Hello hsthompson:
I have researched each of your 20 questions. I have pointed you to
web-based resources that answer each of them, and I've extracted the
most relevant sections of those pages to help you compile your
answers. If the information I've pulled out for you here is not enough
detail for your needs, I suggest that you visit the pages listed for
more details.
1. Why do we measure some distances in astronomy in light-years and
some distances in astronomical units?
Measuring Distance
URL: http://sirtf.caltech.edu/EPO/Field/distance.html
Quote: Astronomical units are usually used to measure distances within
our solar system....Most objects in space are so far away, that using
a relatively small unit of distance, such as an astronomical unit, is
not practical. Instead, astronomers measure distances to objects which
are outside our solar system in light-years.
2. What information does a star's Greek letter designation convey?
Brightness.
STAR NAMES
URL: http://www.astro.uiuc.edu/~kaler/sow/starname.html
Quote: To bring order out the chaos of proper names, around the year
1600 Johannes Bayer, in what is now Germany, applied lower case Greek
letter names <greek.html> to the stars more or less in order of
brightness, rendering the brightest star in a constellation "Alpha,"
the second Beta," and so on. To the Greek letter name is appended the
Latin possessive form <const.html> of the constellation name. Thus the
brightest star in Lyra, Vega (an Arabic proper name), becomes Alpha of
Lyra or Alpha Lyrae (where "Lyrae" means "of Lyra.") The brightness
rule is more often violated than not, however, as Bayer also factored
in the position of the star within the constellation. Though Adhara
<adhara.html> in Canis Major (the Greater Dog) is first magnitude and
the second brightest star in the constellation, it received "Epsilon,"
probably because of its lower (more southerly) position within the
starry figure.
3. How are the celestial poles and equator defined by Earth's
rotation?
Celestial Coordinate System
URL: http://csep10.phys.utk.edu/astr161/lect/time/coordinates.html
Quote: In the celestial coordinate system the North and South
Celestial Poles are determined by projecting the rotation axis of the
Earth to intersect the celestial sphere, which in turn defines a
Celestial Equator. The celestial equivalent of latitude is called
declination and is measured in degrees North (positive numbers) or
South (negative numbers) of the Celestial Equator. The celestial
equivalent of longitude is called right ascension. Right ascension can
be measured in degrees, but for historical reasons it is more common
to measure it in time (hours, minutes, seconds): the sky turns 360
degrees in 24 hours and therefore it must turn 15 degrees every hour;
thus, 1 hour of right ascension is equivalent to 15 degrees of
(apparent) sky rotation.
How Points in the Sky Are Determined
URL: http://astrwww.astr.cwru.edu/reference/skypoints.html
4. What causes precession and why does it move the celestial equator?
precession of the equinoxes
URL: http://www.infoplease.com/ce6/sci/A0840032.html
Quote: precession of the equinoxes, westward motion of the equinoxes
along the ecliptic. This motion was first noted by Hipparchus c.120
B.C. The precession is due to the gravitational attraction of the moon
and sun on the equatorial bulge of the earth, which causes the earth's
axis to describe a cone in somewhat the same fashion as a spinning
top. As a result, the celestial equator (see equatorial coordinate
system), which lies in the plane of the earth's equator, moves on the
celestial sphere, while the ecliptic, which lies in the plane of the
earth's orbit around the sun, is not affected by this motion. The
equinoxes, which lie at the intersections of the celestial equator and
the ecliptic, thus move on the celestial sphere.
5. Why does the moon glow coppery red during a total lunar eclipse?
Why an eclipse paints the moon red
URL: http://www.msnbc.com/news/356781.asp
Quote: THE REASON FOR THE REDNESS...It’s because some sunlight is
still hitting it, and for that you can thank our atmosphere. Particles
in the atmosphere cause the light rays coming from the sun to bounce
around. Some are refracted, or bent. They get redirected through the
atmosphere and out around behind Earth and onto the moon, which is
blocked only from direct sunlight...However, the refracted rays of
sunlight doing the illuminating turn the moon a strange reddish....The
more atmosphere that sunlight travels through, the more the blue and
green parts of the spectrum are scattered....The same thing happens to
sunlight refracted onto the moon during an eclipse. The sunlight hits
the atmosphere on the sides of Earth at a shallow angle and is carried
through a lot of atmosphere until it’s redirected out onto the moon
“hiding” from direct sunlight. The red end of the spectrum is all that
can get through that much interference. So the moon in total eclipse
appears as an eerie, glowing copper ball in the sky.
6. Why don't eclipses occur at every new moon and full moon?
The moon goes around the earth...
