Investigating Astronomy Timothy F. Slater, Roger A. F.docx

Investigating Astronomy
Timothy F. Slater, Roger A. Freedman
Chapter 4
Exploring Our Evolving Solar System
Welcome to chapter 4: Exploring Our Evolving Solar System.
This week we're going to study our solar system in the big
picture view by looking at the major components: the planets,
asteroids, trans-Neptunian objects, and comets. We’ll look at
how our solar system probably formed and how this suggests
that planetary formation is common.
*
Comparing the Planets: OrbitsThe Solar System to Scale* The
four inner planets are crowded in close to the Sun. The four
outer planets orbit the Sun at much greater distances.
*Planets are not to scale!
This diagram shows the orbits of the eight major planets of our
solar system to scale. Please note, however, that the size of the
planets themselves at this scale would be microscopic, so this is
only a scale of the relative sizes of their orbits around the Sun.
There are four inner planets that are all very close to the Sun:

Mercury, Venus, Earth, and Mars. Then there are four outer
planets whose distances from the Sun are significantly greater:
Jupiter, Saturn, Uranus, and Neptune.
*
Comparing the Planets:
Size and CompositionInner planets: rocky materials with dense
iron cores and high average densitiesOuter planets: primarily
light elements such as hydrogen and helium, low average
densities
Now let's compare the planets by size and composition. The
important thing to note here is that the groupings remain the
same, whether we compare orbit size, planet size, or
composition (or even, as we’ll see in a moment, number of
moons). The inner planets are all relatively small and are made
largely of rock with dense iron cores. Their average densities
(the mass divided by the volume) are high. The outer planets, on
the other hand, are all very large and are primarily gaseous
(mostly hydrogen and helium, though with some other
elements). Their average densities are low.
*
Moons are Natural SatellitesAll planets have moons, except
Mercury and Venus.The outer planets have many more moons
than the inner planets.Seven satellites are almost as big as the
inner planets.
The word moon is applied to any natural satellite of a planet or

other solar system body (aside from the Sun). The inner planets
have few if any moons. Earth, of course, has one. Mars has two.
But Mercury and Venus have none. The outer planets all have
many moons (numbered about in the dozens), with a few being
the same size as our Moon or larger, a couple almost rivaling
the size of the planet Mercury.
*
Determining Composition:
Bodies with Surrounding Atmospheres
Some of these planets and moons have atmospheres. How can
we determine what those atmospheres are made of? We can use
spectroscopy to look for the chemical fingerprints of different
elements and molecules. Saturn's moon Titan has a very dense
atmosphere (so much so that we had to send a radar and a lander
there to get a view of the surface… we'll see more about that in
chapter 7). When we look at Titan from Earth, we are seeing
sunlight reflected off of the atmosphere. If we look at Titan
with a spectroscope, we see absorption lines (remember those
are dark lines against a bright background of the spectrum) for
hydrogen, molecular oxygen (O2), and methane (CH4).
However, not all three 3 of those are actually in Titan's
atmosphere. Remember that it is the core of a star that is the
hot, opaque body that emits a continuous spectrum. That
continuous spectrum passes through the atmosphere of the Sun,
where there is hydrogen. So, those hydrogen atoms will absorb
some of the light of the spectrum leaving behind their
fingerprint before the light has even left the Sun. The light then
travels to Titan, reflecting off of its atmosphere. In the process,
some of the photons will be absorbed by the molecules of
methane, leaving their fingerprint on the spectrum. The

reflected light will then continue on, and some of it will arrive
at Earth. But before it gets to our spectroscope, some of it is
absorbed by the oxygen in our atmosphere, and so the oxygen
also leaves its fingerprint.
*
Determining Composition of TitanDips in the spectrum of
sunlight reflected from Titan are due to absorption by hydrogen
atoms (H), oxygen molecules (O2), and methane molecules
(CH4). Only methane is actually present in Titan’s atmosphere.
Astronomers must account for the absorption that takes place in
the atmospheres of the Sun and Earth.
When scientists examine the spectrum, they will see something
like the diagram we see here. This is another way of viewing a
spectrum. Instead of seeing the rainbow of colors with dark
lines, we see a measurement of intensity at each wavelength of
light. Intensity is basically how many photons of that
wavelength are received by the spectroscope. If more photons
are received of a particular wavelength, the graph is higher at
that point. The spectrum will have a curved shape, in general,
with a peak matching the wavelength of maximum emission as
we talked about in chapter 2. But against that general curve, we
will see the dips marking absorption lines. The narrow dip at a
bit under 660 nm is an absorption line marking the presence of
hydrogen. But as we saw, the spectrum acquired that absorption
line before the light even left the Sun. We also see a narrow dip
at just under 690 nm. That is the fingerprint of the oxygen
molecules in Earth's atmosphere. Then we see two wide
absorption lines that are the fingerprints of methane molecules.
From this, we know that Titan's atmosphere is composed of
methane. If it also had hydrogen and oxygen, we wouldn't know
that from this spectrum, because we have to account for the

absorption that occurs in the atmospheres of the Sun and Earth
during the light's journey. This is one of the reasons that we put
telescopes in space and send spacecraft to other planets, as we
get above the interfering effects of the Earth's atmosphere.
*
Determining Composition:
Planets without AtmospheresSpectra of reflected light is
compared to known substances.Infrared light from the Sun,
reflected from the surface of Europa, has almost exactly the
same spectrum as sunlight reflected from water ice.
We can also look at the spectra of light reflected from solid
surfaces, not just those with atmospheres. When light shines on
solid surfaces, some wavelengths are absorbed and others are
reflected. For example, leaves on Earth look green because the
leaves absorb red and violet light and reflect green light. Our
eyes see only the reflected green light. The spectrum of a solid
surface looks a little different than the spectrum of a gas, as the
spectrum of a solid surface will show broad absorption features,
rather than narrow lines and bands. We compare the spectra we
see from the solid bodies we look at with known spectra from
Earth. In the case of the spectrum on this slide, which is from
Jupiter's moon Europa, we see that it looks pretty much like the
spectrum that we know to be from water ice. So, the surface of
Europa is water ice.
*
The Jovian Planets Are Made of Light Elements

