Name ______________________________________________ Date ______________
Partners _________________________________________________________
Astronomy Exercise #1: Celestial Sphere
by M.L.West
Purpose: to become familiar with two coordinate systems which are used to describe the positions
of objects in the sky and to become adept at using a plastic celestial sphere as an analog computer.
Background: Poles have latitude = _______, _______. Equator has latitude = _______
New Jersey has latitude = _________
I. Various coordinate systems, each based on a different plane.
A. Horizon System
Plane: _______________________________________________
Angle along plane: ________________________________ range: _________________
starting at: ________________________________, increasing: _____________
Angle perpendicular to plane: ________________________ range:_________________
B. Celestial Equator System
Plane: _____________________________________________________
Angle along plane: _________________________________ range:________________
starting at: _________________________________, increasing: ____________
Angle perpendicular to plane: _________________________ range:________________
II. Circumpolar Constellations (_____________________________________________________)
A. For an observer living at the Earth's equator (latitude = ______)
Pick a country on the equator and set the celestial sphere on its base so that a tiny person
(post-it note) in that country would be on the top of the world. Reach into the globe and
carefully set the horizon ring parallel with the tabletop (horizontal!). It now represents that
person's horizon plane.
Slowly and carefully turn the Earth knob and notice how stars would appear to rise and eventually
set for the tiny person. How many constellations are circumpolar for an observer at the equator?
______________________________
B. For an observer living at the Earth's north pole (latitude = ______)
How many constellations are circumpolar for an observer at the north pole?
_________________________________
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C. For an observer living in New Jersey (latitude = ________)
How many constellations are circumpolar for an observer living in New Jersey? _________.
Their names are _________________________________________________________________
______________________________________________________________________________
III. Time above the horizon
A. The length of time a star spends above the horizon depends on the star's declination angle
(in the _____________________ System). Notice the thin black hour lines on the sphere.
Set the celestial sphere and the horizon ring for an observer living in New Jersey. Pick a
bright star at declination = 0 degrees. Mark it with your finger. Carefully turn the earth knob
until the horizon ring just touches the star. As you slowly turn the earth knob, count the
hours until this star touches the other horizon and sets. ( ________ hours)
Make a quick prediction (a sketch graph showing ranges, 3 points, trend) as to how you
think this time above the horizon will depend on a star's declination angle:
B. Make a data table by listing vertically the declination angles from -90 degrees to +90
degrees in 10 degree increments. To the right of each angle record the number of hours a
star at that declination would be above the horizon. (To the nearest half hour is sufficient
precision.)
Make a spreadsheet of these data.
C. Make a graph of these data and print it. By eye, draw a smooth curve through these points,
recalling the precision with which they were measured. Do not make a jerky connect-thedots line.
Comment on how well this graph corresponds to your initial prediction above:
IV. The Sun
A. As the Earth travels in its orbit we see the sun "move" against the background of the
___________ constellations of the zodiac. On the plastic celestial sphere the sun's positions are
indicated by black dots with dates near them. For the first day of each month record in your
spreadsheet the sun's constellation, right ascension, and declination.
B. How many hours of daylight per day do you think there are in New Jersey as a function
of the time of year? Make a quick prediction (graph, ranges, 3 points, trend):
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Now you can use your first graph (Hours Seen vs. Declination) to find the number of
hours the sun would be above the horizon at its declination on the first day of each
month. Include these data in your spreadsheet in a new column titled "Hours of
Daylight". Remember to read the smooth line not the raw data points.
Make and print a graph of Hours of daylight vs. Month of the year.
Comment on how well this graph corresponds to your initial prediction above:
V. Discussion Questions
A. Define zenith
Define nadir
B. Why would someone prefer each of these coordinate systems (Horizon, Celestial
Equator)?
C. Make a quick prediction of the hours of daylight vs. month of the year for an observer at
the equator,
and for an observer at the north pole.
Your lab report is due next week and should include these pages, your spreadsheet with
graph of Hours Seen vs. Declination, graph of Daylight Hours vs. Month, and the answers
to the Discussion Questions. You may (in fact, are encouraged to) work with other students,
but each person is to turn in his or her own lab report.