Download Define the following terms in the space provided

SCI-103 Introductory Astronomy Exam #1 Fall 2009
Name_____________________
Class Meeting Time_____________
Grade Summary
HW #1_____________ (out of 20 points)
HW #2_____________(out of 20 points)
Extra-Credit ________
Exam #1 ___________ (6o points total)
Grade So Far _______ (This is a pretty meaningless measure of your performance
given that we are only 3 weeks into the course.)
Define the following key terms in the space provided. (2 Points each for a total of
20 points)
1) Astronomical Unit (AU):
An AU is unit of length based on the average distance between the Earth and
the Sun. 1 AU = 1.5x108 km.
2) Local Group
The Local Group is a small group of three spiral galaxies and several dozen
smaller galaxies of which the Milky Way is a part. It is the galaxy cluster that the
Milky Way belongs to.
3) Right Ascension (RA)
RA is the celestial coordinate on the sky that corresponds to the terrestrial;
coordinate of longitude. In the sky, RA starts at 0 hours RA and proceeds around
the sky up to 24 hours of RA. An hour of RA can be subdivided into 60 minutes
of RA and each of those minutes can be further divided into 60 seconds of RA.
4) Zenith
The zenith is the point on the celestial sphere that lies directly above the
observer’s head at an altitude of 90 degrees away from the horizon.
5) Constellation
A constellation is a well defined area of the sky that has boundaries that follow
lines of RA and dec. There are 88 official constellations that fit together like
pieces of a jig-saw puzzle to completely cover the celestial sphere. Every point
on the celestial sphere is a part of one and only one constellation.
6) Circumpolar star
Circumpolar stars are those stars that lay so close to Polaris (the NCP, really) that
they never dip below the horizon during their diurnal cycle. These are stars that
never set below the horizon because they are closer to Polaris than Polaris is top
the horizon. The number of circumpolar stars, or the size of the circumpolar
region, depends on an observer’s latitude. The number of circumpolar stars
decreases as the observer approaches the equator of the Earth.
7) Precession
Precession is the slow “wobble” of the Earth’s axis caused by the Sun and Moon
pulling on the slightly non-spherical Earth. This “wobble” changes the direction
that the Earth’s axis points in space, but not the magnitude of the angle. As a
result of precession, the NCP slowly cycles around the sky in a circle once every
26,000 years. Aristotle did not write of Polaris when he described the structure of
the universe, because the NCP was not toward Polaris at that time.
8) Solstice
The Solstices (winter and summer) are the two dates when the Sun appears at its
greatest southern and northern declinations of plus or minus 23.5 degrees. After
each solstice, the Sun will begin to appear to drift back toward the celestial
equator. The solstices occur on or near Dec 22 and Jun 22 each year.
9) Meridian
The meridian is a conceptual division of the observer’s sky that separates the
eastern and western halves of the local sky. It is an imaginary line that stars due
north on the horizon proceeds through the zenith and terminates due south on the
horizon. When stars or other astronomical objects cross the meridian they are
said to “transit” the meridian.
10) Sidereal Day
A sidereal day is the period of time required for the celestial sphere to make on
complete rotation on its axis. It is really the period of time the Earth requires
completing one rotation on its axis. The sidereal day is 23 hours, 56 minutes and 4.09
seconds long, about 4 minutes shorter than the solar day.
Address the following two questions on the following two pages in a paragraph
format. Be as detailed and quantitative as you can. (10 Points each for a total of 20
points)
11) Our Universe consists of a series of structures beginning with planets and ending
with the visible universe. List and briefly describe those structures beginning with
the smallest and ending with the largest. When possible give an approximate
dimension for each of the structures.
Your answer should contain the following structures and may include the scale
information in the table below.
Structure
Planets
Solar Systems
Stars
Star Clusters
Galaxies
Galaxy Clusters
Galaxy Super Clusters
Visible Universe
Scale
Earth is about 6,387 km in radius, other planets may
be a bit smaller or larger.
These structures consist of a central star, planets and
many asteroids and comets. Our solar system could
be considered 40 AU in radius, but the comets
extend much farther out.
Stars are much larger than planets. Our Sun is over
100 times the radius of the Earth. There are stars
some what smaller than the Sun and stars that are
vastly larger than the Sun
Star clusters come in two principle forms: open
clusters and globular clusters. They may consist of
several hundred to several hundred thousand stars.
Their diameters are measured in light years.
Galaxies are very large aggregations of stars, gas
and dust. Our Milky Way galaxy is about 100,000
light years in diameter.
Galaxies tend to be grouped in clusters that may
have only a few member galaxies up to ten’s of
thousands of galaxies. Our small galaxy cluster is
called the Local Group.
On extremely large scales galaxy clusters tend to be
grouped together into even larger structures called
super clusters. We are part of the Virgo super
cluster.
Astronomers believe that the Universe is infinite,
but we can theoretically “see” only that portion
whose light has had enough time since the Big Bang
to reach us. This visible universe extends outward
about 13 billion light years from the Earth.
12) The concept of the celestial sphere is a very ancient and still used conceptual model
of the night sky. Describe the celestial sphere, including its “geography”, relevant
time scales, and the position of the Earth and Sun.
Your answer should include the following points.