When we analyze the spectra of the outer planets: Jupiter,
Saturn, Uranus, and Neptune, we see that they are composed
mainly of the lightest elements: hydrogen and helium (mostly
hydrogen). Uranus and Neptune both contain methane, also.
These elements are all in gaseous form on the exterior of these
planets, and so they are often called the gas giant planets.
However, in the interiors of these planets, the pressure is so
high that these gases are actually in liquid form. It is also
thought that each of these four planets has a rocky core.
*
Asteroids100,000+ rocky objects within the orbit of Jupiter
Most orbit the Sun at distances of 2 to 3.5 AU, in the asteroid
beltOrbit the Sun in the same direction as the planetsThe
largest, Ceres, has a diameter of about 900 km (560 mi) Also
called minor planets
Let's move on to the smaller objects in our solar system. First,
the asteroids. There are more than 100,000 of these that orbit
mainly between Mars and Jupiter, though some are closer to the
Sun than Mars, and some are out as far as Jupiter. The area
between Mars and Jupiter, at a distance between 2 to 3.5
Astronomical Units is called the asteroid belt. They orbit the
Sun in the same direction as the planets, which tells us that they
formed along with the planets. The largest asteroid, Ceres, now
has the designation as a dwarf planet (along with Pluto and
others that we'll talk about soon). The asteroid in this picture is
433 Eros (all the asteroids, also called minor planets, have both
a number and a name…the number is sequential in the order of
discovery, and the name is chosen by the discoverer with
approval of the International Astronomical Union).
*

Trans-Neptunian
Objects1,000+ small bodies orbiting beyond the orbit of
Neptune The largest of these are known as dwarf planets Most
orbit within the Kuiper belt at 30 AU to 50 AUInclude Pluto,
Eris, Charon, Makemake, etc.
Another group of small solar system bodies is called trans-
Neptunian objects (trans-Neptunian just means “beyond
Neptune”). This group contains at least 1000 objects orbiting
beyond Neptune. The largest of these are known as dwarf
planets, and most orbit within the Kuiper belt. The most famous
of these is, of course, Pluto. Until a few years ago, Pluto was
considered the ninth planet of our solar system. However, as
we’ve learned more about our solar system, it has become clear
that it doesn’t fit into that category. As we’ve seen in this
chapter, the four inner planets share characteristics such as
being close to the Sun, being made of rock with iron cores,
having few if any moons, and having relatively high average
density. The four outer planets share characteristics such as
being further out from the Sun, being made primarily of light
elements with iron cores, having many moons, rings, and
relatively low average density. Pluto does not fit into either of
those groups. It has a much more eccentric orbit around the Sun,
and its orbit is inclined (or tilted) significantly with respect to
the other planets (as you can see in this diagram). It is made of
ice and has four moons. It does, however, fit into the group of
trans-Neptunian objects quite well. They are all icy and have
eccentric, high inclination orbits. The decision to change
Pluto’s classification was made following the discovery of Eris,
another trans-Neptunian object. Eris was thought at the time to
be bigger than Pluto, so a decision had to be made whether to

classify Eris as a planet or to create a new classification to
include both bodies (and others). As more large trans-Neptunian
objects were found (including Makemake, Haumea, and others),
it became clear that there could potentially be lots more. The
decision to change the classification of Pluto was controversial,
but the most important things to remember are that a)
reclassification happens in science a lot when new information
is acquired, and b) it doesn’t change the fact that Pluto is still
there, is still a fascinating world to investigate, and the New
Horizons spacecraft flew by it in 2015! The New Horizons
images have actually shown us that Pluto is, in fact, bigger than
Eris, and so it now holds the title of “King of the Kuiper Belt.”
*
CometsObjects that result when Kuiper belt objects
collideFragments a few kilometers across, diverted into new and
elongated orbits Oort cloud comets orbit out to 50,000 AUThe
Sun’s radiation vaporizes ices, producing tails of gas and dust
particlesAstronomers deduce composition by studying the
spectra of these tails created by reflected sunlight
Comets include both objects from the Kuiper belt and objects
from even further out, in an area called the Oort Cloud. These
objects can get knocked into elongated orbits as a result of
collisions or other gravitational disturbances, and these
elongated orbits bring them periodically into the inner solar
system. Comets are made mostly of ice. When they pass inside
the orbit of Jupiter, the Sun’s radiation is enough to vaporize
the ice on the surface, producing tails of gas and dust particles.
The solar wind, a stream of charged particles flowing out from
the Sun, pushes these tails away from the Sun. So, no matter
which way a comet is moving, its “tails” are always pointed
away from the Sun. We can learn about their composition by

analyzing the spectra of the tails. We’ve also sent spacecraft to
image them up close, to collect samples from the vaporized ices
and return them to Earth (the Stardust mission), and even to
crash into a comet to see what materials would come up (the
Deep Impact mission).
*
Cosmic “Recycling”The Big Bang produced H and He (some Li
and Be)―still commonAll heavier elements created by massive
stars, dispersed when stars dieOur solar system is recycled “star
dust”
How did our solar system form? Well, we’re going to cover
some of these in later weeks of the course, but the Big Bang
produced light elements. All of the hydrogen in the universe
was produced then, as well as a lot of the helium, and some
lithium and beryllium. But there’s lots more on the periodic
table of the elements than just these four lightest elements. All
the rest of the elements are created inside of massive stars, and
are dispersed into neighboring clouds of gas and dust when
those stars die. Those clouds of gas and dust then become home
to the next generation of stars. These next generations of stars
are then made of more heavy elements than the previous
generations. After several generations, there are enough heavy
elements to create planets. Our solar system, therefore, is made
of recycled “star dust.”
*
The Solar
Nebular HypothesisA cloud of interstellar gas and dust