URL: http://www.telusplanet.net/public/lasa/moon.htm
Quote: . So why don't we see a lunar eclipse every time there is a
full moon? The answer lies in the fact that the moon's orbital plane
is tilted about 5 1/2 degrees with respect to the plane of the earth's
orbit around the sun. If the two planes were exactly parallel, there
would indeed be a lunar eclipse at every full moon (and a solar
eclipse at every new moon). But the effect of the tilted planes is
that in most months, the moon passes above or below the earth's shadow
as it passes through space at full moon.
However, the alignment of the planes is constantly changing, and twice
during each year there is an "eclipse season", a period of a few weeks
during which the alignment of the planes is such that solar or lunar
eclipses (or both) will occur. In most years, there will be two lunar
eclipses.
7. What would you see if you were on the moon and facing Earth when
people on Earth saw a total lunar eclipse?
You'd see a total solar eclipse if you were on the moon during a total
lunar eclipse.
A Total Eclipse of the Sun-- on the Moon!
URL: http://science.nasa.gov/headlines/y2001/ast08jan_1.htm
Quote: Solar eclipses on the Moon happen when our planet lies directly
between the Moon and the Sun. As the full Moon glides into Earth's
shadow, the blinding disk of the Sun temporarily vanishes and the
Sun's faint corona emerges -- much like a solar eclipse on
Earth....That's because a solar eclipse on the Moon is also a lunar
eclipse here on Earth!
8. Why does one cycle of lunar phases take 29.53 days even though the
moon orbits Earth in 27.32 days?
Phases of the Moon
URL: http://www.dc.peachnet.edu/~pgore/astronomy/astr101/moonphas.htm
Quote: The Moon takes 27.32 days to orbit the Earth (with respect to
the stars), but it takes LONGER (29.53 days) to go through a cycle of
phases. WHY?
This is because the Earth-Moon system has moved around the sun by
about 27 degrees over the course of the month. The Moon will have gone
around the Earth once with respect to the stars, but it needs to
travel further to line back up the same way with the sun.
9. Describe the differences between the Ptolemaic, Tychonian and
Copernican models of the universe.
In the Ptolemaic model, the Earth is at the center of the universe and
everything (including the sun) revolves around the Earth. In the
Copernican model, the sun is at the center of the universe and
everything (including the Earth) revolves around the sun. The
Tychonian model is a compromise between the two other models - the
Earth is the center of the universe, the sun revolves around the
Earth, but the other planets revolve around the sun.
The Ptolemaic Model of the Universe
URL: http://www.ifa.hawaii.edu/users/lin/ast110-6/ast110-5_files/frame.htm
Note: See slide 3
The Copernican Model: A Sun-Centered Solar System
URL: http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html
Scientific achievements
URL: http://artzia.com/History/Biography/Galileo/
Quote: "By the time of the controversy, the Catholic Church had in
fact abandoned the Ptolemaic model for the Tychonian model in which
the Earth was at the center of the Universe, the Sun revolved around
the Earth and the other planets revolved around the Sun. This model is
geometrically identical to the Copernican model and has the extra
advantage that it predicts no parallax of the stars."
10. What are Kepler's three laws?
I. The orbits of the planets are ellipses, with the Sun at one focus
of the ellipse.
II. The line joining the planet to the Sun sweeps out equal areas in
equal times as the planet travels around the ellipse.
III. The ratio of the squares of the revolutionary periods for two
planets is equal to the ratio of the cubes of their semimajor axes:
Johannes Kepler: The Laws of Planetary Motion
URL: http://csep10.phys.utk.edu/astr161/lect/history/kepler.html
11. Why were Galileo's telescopic observations of the phases of Venus
critical evidence in favor of the Copernican theory?
Galileo and the Phases of Venus
URL: http://pegasus.astro.umass.edu/a100/handouts/scigal.html
Quote: Copernicus proposed that the Sun was in the center of the Solar
System....If Venus and the Earth both orbited the Sun then the
Copernican model made a definite prediction about what phases it
should show at various places in its orbit....Galileo used the newly
available telescope to observe the phases of Venus. The phases matched
the predictions of the Copernican model. The Copernican model was
validated by Galileo's observations.
Venusian Revolution
URL: http://www.astrosociety.org/pubs/mercury/9601/venus.html
Quote: His observations corroborated the theories of Polish astronomer
Nicholas Copernicus, who, on his deathbed in 1543, released a treatise
declaring the Sun, not Earth, to be the center of the universe.
Copernicus's Sun-centered model overturned the accepted,
Earth-centered view of the heavens, developed by the Greco-Egyptian
mathematician Claudius Ptolemaeus in the second century.
12. How did the geocentric model and the heliocentric model of the
universe explain retrograde motion?