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
The Celestial sphere is a conceptual model of the sky.
The model of the celestial sphere is a very large sphere that carries the stars on
its surface.
The Earth is a very much smaller sphere at the center of the larger celestial
sphere. The Earth does not move in this model.
The celestial sphere appears to rotate once on its axis every 23h 56m 4.09s.
On very long time scales of 26,000 years the celestial sphere appears to
wobble due to precession of the Earth’s poles causing the position of the
celestial poles to cycle around the sky.
The axis of the celestial sphere intersects the celestial sphere at the north and
south celestial poles. These two points on the celestial sphere appear not to
move as seen from the Earth.
The north and south celestial poles are directly above the Earth’s north and
south geographic poles.
Mid-way between the celestial poles lays the celestial equator forming a great
circle around the celestial sphere.
The Earth’s equator lies directly under the celestial equator.
The Sun appears to move on a path around the celestial sphere called the
ecliptic that is inclined to the celestial equator.
Solve the following problems showing your work. (5 Points each for a total of 10
points)
13) Although the Sun contains 99.9 % of the mass of the Solar System, it occupies very
little of its volume. Illustrate how small the Sun is compared to the solar system by
calculating how many Sun’s could be set side-by-side across the Solar system. Use
1.4x107 km for the diameter of the Sun, 80 AU for the diameter of the Solar System,
and of course, 1 AU = 1.5x108 km.
This problem is simply solved using a ratio of the diameter of the solar system
over the diameter of the Sun
80 AU
80  1.5  108 km

 857
1.4  107 km
1.4  107 km
It would take 857 Sun’s side-by-side to cross the Solar System inside Pluto’s
orbit.
Note: I made an error in writing this question. The diameter of the Sun is not 1.4x10 7 kilometers
as I stated in the question, but 1.4x107 meters. Thus the real answer to the question is not “It
would take 857 Sun’s side-by-side to cross the Solar System inside Pluto’s orbit”. Actually, it
would take 857,000 Sun’s side-by-side to cross the Solar System inside Pluto’s orbit. The Solar
System is much much much larger than a single star.
14) Imagine that your are observing the stars at an observatory located outside Miami,
Florida (latitude = +26 degrees N),
A) At what altitude would Polaris appear above the northern horizon?
The altitude of Polaris (NCP, really) just equals your latitude. Thus, Polaris
would appear +26 degrees above the northern horizon.
B) Would a star with a declination of +63 degrees be circumpolar? Explain.
Since Polaris (the NCP, we assume) is 26 degrees above the horizon, the
circumpolar stars must be closer to Polaris than 26 degrees, otherwise they will
dip below the horizon on their diurnal cycle. Polaris has a declination of +90
degrees so any star with a declination greater than (90 - 26) = 64 degrees will be
circumpolar. So a star at 63 degrees declination will be circumpolar, but just
barely.
C) What would be the declination of a star that appeared at your zenith?
The declination of the zenith just equals your latitude, so the declination of the
zenith in Miami will be 26 degrees dec.
D) Would you be able to observe stars as far south as -50 degrees declination?
Explain.
The declination of the southernmost visible stars is determined by subtracting 90
degrees from the declination of the zenith. In Miami, the zenith has dec=26
degrees, so the southernmost visible declination is (26-90) = -64 degrees
declination. So the star at -50 degrees dec will be visible.
Answer the following Multiple Choice Questions by circling the correct
response. (2 Points each for a total of 10 points)
15) Imagine you are taking a leisurely sail down the inter-coastal waterway from New
York to Miami. Each night on your journey of a few weeks, you spend some time
observing the stars. Which of the statements below would represent one of your
observations?
A) The region of the sky that was circumpolar diminished each night.
B) Polaris was seen higher in the sky on each succeeding night.
C) The northern stars were above the horizon longer each succeeding night.
D) Stars near the celestial equator were above the horizon for about 8 hours.
E) All of the above were observed
Use the drawing below showing the Sun and several constellations to answer the next two
questions.
Sun
Gemini
Taurus
Cancer
Leo
Pisces
 East
South
West 
16) If you could see stars during the day, the drawing above shows what the sky would
look like at noon on a given day. The Sun is near the stars of the constellation
Gemini. Near which constellation would you expect the Sun to be located at sunrise?
A) Leo
B) Cancer
C) Gemini
D) Taurus
E) Pisces
17) Imagine that you are visiting OCC’s sister University of Namibia located next to the country of
South Africa. You step outside at night to see the stars. Where would you look to see Polaris?
A) North, near the horizon (0 altitude)
B) Near the zenith (90 altitude)
C) North, near 43 altitude
D) Northwest, in the direction towards the U.S.A.
E) Polaris cannot be seen from this location.
18) The long exposure image of star trails shown below was taken while looking toward the east. From
which of the locations listed below was the image obtained?
A) Fairbanks, Alaska (Latitude 65oN)
B) Syracuse, New York (Latitude 43oN)
C) Miami, Florida (Latitude 26oN)
D) Phitsanulok, Thailand (Latitude 0oN)
E) Sydney, Australia (Latitude 34oS)
EASTERN HORIZON
19) Polaris has not always been the “North Star”. Which of the statements below best describes the
reason why Polaris has not always been the “North Star”?
A) The stars are not really fixed to the celestial sphere and the motion of Polaris through space will
move it away from the celestial pole and, in time, another star will move into the space to
become the “pole star”.
B) The rotation period of the Celestial Sphere is slowly increasing causing Polaris to slowly move
away from the celestial pole.
C) The Celestial Sphere is slowly tipping over so that the North Celestial Pole will become the
South Celestial Pole in about 26,000 years.
D) The Earth’s rotation axis “wobbles” around a cone shape every 25,800 years.