Download Motions in the Night Sky and the Celestial Sphere

Fa10
Motions in the Night Sky and the Celestial Sphere
Section I: Motions in the Night Sky
Description:
The celestial sphere is a scientific model of the universe. It is not representative of the entire
universe, rather it mimics the motions of objects in the night (and daytime) sky. The celestial
sphere model cannot be used to accurately explain the motions of objects in the sky, since it is
not an entirely accurate representation of the physical universe. A planetarium or planetarium
software (like Starry Night Pro, or the World Wide Telescope) also constitute scientific models
of the universe that are not entirely accurate representations of the physical universe in the same
way a celestial sphere is not an entirely accurate representation of the physical universe.
This exercise incorporates real student observations and a representation (model) of the universe
(a celestial sphere, a planetarium or planetarium software) and examines the usefulness of these
models while identifying the model’s shortcomings.
Introduction:
A scientific model is one that is based on scientific observations and represents a physical system
that accurately reflects at least one aspect of the physical system. For example a scientific model
of the solar system may include an accurate representation of the planets’ sizes relative to one
another, but not their distances from one another. Such a model would still be considered a
scientific model. Several scientific models have been developed that mimic, with some accuracy,
the motions of objects in the sky. In this exercise, you will observe the motions of the stars in
your night sky and compare your observations to one of the several models astronomers have
developed to mimic these motions. Then, you will critically analyze the usefulness of the model
to which you compared your observations.
First, you will make some observations. You already know that the Sun rises in the morning and
sets in the evening, bringing on night. At night we can observe stars, the Moon and some planets
without the aid of a telescope or binoculars. You will need to find a location that is dark enough
so that you can see at least 10 stars. It would be best if the stars you can see are in a memorable
pattern or part of a constellation, but depending on your proximity to a large city you may or
may not be able to find a spot where you can see the Big Dipper, for example.
Part I: Observations of the Night Sky
1. Before you make any observations, write down a prediction: Considering that the
Sun and Moon both rise and set, predict what the motions of the stars you observe
will be. Try to be as complete as you can with your prediction. Identify the direction of
the motion you predict as well as the rate of motion.
2. Make and record an observation of the night sky. If you are in the northern
hemisphere, face any direction but north. Observe the night sky and make a drawing
representing what you observe. Include some of your horizon as well as at least ten stars
or patterns or other celestial objects you see in the night sky. It’s not important that you
can identify the constellations or stars, but you can try to do that if you want to. Note the
time of your observation and include cardinal directions (North, South, East or West) on
your drawing.
3. Make and record a second observation of the night sky. Once you have finished your
drawing of your first observation., stop and do something else. Come back to the same
location, facing the same direction after about 1 hour and make a new drawing of what
you see. Try to find at least some of the same stars or patterns as before. Again, be sure to
include some of your horizon as well as the stars and other celestial objects you see in the
night sky. Note the time of your observation and include cardinal directions (North,
South, East or West) on your drawing. (It is not necessary, but you could continue to
make observations one hour apart for as long as you are able to get more accurate data.)
4. Compare the two observations you made. Knowing the time between the observations,
compare the two and determine whether you observed motion. Then determine the
direction and rate of the motion you observed. (About how far did the stars move and in
what general direction?) To determine whether you’ve analyzed your data sufficiently,
determine the time it would take for a star you observed to set (disappear beyond the
horizon).
Part II: Using an Astronomical Model
5. Use an astronomical model to check your observations. Set up a celestial sphere,
planetarium or planetarium software to show the night sky at the time and day that you
made your first observation. Now you can check to determine what objects you saw.
Identify at least 10 objects (stars or planets or other celestial objects) from your drawing
using the model you chose. Determine the time it would take for a star you observed to
set (disappear beyond the horizon).
a. Celestial Sphere. Using a celestial sphere, you will need to move the Sun to the
correct position for the day that you made your observation, then rotate the sphere
to the time that you made your observation. You can then check your observation
by fixing the horizon to match your latitude. On the sphere, larger dots are
brighter stars. You will not be able to identify planets or the Moon with this tool.
b. Planetarium. The planetarium can be set to mimic exactly the time and day that
your observation was made. The horizon in the planetarium will be more
extensive than what you observed. Brighter objects will appear larger in the
planetarium “sky”. Most planetaria include constellation outlines and a “moon”.
You may need to use a star chart to determine the names of the stars you
observed.
c. Planetarium Software. Starry Night ProTM, World Wide TelescopeTM and Sky
GazerTM all have the capability of being set up to show the night sky at any time
and for any location on Earth. Also, each of these will label constellations and
allow the user to determine the names of any objects, including galaxies, nebulae,
planets and stars.
2