contracts because of its own gravity. The cloud flattens and
spins more rapidly around its axis.A central condensation
develops that evolves into a glowing protosun.The planets form
out of the surrounding disk of gas and dust.
The solar nebular hypothesis, which is the idea of how planetary
systems form, states that a slowly spinning cloud of interstellar
gas and dust begins to contract when a knot of material gets
massive enough for its gravity to start pulling on nearby
material. As the cloud flattens and condenses, it begins to spin
more rapidly. The central region that is denser eventually
evolves into a protosun. The surrounding disk of gas and dust
eventually forms planets, as collisions gradually form larger and
larger objects.
*
Protoplanetary DisksRapid rotation flattens the nebula.~100,000
years after contraction begins, a rotating, flattened disk
surrounds what will become the protosun. Also called a proplyd,
planets form from its material.Explains why orbits all lie in the
same plane, in the same direction.
The rapid rotation in this disk is what causes the nebula to
flatten. This object, a flattened disk surrounding a protosun is
called a proplyd, or protoplanetary disk. We see these proplyds
in the sky (a lot of them in the Orion Nebula). This solar
nebular hypothesis explains why orbits in our solar system all
lie essentially in the same plane, orbiting the Sun in the same
direction.
*

Temperatures in the Solar NebulaTemperatures varied across
the solar nebula as the planets were forming. A general decline
in temperature with increasing distance from the center of the
nebula. Beyond 5 AU from the center of the nebula,
temperatures were low enough for water to condense and form
ice.Beyond 30 AU, methane (CH4) could also condense into ice.
The temperature varied throughout the solar nebula during the
planetary formation stage, gradually declining as distance from
the center increased. Beyond 5 Astronomical Units out from the
center, temperatures were low enough to form water ice (that’s
why we see ice on the moons of the outer planets). Further out,
beyond 30 AU, methane also condenses into ice (we see
methane ice on the trans-Neptunian objects.
*
Planetesimals Become Protoplanets,
then Rocky Planets
Since we can’t actually watch a solar system forming, scientists
can use computer simulations to test out hypotheses. These
diagrams show the results of one such simulation. It starts with
100 planetesimals (small bodies) orbiting the Sun. As these
objects collide, more massive objects form. After about 30
million years, there are 22 larger planetesimals, and after a total
of 441 million years, there are four planets. Our solar system’s
protoplanetary disk started out with many more than 100
planetesimals, but the simulation does show the hypothesis
stands up.
*

Outer Planet Formation:
Capturing an Envelope of Gas
Cold, slow moving gases were gravitationally attracted to the
Jovian planet cores.
How did the outer planets become so different from the inner
planets? Out at their distance, cold, slow moving gases were
attracted to the planetesimals there by gravity. The planetesimal
continued to attract both rocky and gaseous material until the
rocky mass and gaseous mass were about equal. At that point, it
would actually have been able to much more rapidly attract
more gas, eventually growing significantly with a very thick
hydrogen-rich atmospheric envelope.
*
Final Stages of Solar System EvolutionOur unstable young Sun
ejected its thin outermost layers into space―a brief but intense
burst of mass loss called a T Tauri wind.The T Tauri wind
swept the solar system nearly clean of gas and dust. The planets
stabilized at roughly their present-day sizes.
In the final stages of the evolution of our solar system, the Sun
was still quite unstable, throwing bursts of material out into
space. This is called a T Tauri wind. The name of this is based
on the first such object seen, a protostar in the constellation
Taurus the Bull, called T Tauri. In a young solar system, this T
Tauri wind sweeps the rest of the solar system clear of leftover
gas and dust (leaving behind more massive objects). It was at

this point that the planets stabilized at about their current sizes.
Check out page 108 in your text for an overview of this whole
process: from a rotating cloud of gas and dust into a solar
system. Since we see things like proplyds and T Tauri stars
elsewhere in our galaxy, it reinforces the idea that planetary
formation may be common. Further enhancing this idea is the
discovery over the past decades of extrasolar planets: planets
orbiting stars beyond the Sun.
*
Searching for Extrasolar Planets (or Exoplanets)
Astrometric Method
Radial Velocity Method
The first planets around other stars were found in the mid-
1990s. Since then, the number of exoplanets (also called
extrasolar planets) has grown significantly. The number of
candidate planets (not yet confirmed) is in the thousands, and
the number of confirmed planets is growing too rapidly to
bother mentioning a number here, as it’ll be outdated before you
read this. Check out the website www.exoplanets.org for the
current numbers.
For the most part, these planets have been discovered because
of their influence on their parent stars. The first exoplanets
were discovered because of their gravitational interaction with
their star. A star with a planet (or planets) orbiting it will orbit
a common center of mass. We can’t see the planet (or planets),
as they do not give off any light of their own and the light of
their star washes out any view of reflected light off the planets.
But we can see the star appear to wobble. If the motion is across
our line of sight, we can detect this by seeing the star visibly
wobble as we measure its position in the sky very precisely.

