = 64
11. Complete the table below:
Star Name
Parallax
Distance
(arcsec)
(parsecs)
11.1
Arcturus
0.090
0.288
Procyon
3.472
166.67
Hadar
0.006
Rigel
0.004
250
12. Draw Sun, Moon and Earth configurations that would cause a) neap and b) spring tides.
Chapter 7 (Comparative Planetology)
13. List the planets in order from nearest to farthest from the Sun.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
14. Describe Bode’s Law.
A pattern of roughly determining the distances to each planet in the Solar System from the Sun (in AU)
Start with 0, 3, 6, 12, 24, 48, 96, 192, 384
Then add ‘4’…4, 7, 10, 16, 28, 52, 100, 196, 388
Divide by ‘10’…0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, 19.6, 38.8 (note: 2.8 AU is the distance to the Asteroid Belt)
15. Construct a Venn Diagram in order to compare and contrast the properties of terrestrial and jovian planets. Properties
to consider in your analysis: mass, density, chemical composition, moon and rings, distance from the Sun, outer (or
surface) temperatures.
Terrestrial






Jovian

Lower mass
Higher density
Heavier
elements/compounds
(i.e. iron, silicates)
Few or no moons/no
rings
Closer to Sun
Higher
surface/atmospheric
temperatures



Both planets in Solar
Systems
Travel around Sun in
nearly circular orbits
Have cleared out debris
in their orbital path over
time
Have nearly a
spheroidal shape due to
gravity






Higher mass
Lower density
Lighter
elements/compounds
(i.e. hydrogen, helium)
Numerous moons/rings
Farther from Sun
Lower
surface/atmospheric
temperatures
16. Define planet. Why is Pluto not a planet?
A planet is any object that orbits a star, is rounded due to its own gravity and has cleared out debris in its
orbit. Pluto is not a planet because it has not cleared out material in its orbit (i.e. is Charon its moon or part
of a double-moon binary system? Is Pluto a Kuiper Belt object) and not sufficiently spherical
17. Where are asteroids mostly found in the Solar System?
Asteroid Belt
18. Contrast the Asteroid Belt and the Kuiper Belt, in terms of their location in the Solar System and the objects they
contain.
The Asteroid Belt between Mars and Jupiter contains numerous small, rocky planetoids. The Kuiper belt,
which extends from Neptune's orbit out to 50 AU, is thought to be the source of short-period comets, which
are icy, “dirty snowballs” composed mostly of silicates and water.
19. Contrast the Kuiper Belt and the Oort Cloud, in terms of their distance from the Sun and the objects they contain.
The Kuiper belt, which extends from Neptune's orbit out to 50 AU, is thought to be the source of short-period
(orbital period of less than 200 years) comets, which are icy, “dirty snowballs” composed mostly of silicates
and water. The Oort cloud, is also a comet reservoir of long-period comets (orbital period greater than 200
years), and is perhaps as far as 50,000 AU from the Sun.
20. What is the relationship between the extent to which a planet or satellite (i.e. moon) is cratered and the amount of
geologic activity on that planet or satellite?
Geologic activity on a planet or satellite will erase evidence of cratering.
21. Why do smaller planets retain less of their internal heat?
The smaller the planet or satellite, the greater its surface area relative to its volume, and it radiates energy
more readily into space.
22. Why is a large planet more likely to have a magnetic field than a small planet?
A larger planet has more temperature and pressure in its core that causes material to become ionized (or lose
electrons) and thus charged. Movement of this material as magma develops an electric current which
produces a magnetic field.
23. How are magnetic fields in terrestrial planets produced? In Jovian planets?
In terrestrial planets, magnetic fields are produced by magma currents in liquid iron-nickel core; in jovian
planets, magnetic fields are produced by liquid metallic hydrogen
Chapter 8 (Origin of the Solar System)
24. What are 3 key properties of the Solar System that must be accounted for when developing any model of its
formation?
Sizes and compositions of terrestrial vs. jovian planets
Directions and orientations of planetary orbits
Sizes of terrestrial planet orbits vs. jovian planet orbits
25. Describe the nebular theory of Solar System formation. Be sure to elaborate how
a. Terrestrial planets and asteroids were formed (i.e. planetesimal accretion).
b. Jovian planets and comets were formed (i.e. core accretion and disk instability models).
Note: a diagram would assist any response to the question.
c.




Solar Nebula Forms
A huge cloud of cold gas and dust.
Many times larger than our present solar system.
Probably spinning very slowly.


Formation of the Protosun
Under the influence of its own gravity, the solar nebula condensed into a dense central region (the
protosun) and a diffuse outer region (the protoplanetary disk).
Began to spin faster, flattened out, and central region heated up.








Planetesimals
Instabilities in the rotating disk caused regions within it to condense into rings under the influence of
gravity.
Gradually, planetesimals formed in these rings through accretion and collisions.
Terrestrial or Rocky Planets
The planetismals attracted each other by gravity (accretion) and collided to build planets.
Closest to the protosun, only rocky material and metals with higher condensation temperatures, and so
the planets in this region are made mainly of these materials. Lighter elements, like hydrogen and
helium, were driven out towards the outer regions due to solar wind from protosun.
Leftover planetesimals that did not accrete into planets became the asteroids.