6. Compare your observations to your prediction. Answer the following questions
regarding your observations and prediction. Each answer should be at least a few
sentences, not just a single word or sentence. Try to give as complete an answer as
possible.
a. Did the stars move as you expected? (If you did not expect them to move,
rephrase this question to read: Did the stars stay stationary as you expected?) If
the answer is no, describe in detail how the motion of the stars differed from your
prediction.
b. Compare the motions of the stars that you observed to the motion of the Sun.
Use the planetarium software, the celestial sphere, or a planetarium to simulate
the motion of the Sun.
c. Describe, in detail, using only your observations and the model you have
(celestial sphere, planetarium or planetarium software) the reason for the
apparent motions of the stars across the sky. It is not important that this answer
is scientifically correct. It is more important that you base your explanation on the
model you are using and the observations you made. (Are the stars moving around
Earth, or is Earth’s motion causing the stars to appear to move? What does the
model you are using make it seem like and why?)
d. Describe, in detail, using resources in your textbook, in the library or on the
internet, the reason for the apparent motion of the Sun across the sky. For
this response, be sure to cite the resources you use. This answer should be a
scientifically correct explanation for the apparent motion of the Sun across the
sky. Check with your instructor to make sure you have the correct scientific
explanation before you proceed to the next question. Would this reason explain
the star motions you observed?
Part III: Evaluation and Final Product
7. Evaluate the model by answering the questions below.
a. Compare the model you have (celestial sphere, planetarium or planetarium
software) to the correct scientific explanation. Does the model demonstrate a
correct explanation for the motions of the Sun and stars?
b. Evaluate the usefulness of the model for demonstrating the motions of
objects in the sky. Does the model replicate the motions accurately? Is the model
easy to use?
c. Evaluate the usefulness of the model for explaining the reasons for the
apparent motions of objects in the sky. Does the model create any
misconceptions about the motions of objects in the sky?
OPTIONAL Step 8. Your final product should be a paper that describes the experiment
(what you set out to understand and how you set out to understand it), the hypothesis
(your prediction), the data collection (how you made your observations), the data (your
observations), the analysis (how you compared your data to a model and your answers
to questions 6a-d), and your conclusions (your answers to the questions posed in step 7).
3
Section II: Celestial Sphere
OBJECTIVE: Last semester (Phys 1411) we familiarized ourselves with star charts and how to
use them to find objects relative to others, such as how to find a planet relative to the stars and
constellations. This is due to the fact that many solar system objects do not require use of
coordinates to find, nor do they require the use of a telescope to observe. We also noted the
motions of the nighttime sky.
This semester we are studying objects that are much smaller, many of them requiring the use of a
telescope to observe. Thus, we will find it necessary to understand more deeply how to use and
read star charts, and how to use and read coordinates.
BACKGROUND
Finding the stars in the nighttime sky is very similar to finding a location on Earth: both use a
coordinate system of some sort. In both cases there is more than one system that can be used,
though there is one which is predominately preferred.
Earth
On Earth, the preferred coordinate system uses latitude and longitude. This system uses the
Earth’s equator, 0° latitude, as the base line for one coordinate and the Prime Meridian, 0°
longitude which runs through Greenwich, England, for the other. Lines of latitude form circles
around the globe, decreasing in size as they near one of the two poles. They are described as
North or South of the equator, and range in measurement from 0° at the equator to90° at the
poles. Both of Earth’s axial poles correspond to both of the celestial poles.
Lines of longitude originate at one pole and spread-out as they near the equator where they reach
the maximum distance between the lines. As the lines continue past the equator, the distance
between them begins to shrink until they all converge at the opposite pole. They are described as
East or West of the Prime Meridian (where the meeting of East and West on the opposite side of
the globe occurs at the International Date Line). Lines of longitude range from 0° at the Prime
Meridian to 180° which is generally the International Date Line. (Technically, the IDL wiggles
back and forth for political reasons, not always following the 180° parallel, or line of longitude.)
Heavens
In the nighttime sky, many of the same ideas and terms are used, though they may not be defined
the exact same way. The celestial equator is the Earth’s equator “broadcast” onto the nighttime
sky. Thus, an observer from space would notice that the celestial equator and Earth’s equator
“line-up,” or lay one on top of the other. Lines of declination are very similar to the lines of
latitude on Earth; the celestial equator is the base line with lines of declination forming circles
which decrease in size as they near a pole. They, too, are measured from 0° at the celestial
equator to 90° at the poles. However, in space “north” and “south” have no meaning, so lines of
declination are described as “+” or “-” where “+” would be similar to “north” on Earth (towards
the North Celestial Pole) and “-” would be in the direction towards the South Celestial Pole.
Declination is measured in degrees. Each degree is subdivided into 60 minutes (or 60’), and each
minute is subdivided into 60 seconds (or 60”). The abbreviation for declination is dec. and the
symbol for declination is  (lower-case delta).
4
Lines of right ascension are similar to the lights of longitude on Earth. Right ascension lines start