This is called the astrometric method. If the star’s motion is
towards and away from us, then we can use the radial velocity
method and look for alternating redshifts and blueshifts in the
spectrum as the star moves. Remember that an object moving
closer to us will have spectral lines that appear blueshifted
(shifted towards shorter wavelengths), and an object moving
further away from us will have spectral lines that appear
redshifted (shifted towards longer wavelengths). So, both the
astrometric method and the radial velocity method are
measuring the gravitational influence of planets on their parent
stars, but they measure a different component of the motion.
These methods have been mostly finding very large planets
(around the size of Jupiter and bigger) that are relatively close
to their stars (remember that the gravity is stronger with large
objects close together).
*
Searching for Extrasolar Planets (or Exoplanets)
Transit Method
Image credit: http://www.euhou.net/
Another method for finding extrasolar planets is the transit
method. If we are looking at a star system edge on, sometimes
the planet and its star might line up from our perspective. When
the planet moves in front of its star, it blocks a tiny portion of
the star’s light. It might be a tiny portion, but we are still able
to detect the small dip in light. If we see this happen multiple
times, we can confirm that there is a planet (or planets) there,
and we can also calculate the orbit(s). This is the method that
was used by the Kepler spacecraft, which stared continuously at
more than 100,000 stars in a small patch of the summer sky,
waiting for that tell-tale dip in a star’s light. Thousands of
planets have been found in the Kepler data. Back when I was in

college, we had one solar system to look at and could only
speculate about finding more. We could guess that others had
formed, but we had no proof. We now know that planetary
formation is common, and we are likely to continue to find
many more extrasolar planets, especially as the technology
improves, allowing us to find smaller planets and also planets
that are further away from their parent stars. In early 2013, the
Kepler satellite even found a planet smaller than Mercury
(about 1/3 the size of Earth)!
Speaking of Earth…next week in chapter 5, we will look at the
Earth itself. That’s it for chapter 4. See you next time.
*
Sierra Nevada Sustainability Tour Reflection
Fall 2016
30 Extra Credit Points Possible (Applied to Exam Category of
Grade)
Due at 11 p.m. on Monday, November 29
The purpose of this assignment is to reflect upon what you
learned from the Sustainability
Tour and learn more about the sustainability plan of Sierra
Nevada Brewing Co. This report
should be a minimum of two-typed pages with 1.0” line spacing.
The two-page requirement
does not include the cover page, reference page, and any images
or diagrams. All reports
must be submitted to Blackboard by 11 p.m. on Monday,
November 29. It is strongly
recommended that you begin working on your report
immediately so that you do not

forget any of the information from the tour.
Deliverables:
The written report should address Sierra Nevada Brewing Co.’s
sustainability plan. In this
report you must:
1. Give an introduction to the company that includes a company
history.
2. Give an overview of Sierra Nevada’s sustainability plan that
includes facts and
quotations from the tour and other reliable sources. Use the
Sierra Nevada
Sustainability Report to inform your research in this section.
The report is available
at http://www.sierranevada.com/brewery/about-us/sustainability
3. Conclude with discussion the CSU, Chico Graduation Pledge
and what it means to
you. The social and environmental is a pledge to yourself that
you have the option to
complete as a graduating senior.
““I pledge to explore and take into account the social and
environmental consequences
of any job I consider and will try to improve these aspects of
any organizations for
which I work.”
Is the pledge something you will take into consideration with
your future career? If

so, how do you foresee that you will implement the graduation
pledge?
More information on the Graduation Pledge of Social and
Environmental
Sustainability is available at:
https://www.csuchico.edu/commencement/info-undergrads/life-
after-grad.shtml
http://www.graduationpledge.org/
4. APA formatting should be used for this report. For more help
with APA formatting
and citations, refer to the Purdue OWL website:
https://owl.english.purdue.edu/owl/section/2/10/
http://www.sierranevada.com/brewery/about-us/sustainability
https://www.csuchico.edu/commencement/info-undergrads/life-
after-grad.shtml
http://www.graduationpledge.org/
https://owl.english.purdue.edu/owl/section/2/10/
Investigating Astronomy
Timothy F. Slater, Roger A. Freedman
Chapter 3
Analyzing Scales and Motions of the Universe
Welcome to week 3. In this chapter we’re going to be looking at
the work of some famous astronomers in history, including

Copernicus, Galileo, Kepler, and Newton. The work they did led
to our understanding of how objects (especially those in space)
move.
*
Eratosthenes and Aristarchus
Using simple tools and basic geometry to measure: the size of
the Earth Moon and Sunand the distances to the Moon and Sun
First, we look back to ancient Greece. Greek science is known
for a number of achievements. Greek astronomers knew that the
Earth was round, for example. They saw that during a lunar
eclipse the Moon moved into the Earth's shadow. They saw that
the shadow had a round edge, and that it always had a round
edge. They knew that a sphere is the only shape that casts a
round shadow from any angle, so they knew the Earth was a
sphere 2000 years before the days of Christopher Columbus.
The Greek astronomer Eratosthenes figured out a way to
measure the circumference of the spherical Earth. He knew of a
town where the Sun shined straight down water wells at noon on
the summer solstice, and he knew that the same thing did not
happen in his own city of Alexandria, due north. So he
measured the angle of shadows on that day in Alexandria, and
using the distance between the two cities extrapolated out to the
circumference of the Earth. The only problem for our analysis
of his work is that he used a unit of distance, the stade, and we
don't know the exact length of that unit. Based on our best
guess, Eratosthenes' calculated value would've been about
42,000 km, amazingly close to the known value of about 40,000
km.
Another Greek scientist, Aristarchus, used geometry to calculate

the relative distances to the Sun and the Moon. His calculations
found that the Sun is 20 times further than the Moon, but it
turns out, his measurements were off, as the Sun is actually 390
times further than the Moon. That error caused his calculations
of the relative size of the Sun and Moon to be incorrect as well,
but it's still amazing that the thinking was advanced enough
2000 years ago to have the basic ideas right.
*
The Greek Geocentric Model
An Earth-centered, or geocentric,
model of the universe
As I mentioned earlier, the ancients believed that everything in
the universe went around the Earth, like our imaginary celestial
sphere model. This model of the universe is known as
geocentric, meaning centered on the Earth. In this model, the
stars were fixed on the celestial sphere, and the Sun, Moon and
other planets revolved around the Earth inside the sphere.
*
The Problem of Retrograde Motion
The “merry-go-round” model doesn’t explain retrograde
motion―periods when the planets appear to move backwards in
the constellations.
However, this model (which was depicted as a merry-go-round
on the previous slide) cannot explain retrograde motion, which
occurs when planets appear to stop moving, back up, stop again,
and then resume their forward motion.