Retrograde Motion
URL: http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html
Quote: Ptolemaic (geocentric) Explanation: The model of the solar
system developed by Ptolemy (87 - 150 A.D.) was a refinement of
Aristotle's (384 - 322 B.C.) universe. This model consisted of a
series of concentric spheres, with the Earth at the center
(geocentric). The motions of the Sun, Moon, and stars was based on
perfect circles. To account for the observed retrograde motion of the
planets, it was necessary to resort to a system of epicycles, whereby
the planets moved around small circular paths that in turn moved
around larger circular orbits around the Earth.
Copernican (heliocentric) Explanation: Copernicus replaced the
geocentric universe of Ptolemy with one that was centered on the Sun
(heliocentric), with only the Moon orbiting the Earth. His model was
still based on circular orbits (and therefore still required further
refinement), but it was able to achieve superior precision than the
Ptolemaic model without the need for epicycles or other complications.
The explanation for retrograde motion in this system arises from the
fact that the planets further from the sun are moving more slowly in
their orbits than those closer to the sun.
Note: Go to the page and see the animations.
13. What are Newton's three laws of motion?
I. Every object in a state of uniform motion tends to remain in that
state of motion unless an external force is applied to it.
II. The relationship between an object's mass m, its acceleration a,
and the applied force F is F = ma. Acceleration and force are vectors
(as indicated by their symbols being displayed in slant bold font); in
this law the direction of the force vector is the same as the
direction of the acceleration vector.
III. For every action there is an equal and opposite reaction.
Newton's Three Laws of Motion
URL: http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html
14. Explain how the circular orbit of the moon can be described by two
straight-line motions, one tangential to the orbit and one directed
toward Earth.
The Mathematics of Circular Motion,and Multiple Star Systems
URL: http://www.eso.org/outreach/eduoff/catchastar/cas-projects/uk_castor_1/mech.html
Quote: Before considering the more involved example of a multiple star
system, consider a moon moving in a simple circular orbit around a
planet, which can be considered stationary. The planet's gravitational
field results in the moon having an acceleration towards it; the moon
is constantly falling towards the planet. However, the moon also has
its own velocity, which is tangential to the circle around which it is
moving, and so at right angles to the acceleration of gravity exerted
on it by the planet. If the motion is treated in a series of large
steps, as in the diagram below, can be seen why the resulting motion
is an orbit....At each step, the orbiting moon is falling toward the
planet under gravity. It is also moving, with its own velocity,
tangential to the orbit. The result is that it falls not toward the
planet, but into the next step of its motion. These steps can be
reduced in size until they are infinitely small, which produces a
circular orbit.
Note: Go to the page to see the diagrams.
15. How do spectral windows limit observations made from Earth's
surface?
The ISO Mission - A Scientific Overview
URL: http://www.iso.vilspa.esa.es/outreach/bck_grnd/bull2/kess184.htm
Quote: The scientific potential of infrared astronomy has been amply
demonstrated by observations made from both ground-based telescopes
and those on high-flying aircraft and balloons. However, Figure 2
shows the two main limitations to these observations. Firstly, the
Earth's atmosphere is totally opaque at many wavelengths, absorbing
all the incoming radiation and thus preventing the astronomer from
viewing the celestial object. Work from ground-based telescopes is
only possible through a number of narrow spectral 'windows'. Even at
altitudes of 30 to 40 km, which are typical for balloon-borne
telescopes, the atmosphere is not totally transparent.
Module 2: The Tools of Astronomy
URL: http://www.umuc.edu/virtualteaching/module1/umuc_ex/content/mod2.html
Quote: However, not all parts of the electromagnetic spectrum can get
through the earth's atmosphere and make it to the ground. In fact, the
only two kinds of electromagnetic radiation than can easily make it to
the ground are visible light and radio waves. (Although a little UV
radiation gets through the atmosphere—enough to give you a sunburn or
skin cancer—most of it is blocked by the ozone layer.) The regions of
the electromagnetic spectrum to which the atmosphere is transparent
are sometimes referred to as spectral windows, through which we can
detect radiation from astronomical objects. To detect other types of
radiation, you have to get your telescope above the atmosphere....The
reason that certain parts of the electromagnetic spectrum cannot get
through the earth's atmosphere is that various gases in the atmosphere
absorb those wavelengths of light. You may already know that ozone
(O3) blocks a lot of the ultraviolet radiation coming through the
atmosphere. The ozone also absorbs x-rays and gamma rays. Water vapor
(H2O) and oxygen (O2) absorb microwaves, and water vapor and carbon
dioxide (CO2) absorb a lot of infrared radiation.
16. Explain how the law of gravitation enables us to measure the
masses of astronomical bodies.