at one celestial pole and spread-out as they near the celestial equator, where they reach maximum
distance between the lines. However, the Earth’s Prime Meridian does not correlate with the
vernal equinox. The base line for right ascension is the vernal equinox; an event that occurs on or
near March 21st every year. This is where the ecliptic crosses the celestial equator. However,
unlike longitude, right ascension is measured in hours, minutes, and seconds. Whereas there are
360° of longitude lines, there are only 24 hours of right ascension. And whereas longitude is
described as East or West, right ascension is in “military time,” meaning there is no directional
description. The abbreviation for right ascension is RA and its symbol is  (lower-case alpha).
STAR CHART BASICS
Because a star map cannot make the ink darker as a way to describe how bright an object is, they
will make the dot larger. Often, the Bayer designation is listed next to the stars. While the
brighter stars have names, such as Sirius, most stars do not. Johann Bayer in 1603 suggested a
naming method that would give a name to many more stars. He started with the Greek letters,
, and placed them in the front followed by the genitive of the Latin name for the
constellation region the star was found in. Usually, the letters closer to the beginning of the
Greek alphabet were used for the brighter stars for each constellation. For example, Sirius is the
brightest star in Canis Major. Thus, its Bayer designation is  CMa. Sometimes a two-digit
number may be found next to the object. This is its magnitude. Remember, the brighter the
object, the small or more negative the number. Often objects are described by a shape on a star
map. For example, galaxies are often ovals. Each map will have a legend, or key, for the shapes
used and for how the size of the dot corresponds to the magnitude. (Star charts ONLY use
apparent magnitude, NEVER absolute magnitude.) Remember, the cardinal directions East and
West are always backwards on a star map. This is because the map is intended to be used over
one’s head, not on a tabletop. Once the map is placed over one’s head, we notice that all four
cardinal directions line up correctly. Another item to look for on a map is the time the map is
intended to be used. Some maps require a simple calculation to determine the local solar time.
Many maps, however, are set for some civilian time. Another feature found on many maps is the
location of the ecliptic. This is where one will look to find the Sun, Moon, and planets. It is also
where the zodiacal constellations are found.
PART IV: EXERCISES
DIRECTIONS: Use the star charts passed out in class, a pencil, and an erasure to answer the
following questions.
8. The Earth coordinates for Dallas, Texas are approximately 32° 47’ and 96° 48’. How far
above my northern horizon should I expect to find Polaris? ______________________________
9. What would I expect the radius (in degrees) of my circumpolar region be for Dallas, Texas?
______________________________________________________________________________
5
10. Sketch an outline of your circumpolar region on Chart 1. Then list the 6 constellations that
are located within the circumpolar region for Dallas, Texas.
____________________________________
______________________________________
____________________________________
______________________________________
____________________________________
______________________________________
11. How many degrees exist between a pole and the celestial equator? _____________________
12. How many degrees above my southern horizon is the celestial equator?__________________
13. How high above the northern horizon is my zenith from Dallas, Texas? _________________
14. What is the declination of my zenith point, as viewed from Dallas, Texas? _______________
15. How many hours are in one day? ________________________________________________
16. How many hours of sky can I see any time I view the nighttime sky? ___________________
17. How many hours exist between the meridian and the eastern horizon? __________________
18. What is the RA of tonight’s meridian? ___________________________________________
19. What is today’s date? _________________________________________________________
20. What constellation is the Sun in today? ___________________________________________
21. When is your birthday? (month and day only) _____________________________________
22. What constellation is the Sun in on your birthday? __________________________________
23. Between what dates will Orion be on the meridian at 8pm local time?
______________________________________________________________________________
24. What star is located at 6 hrs 44 mins right ascension and -16.7° declination? _____________
25. What star is located at 5 hrs 14 mins right ascension and +46.0° declination? _____________
26. What star is located at 5 hrs 13 mins right ascension and -8.2° declination? ______________
27. What star is located at 13 hrs 23 mins right ascension and 55.1° declination? _____________
28. What star is located at 16 hrs 28 mins right ascension and -26.4° declination? ____________
29. What are the coordinates for the star Castor in the constellation Gemini?


6
30. What are the coordinates for the star Dubhe in the constellation Ursa Major?


31. What are the coordinates for the star Deneb in the constellation Cygnus?


32. What are the coordinates for the star Fomalhaut in the constellation Piscis Austrinus?


33. What are the coordinates for the star Mimosa in the constellation Crux?


34. What is the declination for the vernal equinox? ____________________________________
35. What is the declination for the autumnal equinox? __________________________________
36. What time (on the clock) are the maps made for? ___________________________________
The diagram below is to help students who are more visual “to see” the geometry of the horizon
(horizontal circle or oval), the North Celestial Pole (arrow pointing up and to the right), and the
Celestial Equator (the semi-circle or semi-oval going up and to the left). It does not require a
response.
7