*
The Ptolemaic
System
To explain retrograde motion, Ptolemy created a complicated
system of spheres on spheres.
To try to solve this problem, the Greek astronomer Ptolemy
created a very complicated system in which he imagined the
planets moving in smaller circles as they moved on larger
circles around the Earth. To his credit, this system did seem to
explain retrograde motion. It just happened not to be the correct
explanation.
*
The Heliocentric Model and Retrograde Motion
So, what actually causes retrograde motion? It is an apparent
motion, due entirely to our perspective on Earth. What happens
is that the planet that is closer to the Sun is moving faster than a
planet that is further from the Sun and, like a runner in lane one
of a racetrack, will lap the other planet. For a brief time, then,
the other planet will look like it's going the other direction. We
see the same thing when driving, as a car that we are passing
briefly looks stationary as we draw up to it, and then looks like
it is going backwards for a moment. If you look at this diagram,
you can match the position of Earth and Mars at each number,
and follow the yellow line to see where against the background
of stars Mars appears to be as seen from Earth.

*
Copernicus and the OrbitsConjunction: a planet and the Sun
lining up, as viewed from the Earth.Opposition: a planet and the
Sun are on opposite sides of the Earth.
Elongation: the angle between the Sun and a planet, as viewed
from Earth.
The Polish scientist Copernicus was the first to publish the idea
that the Earth actually goes around the Sun, rather than the
other way around. This was such an unpopular idea that he did
not choose to publish it until the year he died. Even then, it
remained an unpopular idea for some time.
If you look at this diagram (which is more easily seen on page
73 of your textbook) you'll see some useful vocabulary.
Conjunction occurs when a planet and the Sun are lined up as
seen from Earth. For the inner planets, Mercury and Venus, this
happens both when they are in between the Sun and the Earth
and when they are on the opposite side of the Sun from Earth.
During conjunction, a planet cannot be seen, as it is either lost
in the glare of the Sun or hidden behind it. A very rare event is
called a transit, when Mercury or Venus is actually seen to
cross the face of the Sun as seen from Earth. Obviously, this
cannot be seen without proper eye protection. This happened on
June 6, 2012 as Venus crossed the face of the Sun. The
alignment with Venus is such that two transits will occur eight
years apart, and there was a previous one in 2004. If you didn’t
catch the one in 2012, sorry to say you’re out of luck because
the next one is not until 2117! Transits of Mercury happen more
frequently, about 13 or 14 transits per century.
Opposition refers to a planet being on the opposite side of the

Earth from the Sun. A full moon can be said to be in opposition,
as we see the near side in full sunlight. Mercury and Venus,
being inside the orbit of the Earth, can never be in opposition.
Therefore, we can never see them as being “full.”
Elongation is the angle between the Sun and the planet as
viewed from Earth. Greatest elongation would be the furthest
angular distance that the planet can appear to be. Mercury, for
example, as the closest planet to the Sun has a smaller greatest
elongation than any other planet, which means that it is never
seen in our sky very far away from the Sun, so it is only seen
just after sunset or just before sunrise.
*
The Phases of Venus
Galileo’s discoveries of moons orbiting Jupiter and phases of
Venus strongly supported a heliocentric model.
In 1609, Italian astronomer Galileo Galilei learned of a new
invention that allowed one to see far-off objects magnified. He
built one for himself and pointed it at the sky. A little known
fact is that he actually was not the first to point a telescope at
the sky (Englishman Thomas Harriot did so earlier in the same
year), although Galileo was the first to publish. That fact about
science is still true today–it often doesn't matter who was first
to do something, only who is first to publish. Anyway, Galileo
saw amazing sites through his telescope. He saw that Venus
goes through a cycle of phases, much like the Moon. However,
and this is critical, he saw that it was never full. That means it
is never in opposition, and so must orbit the Sun inside the orbit
of the Earth. This was one nail in the coffin of the geocentric
model of the universe.
*

The Moons of Jupiter
Observations of Jupiter and its moons showed that there are
objects that do not orbit Earth.
Galileo also looked at Jupiter. One of the things he saw was that
there were four, starlike objects that seemed to move back and
forth from one side of Jupiter to the other. Sometimes all four
were visible and sometimes not. One of his original drawings is
seen in this image. He concluded that they were four moons
orbiting the planet. This was another strong bit of evidence in
favor of the heliocentric model and against the geocentric
model. In the geocentric model, EVERYTHING orbited around
the Earth. But Galileo's observations of Jupiter showed quite
clearly that those objects were orbiting Jupiter, and therefore
NOT everything orbited the Earth.
*
Elliptical Orbits and Kepler’s First LawThe orbit of a planet
about the Sun is an ellipse with the Sun at one focus.
Mercury has the most eccentric orbit at 0.207.
Before Galileo's observations, there was a Danish scientist
named Tycho Brahe. While he didn't have a telescope, since one
hadn't yet been invented, he did develop instruments with which
he could make very precise measurements of the positions of
planets. He was a bit odd, and it is said that he had a gold and
silver prosthesis on his nose, having lost the bridge of his nose
in a duel. He had an assistant named Johannes Kepler. When
Tycho died, he left his data to Kepler. Interestingly, while