Physics
URL: http://www.physics.rutgers.edu/~pryor/ph109/notes32-44.pdf
Quote: Masses of astronomical objects: The importance of Newton's form
of Kepler's Third
Law is that it allows us to deduce masses (or at least sums of masses)
for astronomical
bodies (planets, stars) from the observable sizes and periods of
orbits. Often the orbit-
ing satellite is much smaller than the object it circles and its mass
can be safely dropped
from the sum. This relation is essentially measuring the strength of a
body's gravita-
tional force and then deducing the body's mass from the Universal Law
of Gravitation.
This is the only way to measure masses for astronomical bodies.
Note: Go to page 9 of 14 in this document to see the details.
17. Explain how Newton’s laws of motion and the law of gravity
account for Kepler’s 3rd laws.
Sir Isaac Newton: The Universal Law of Gravitation
URL: http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html
Quote: If you think about it a moment, it may seem a little strange
that in Kepler's Laws the Sun is fixed at a point in space and the
planet revolves around it. Why is the Sun privileged? Kepler had
rather mystical ideas about the Sun, endowing it with almost god-like
qualities that justified its special place. However Newton, largely as
a corollary of his 3rd Law, demonstrated that the situation actually
was more symmetrical than Kepler imagined and that the Sun does not
occupy a privileged position; in the process he modified Kepler's 3rd
Law....Because for every action there is an equal and opposite
reaction, Newton realized that in the planet-Sun system the planet
does not orbit around a stationary Sun. Instead, Newton proposed that
both the planet and the Sun orbited around the common center of mass
for the planet-Sun system.
Note: See this page for the actual modified formulas.
18. Newton's Law # 2 tells you - F=ma. If F=0, then a=0. But if a=0,
then velocity is not changing and object keeps going in a straight
line with constant velocity. In other words - Absence of force
implies no change in motion. Isn't that the statement of the first
law? Why would the greatest genius that ever lived make such a trivial
error?
It isn't an error. For an explanation of why, see:
Explanation of the conceptual questions from LAB-4:
URL: http://www.physics.rutgers.edu/ugrad/193/WS04_soln.pdf
Quote: If you will think more deeply, you will see that the 1st and
the 2nd laws are not redundant and each one of them has its own
purpose. The first law, also called the law of Inertia and was first
discovered by Galileo (1567-1642), states that an object continues in
the state of uniform motion (moves constant velocity) or stays at rest
if no other objects interact with it or if the sum of all these
interactions (forces) is zero. This law defines a very special frame
of reference – inertial frame of reference.
If the sum of all forces (corresponding to external interactions)
exerted on the object is zero, then in an inertial reference frame the
object’s velocity remains unchanged.
Newton’s law only work for INERTIAL FRAMES of REFERENCE and the 1st
law defines what an INERTIAL FRAME of REFERENCE is! The second law
defines what acceleration will the object, in inertial frame of
reference, will have under the influence of known forces.
19. A 2000-kg car is moving with a speed of 20 m/s on a circular arc
of 200-m radius. Determine its centripetal acceleration and the force
from the pavement that keeps it on a circular path.
The formula for centripetal acceleration is:
a[c] = v^2/r
so, for this example, it is
a[c] = (20 m/s)^2 / 200 m
= 400/200 m/s^2
= 2 m/s^2
We also know that F=m*a. So the force from the pavement keeping the
car on its circular path is:
F = m*a
= 2000 kg * 2 m/s^2
= 4000 N
Note-A-Rific: Centripetal Acceleration
URL: http://studyphysics.iwarp.com/20/unit2/circle/accel/note.htm
Lesson 3: Newton's Second Law of Motion
URL: http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l3a.html
20. You (75 kg) are pushing a satellite (2000 kg) in the space. If
the acceleration of the satellite is .01 m/s2, what is your
acceleration?
Since every action has an equal and opposite reaction, the forces
exerted must be equal.
The force you are exerting on the satellite is:
F = m * a
= 2000 kg * .01 m/s^2
= 20 N
Therefore, the satellite is exerting a force of 20 Newtons back on
you:
F = m * a
20 N = 75 kg * a
a = 20/75 m/s^s
a = .27 m/s^2
Therefore, your acceleration is .27 m/s^2.
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websearcher-ga
Search Strategy (on Google):
measure distances "light years" "astronomical units"
"greek letter" stars brightness
"celestial poles" "celestial equator" rotation
precession "celestial equator"
moon red "lunar eclipse"
lunar eclipses "new moon"
"total lunar eclipse" "solar eclipse" "on the moon"
"29.53 days" "27.32 days"
Ptolemaic model universe
copernican model universe
Tychonian model universe
kepler's three laws
galileo observations phases venus Copernicus
geocentric heliocentric "retrograde motion"
newton's three laws
moon "circular motion" tangential
"spectral windows" earth opaque
gravity "measure the mass" "astronomical bodies"
newton's gravity "Kepler's third law"
newton's "first law" "second law" redundant
"centripetal acceleration" formula
units force mass acceleration |