26.
27.
28.
a.
b.
c.
Jovian or Gas Giants
In the outer part of the disk where temperatures are lower, lighter elements with lower condensation
temperatures accreted to form planets
 Jovian planet formation occurred either through core accretion or disk instability models
o Core accretion model: Initially core of Jovian planets formed by accretion of solid materials;
then, gas accreted onto solid core to form gas giant.
o Disk instability model: Gases rapidly accrete in denser regions of the outer protoplanetary disk
and condense to form Jovian planets without a solid core.
 The remaining gases/ices that did not accrete to into gas giants were ejected to the furthest regions of the
disk to form the comets (in either the Kuiper Belt or Oort Cloud).
What role did condensation temperatures play in the formation of the planets?
The condensation temperature of a substance is the temperature at which the substance solidifies from gas.
In the inner part of the solar system where the temperature was high, only substances with highcondensation temperatures could become planets (i.e. heavier materials like iron, nickel and silicates formed
the terrestrial planets). In the outer part of the solar system, substances with low-condensation temperatures
could also become part of planets (i.e. lighter materials like hydrogen and helium formed the jovian planets).
The Solar System is approximately 4.6 billion years old. How is the age of the Solar System determined?
Radioactive elements are present in meteorites which is material leftover from the origin solar nebula that
formed the Solar System. Radioactive elements decay at a set rate (i.e. half-life). Knowing this rate, and the
amount of a radioactive sample in a meteorite, makes it possible to find the age of meteoritic material.
Meteorites have been found to be 4.6 billion years old.
Describe 3 physical processes that are essential in the nebular theory of solar system formation.
Heating, as the protosun becomes the current Sun
i. In-falling materials converts gravitational energy into thermal energy (heat) (i.e. Kelvin- Helmholtz
contraction).
ii. The dense materials collides with each other, causing the gas to heat up.
iii. Once the temperature and density gets high enough for nuclear fusion to start, a star is born.
iv. This process accounts for the chemical differentiation and location of the terrestrial and jovian
planets.
Spinning, which causes smoothing of the random motions
i. Conservation of angular momentum causes the in-falling material to spin faster and faster as they get
closer to the center of the collapsing cloud.
ii. This process accounts for the direction and orientation of planetary orbits.
Flattening  Protoplanetary disk
i. The solar nebula flattened into a disk.
ii. Collision between clumps of material turns the random, chaotic motion into a orderly rotating disk.
29. Describe 5 types of extrasolar planets.
a. Hot Jupiter - A type of extrasolar planet whose mass is close to or exceeds that of Jupiter (1.9 × 1027 kg), but
unlike in the Solar System, where Jupiter orbits at 5 AU, hot Jupiters orbit within approximately 0.05 AU of
their parent stars
b. Hot Neptune - An extrasolar planet in an orbit close to its star (normally less than one astronomical unit
away), with a mass similar to that of Uranus or Neptune
c. Pulsar Planet - A type of extrasolar planet that is found orbiting pulsars, or rapidly rotating neutron stars
d. Gas Giant - A type of extrasolar planet with similar mass to Jupiter and composed on gases
e. Super-Earth – A gaseous extrasolar planet with a mass higher than Earth's, but substantially below the mass
of the Solar System's gas giants
30. Describe 6 methods of detecting extrasolar planets.
a. Transit Method - If a planet crosses ( or transits) in front of its parent star's disk, then the observed visual
brightness of the star drops a small amount.The amount the star dims depends on the relative sizes of the
star and the planet.
b. Astrometry -This method consists of precisely measuring a star's position in the sky and observing how that
position changes over time. If the star has a planet, then the gravitational influence of the planet will cause
the star itself to move in a tiny circular or elliptical orbit. If the star is large enough, a ‘wobble’ will be
detected.
c. Doppler Shift or Radial Velocity - A star with a planet will move in its own small orbit in response to the
planet's gravity. The goal now is to measure variations in the speed with which the star moves toward or
away from Earth. In other words, the variations are in the radial velocity of the star with respect to Earth. The
radial velocity can be deduced from the displacement in the parent star's spectral lines (think ROYGBIV) due
to the Doppler effect.
d. Pulsar Timing - A pulsar is a neutron star: the small, ultra-dense remnant of a star that has exploded as a
supernova.•Pulsars emit radio waves extremely regularly as they rotate. Because the rotation of a pulsar is
so regular, slight changes in the timing of its observed radio pulses can be used to track the pulsar's motion.
Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulsetiming observations can then reveal the geometry of that orbit.
e. Gravitational Microlensing - The gravitational field of a star acts like a lens, magnifying the light of a distant
background star. This effect occurs only when the two stars are almost exactly aligned. If the foreground
lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the
lensing effect.
f. Direct Imaging - Planets are extremely faint light sources compared to stars and what little light comes from
them tends to be lost in the glare from their parent star. It is very difficult to detect them directly. In certain
cases, however, current telescopes may be capable of directly imaging planets.