Tycho was brilliant at observing, he was not enough of a
mathematician to be able to use his own data to its fullest
purpose. Kepler, on the other hand, was an excellent
mathematician. He was able to use Tycho's data to establish
three laws of planetary motion. His first law states that
planetary orbits are in the shape of an ellipse (a squashed
circle). Ellipses have two foci (singular: focus), and Kepler
established that the Sun lies at the focus of planetary orbits. The
amount of squash in an ellipse is given by its eccentricity. A
circle is not squashed at all, and so it has an eccentricity of 0.
The higher the eccentricity, the more squashed the ellipse. Of
the eight major planets, Mercury has the most eccentric orbit at
0.207. The orbits of the planets (especially Earth) are actually
quite close to being circles, but not quite.
*
Orbital Speeds and Kepler’s Second LawA line joining a planet
and the Sun sweeps out equal areas in equal intervals of time.A
planet moves fastest when closest to the Sun.
Kepler’s second law states that a line between a planet and the
Sun sweeps out equal areas in equal time. Looking at this
diagram, the planet in question starts at point A and moves to
point B. Shade the region between those two lines. Then,
imagine the planet starting at point C and then moving to point
D (a further distance than from A to B). If you then measure the
area of the shaded region between C and D and find it to be
identical to the area of the shaded region between A and B, then
it will have taken exactly the same amount of time to travel
from A to B as it does from C to D. Since the distance from C to
D is longer, the meaning of this is that a planet moves faster
when it is closer to the Sun. There are two points marked on
this diagram, perihelion and aphelion. Perihelion is the point in

a planet's orbit when it is closest to the Sun, while aphelion is
the point when it is furthest. Orbiting objects move fastest at
perihelion and slowest at aphelion. You might also see the
words perigee and apogee. Those have the same meaning,
except for objects orbiting the Earth. Kepler’s Laws apply to
objects orbiting the Earth, too (or anything else that can be
orbited).
*
Orbital Periods and Kepler’s Third Law
The greater the distance between the Sun and planet, the slower
the planet travels.
P2 ≈ a3
P: planet’s period, in years
a: planet’s semimajor axis, in AU
Kepler's third law showed that the further away from the Sun a
planet is, the slower it moves. This one can be written in the
form of an equation, in which the square of the planet's orbital
period (in years) is approximately equal to the cube of the
planet's semi-major axis (in astronomical units…where 1
astronomical unit equals the distance between the Sun and
Earth). Orbital period is the time it takes an object to complete
one orbit. Semi-major axis is half of the long axis of an ellipse.
Of the planets in the solar system, Mercury travels the fastest,
since it is the closest to the Sun. The next fastest is Venus, then
Earth, then Mars, and so on. Note that this equation only says
these values are approximately equal to each other, as there’s
another version of this equation that takes mass into account for
a more precise relationship.
*

Newton’s Laws
An object remains at rest, or moves in a straight line at a
constant speed, unless acted upon by a net outside force.
F = ma
Whenever one object exerts a force on a second object, the
second object exerts an equal and opposite force on the first
object.
And now we have reached the great Sir Isaac Newton, a British
mathematician and scientist. Newton discovered three laws of
motion.
The first states that an object at rest remains at rest, and an
object in motion remains in motion, unless acted upon by an
outside force. This is also known as inertia…and doesn’t really
have much to do with why it’s so hard to get up off the couch
when watching television.
His second law states that the net force on an object is equal to
the mass of the object times the acceleration of the object.
F=ma. Acceleration is the change in velocity. If you are
traveling down the highway with the cruise control on, you are
not experiencing any acceleration. But if you are driving with
your foot either on the brake or on the gas pedal, then you are
accelerating (braking is just a negative acceleration).
Newton's third law of motion says that for every action there is
an equal and opposite reaction. In other words, if an object
exerts a force on another object the second object exerts an
equal and opposite force on the first object. Astronauts
experience this a great deal in space. For example, if you do not
anchor your feet when turning a wrench in space, instead of

turning the nut you will turn your body the opposite direction.
That's why there are so many handholds and footholds in and
around the International Space Station.
Check out section 3-5 of your text for a bit more detail on these
laws.
*
Gravity Explains Kepler’s Laws
Newton also managed to explain why Kepler's laws of planetary
motion work. He realized that there was an attractive force
between two objects. If you put a ball on a string and spin it
around, the force of the string pulling on the ball is analogous
to the force of gravity. If you use a smaller string, you have to
swing a bit harder to get the ball to move in a circle than if you
have a larger string. Similarly, gravity is stronger when the
distance between the Sun and a planet is smaller.
*
Newton’s Law of Universal GravitationF : gravitational force
between two objectsm1 : mass of first objectm2 : mass of
second objectr : distance between objectsG : universal
constant of gravitation
Here is Newton's Law of Universal Gravitation. If F represents
the gravitational force between two objects of mass m1 and m2
that are a distance r apart, Newton's law tells us that the
gravitational force is directly proportional to the product of the
masses and indirectly proportional to the square of the distance

between them. Directly proportional means that as one number
goes up the other goes up as well. So, the gravitational force
being directly proportional to the product of the masses means
that if either or both of the masses gets bigger, so does the
gravitational force between them. Conversely, if the masses are
smaller, the gravitational force is smaller. Indirectly
proportional means that as one number goes up the other goes
down. Thus, the gravitational force being indirectly
proportional to the square of the distance means that as the
distance gets bigger the gravitational force gets smaller, and as
the distance gets smaller the gravitational force gets bigger.
Because the distance is squared in this equation, that means that
the gravitational force drops off more quickly as the distance
increases. The letter G in this equation is a universal constant
(which means it applies the same everywhere we look in the
universe). It's a very small number, which you can see in your
book.
*
An Explanation of OrbitsA: A ball dropped from a great height
falls straight down. B & C: A ball thrown with some horizontal
speed. E: A ball thrown with the “right” speed orbits in a
perfect circle.D & F: Balls thrown with speed a little too slow
and a little too fast orbit in an ellipse.
Let's look very quickly at what orbiting actually means. Look at
the lines in this diagram (it's on page 83 if you want to see it
more clearly). If you drop a ball from a height, it will fall
straight down (as shown by line A). Lines B and C show that if
you throw a ball with some horizontal speed, it will move out
before falling down to the ground. If you throw a ball fast
enough, it would actually go into orbit because it would have
enough forward speed to keep missing the Earth as it falls. If it

is thrown with just the “right” speed, as with line E, it will go
into a circular orbit. If the speed is a little slower or faster than
that “right” speed, then it would orbit in an ellipse (lines D and
F). So an object that is in orbit is actually falling, but it has
enough forward velocity to not hit the ground. This is why
astronauts feel weightless… they're actually in a constant state
of freefall.
And that’s it for chapter 3. See you next week!
*
BEFORE YOU BEGIN - This is important for all of the labs.
Create a new document for your responses with the following
naming convention:
Lastname_Firstname_Sun_Lab.doc.
For example, Smith_John_Sun_Lab.doc.
Type your responses into your blank document, being careful to
include headings for the six sections of the lab (Phase I through
Phase VI) and any question numbers. To submit your
assignment, upload this document plus any additional
documents you may have, such as screenshots of your data (or
you can paste those directly into your document). All submitted
labs will be automatically scanned by the anti-plagiarism
Turnitin.com.
Phase I—You will be guided through exploring an interactive
sky chart at www.heavens-above.com. (Please note that if you
ever want to go play with the Heavens Above web site, which
has lots of cool things about the sky, including how to find and
identify satellite passes, do NOT forget the hyphen in the web
address...'nuff said.) Follow the instructions in the Exploration
Part A and Exploration Part B sections, and enter your answers
directly in the Word document.

Phase II—You will now use the site to answer some questions
and then analyze a provided generalization based on your
evidence.
Phase III—You are given some data collected from the site, and
asked to come up with a conclusion based on that data (in other
words, an evidence-based conclusion). Make sure that you
mention the specific pieces of data that you are using for
evidence.
Phase IV—In this phase, you are given a research question and
asked to come up with a step-by-step method of collecting the
evidence needed to answer this question. You do not need to do
the whole data collection process, but you do need to explain
what someone would need to do (again, further instructions are
in the lab document). You must use heavens-above.com for this
(and all) phases of the lab. Do not make your instructions be
about outdoor observations. Write it so that someone else could
follow your step-by-step procedure to successfully collect the
needed data.
Phase V—Now, based on what you have done in the first four
phases, you will come up with your own answerable research
question that can be answered using the Heavens Above (or
similar) site. Don't worry, the question does not need to be
complex or sophisticated, but it does need to be answerable, by
you, using this web site. You will write your question, the
procedure to collect the evidence (like in phase IV), collect the
data (use the data table in phase III as a rough guide), and come
to an evidence-based conclusion (like you did in phase III).
Phase VI—Finally, you will write a short (50-word) summary
(details in the lab document). This should not be information
from your textbook or other sources.
Lastly, upload your completed lab document to the assignments

area (remember that your name must be in the name of the file).
Questions—Post any questions about the lab to the Lab Q&A
Forum. Please include the lab number in your subject line. You
can also send me a message with questions.
Additional fun—As mentioned above, the Heavens Above site
can help you find satellite passes (including the International
Space Station and the Hubble Space Telescope) over your
location. I encourage you to play around and see what you can
find to see in the night sky.
Assignment Instructions
Instructions: All details for completing this lab are in the
weekly lesson. Download the provided Word document and
carefully follow all instructions both in the lesson and in the
Word document. You will enter your work in a separate file,
following the file naming convention in the lesson description.
Submission: Upload your Word document, or you may also save
and upload your work as a PDF file. Any appropriate sketches,
diagrams, etc. may be pasted into the Word document or
uploaded as a separate image file.
Rubric: Your score will be based on the following rubric. As
you can see, phases V and VI constitute the majority of points
for the lab. If you are having difficulty, don't leave a section
blank...ask questions! You may post questions in the Lab Q&A
forum (in which case your classmates and/or the instructor can
respond, and the answer will benefit the whole class), or
directly in a message to the instructor.
Phase I - done/not done (if not done, 1 point will be subtracted)

Phase II - 1 point
Phase III - 1 point
Phase IV - 1 point
Phase V - Research question - 1 point
Phase V - Procedure - 1 point
Phase V - Data - 1 point
Phase V - Evidence-based conclusion - 1 point
Phase VI - Summary - 3 points
Observing the Sun’s Position and Motion
Big Idea: Sky objects have properties, locations, and predictable
patterns of movements that can be observed and described.
Those motions explain such phenomena as the day, the year, the
seasons, phases of the moon, and eclipses.
Goal: Students will conduct a series of inquiries about the
motion of the Sun in the sky using prescribed Internet
simulations and learn how the Sun follows different pathways at
different times of the year.
Computer Setup:
Access http://www.heavens-above.com/ and
a) Find the CHANGE YOUR OBSERVING LOCATION link
under Configuration and set your observing location and time
zone. If you use the search feature, you should just be able to
click “Update” at the bottom of the screen.
b) Find INTERACTIVE SKY CHART link under Astronomy.
You can also use SKY CHART (OLD VERSION), but the
interactive version lets you mouse over objects to see pop-up
information about them. It also has a print to PDF function,
which can be useful for capturing your data in the later parts of
the lab.

Phase I: Exploration PART A:
1) On a map of the United States, north is toward the top of the
page and west is to the left. On all of the star charts, north is
toward the top of the page and west is to the right. How do you
account for this difference?
2) This is the current sky. Find the YELLOW star marking the
current location of the SUN. Which constellation is it closest to
right now?
3) Change the time by increasing it one hour and pressing
update. Exactly how has the Sun’s position change on the map?
4) Slowly increase the time to later and later in the day. This
system uses 24-hr “military time” or “Zulu” time. So, 6pm is
actually entered as 18-hours. Determine EXACTLY what time,
hours and minutes, that the Sun will set tonight. It should be the
time when the Sun disappears below the western horizon (test
by clicking the – button for the minutes…if the Sun reappears,
then you were on the right time).
Sunset: __________
5) Which constellation was the Sun closest to when it set?
6) Is this the same or different than where the Sun was earlier in
the day?
7) What generalization can you make about the relative speeds
that the Sun and the stars move through the sky over the course
of a day?
8) What generalization can you make about the direction the
Sun and the stars move through the sky over the course of a
day?
9) Describe precisely how you would test to see if this

generalization is true during the night time too.
10) What is the physical cause of your generalization (what is
happening physically in the world that causes what you see)?
Phase I: Exploration PART B:
When looking at the star map set for SUNSET TONIGHT:
11) on what part of the map (left, right, top, bottom or center) is
the star group that appears highest in the night sky? What is the
name of this star group?
the star group that appears near the southern horizon? What is
the name of this star group?
the star group that appears near the eastern horizon? What is the
name of this star group?
When looking at the star map set for THREE HOURS after
tonight’s sunset:
the star group that now appears highest in the night sky? What
is the name of this star group?
15) Where did the stars that used to be at this position move to?
the star group that now appears near the southern horizon? What
is the name of this star group?
the star group that now appears near the western horizon, where
the Sun sets? What is the name of this star group?

the star group that now appears near the eastern horizon, where
the Sun rises? What is the name of this star group?
22) If you were to change the time to midnight, predict what
would be different about the positions of the stars.
23) What generalization can you make about how the stars
change position over the course of the night?
Phase II – Does the Evidence Match the Conclusion?
24) From before, precisely what time (hours and minutes) will
the sun set below the western horizon tonight?
25) Using the sky chart, precisely what time the sun will set one
month from now?
26) Using the sky chart, precisely what time the sun will set two
months from now?
27) Using the sky chart, precisely what time the sun will set
three months from now?
28) Using the sky chart, precisely what time the sun will set six
months from now?
nine months from now?
twelve months from now?

31) If a student proposed a generalization that “sunset time
changes about one hour per month, setting earlier and earlier in
the fall and then setting later and later in the spring,” would you
agree, disagree with the generalization based on the evidence
you collected? Explain your reasoning and provide evidence
either from the above questions or from evidence you yourself
generate using the star map program.
Phase III – What Conclusions Can You Draw From the
Evidence?
Most of us would agree that the sun sets in the general direction
of west. What conclusions and generalizations can you make
from the following data collected by a student in terms of HOW
DOES THE DIRECTION OF THE SUNSET CHANGE? Explain
your reasoning and provide evidence to support your reasoning.
Date
Sunset Time
Azimuth (west = 270()
Direction
August 15
7:56 pm MDT
289(
Northwest
September 15
7:06 pm MDT
274(
West
October 15
6:16 pm MDT
258(
West Southwest
November 15
4:37 pm MST
245(

Southwest
December 15
4:28 pm MST
238(
South Southwest
Evidence collected in standard time from http://www.heavens-
above.com/ using SUN AND MOON DATA FOR TODAY under
the Astronomy section and/or
http://aa.usno.navy.mil/data/docs/AltAz.php for Laramie, WY
32) Evidence-based Conclusion:
Phase IV – What Evidence Do You Need?
Imagine your team has been assigned the task of writing a news
brief for your favorite news blog about how the noon-time sun’s
altitude above the southern horizon changes over the course of
the semester. Describe precisely how and what evidence you
would need to collect in order to answer the research question
“How does the noon-time sun’s position above the southern
horizon change over the semester?” Your procedure MUST use
this heavens-above.com web site, you do NOT need to use any
other resources. You do not need to collect data for this phase,
but you should write the instructions such that someone else
could follow your instructions to successfully collect the
relevant data.
33) Create a detailed, step-by-step description of evidence that
needs to be collected and a complete explanation of how this
could be done—not just “measure the position of the Sun,” but
exactly what would someone need to do, step-by-step, to
accomplish this.
Phase V – Formulate a Question, Pursue Evidence, and Justify
Your Conclusion
Your task is design an answerable research question, propose a

plan to pursue evidence, collect data using heavens-above (or
another suitable source pre-approved by your lab instructor),
and create an evidence-based conclusion about some motion or
position of the sun in the sky that you have not completed
before. This question doesn’t need to be complex. Think about
the observations you’ve learned about so far in the lab. The best
research questions are those that can’t be answered by a simple
yes/no, or a single number or characteristic. Look at ways you
can compare/contrast or otherwise analyze a collection of data.
The questions in phases II through IV are good examples. Your
question can be similar, but must be different from those. If you
have difficulty, ask a question in the Lab Q&A Forum or send a
message to your instructor.
Research Report:
34) Specific Research Question:
35) Step-by-Step Procedure to Collect Evidence:
36) Data Table and/or Results:
37) Evidence-based Conclusion Statement:
Phase VI – Summary
PRINT YOUR NAME
38) Create a 50-word summary, in your own words, that
describes how the sun’s motion and position changes over the
day and over the year. You should cite specific evidence you
have collected in your description, not describe what you have
learned in class or elsewhere. Feel free to create and label
sketches to illustrate your response.
5

Investigating Astronomy Timothy F. Slater, Roger A. F.docx

Investigating Astronomy Timothy F. Slater, Roger A. F.docx

Recommended

Recommended

More Related Content

Similar to Investigating Astronomy Timothy F. Slater, Roger A. F.docx

Similar to Investigating Astronomy Timothy F. Slater, Roger A. F.docx (20)

More from normanibarber20063

More from normanibarber20063 (20)

Recently uploaded

Recently uploaded (20)

Investigating Astronomy Timothy F. Slater, Roger A. F.docx