LABORATORY EXERCISE 1:  BUILDING AND USING A CELESTIAL SPHERE

 

 

PURPOSE

To construct and use a Celestial Sphere to show the motion of the Sun and stars in the sky.

 

 

 

               MATERIALS

               2 plastic hemispheres                                                                scissors

               star chart sheets - north and south                                                colored map pin

               protractor (the larger the better)                                                    transparent tape

               colored marking pens (transparency pens work best)                        ruler

               ecliptic strips (attached)                                                             thumb tack

               small Earth globe                                                                     styrofoam block

               dowel

               straw

               construction paper

 

PROCEDURE

1.     Cut out the two star charts with the scissors.  Cut along the outside lines only.  The star chart will look like a flower with eight black petals.

2.     Place the chart of the southern sky inside one of the plastic hemispheres with the printed side facing up.  CAREFULLY align the chart so the ends of the ecliptic (the line that crosses each of the chart's "petals") touch the base of the hemisphere at two opposite ridges.  Secure the chart by placing the other hemisphere over the star chart and pushing it against the first hemisphere (see Figure 1). Make sure that the ridges of both hemispheres match.

3.     Check this hemisphere/star chart sandwich" with the instructor or lab assistant before continuing to the next step.

4.     Tape the edges of the two hemispheres together.

5 .    Mark the stars on the inside of the inner hemisphere with the marking pen.  Also, draw the lines that mark the ecliptic and some brighter constellations. (You may wish to use different color pens for the ecliptic and the constellation lines.) The brighter stars are indicated by the bigger symbol. (The "magnitude" of a star is an indication of its brightness.  A zero magnitude star is the brightest and the fourth magnitude star is the dimmest on this chart.)

6.     When you have marked all the stars, separate the hemispheres and remove the star chart.  Repeat Steps 2-4 with the northern star chart and the unmarked hemisphere (the "outer" hemisphere in Figure 1).  Confirm that the ecliptic lines touch the base at opposite ridges. (Use the hemisphere you have already marked to secure the chart in place.)

7.     Check this hemisphere/star chart sandwich" with the instructor or lab assistant before continuing to the next step.

 

 

 

 

Adapted from Project STAR, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138

8.   Look into the opening of the northern hemisphere (see Figure 2).

 

       1.1  What star is found in the center of the northern hemisphere?

 

 

 

       1.2  What pattern of stars can be used to help you locate this star?

 

 

 

The constellations with lines on the northern hemisphere are Leo, Gemini, Taurus, Pegasus, Cassiopeia, Cygnus with the Summer/Fall Triangle, the Big Dipper (an asterism - an easy to recognize pattern, not a constellation), and northern half of Orion.  The lined constellations on the southern hemisphere are Scorpius, Sagittarius, Canis Major, the Southern Cross, the southern half of Orion, and the stars Alpha and Beta Centauri.

 

9.     Cut out the two strips of dates on the attached ecliptic strips. The strips should appear as shown in Figure 3. Tape the MAR to JUN to SEP strip into the northern hemisphere.  The ecliptic line on the hemisphere should pass through the center line of dates on the strip.  The S mark in JUN should line up with the middle ridge of the northern hemisphere.  Tape the SEP to DEC to MAR strip into the southern hemisphere.  The ecliptic line should run through the middle of the strip and the W in DEC should line up on a ridge.  Tape the strips in at least three places so when a pin is pushed through the plastic into the paper strip, the strip will not pop into the hemisphere.

 

 

 

 

 

 

 

 

 

 

 

                                 

                                                      

 

10.     Slide the Earth globe to the center of the wooden dowel.  Cut the drinking straw into two pieces, each 7.5 cm long.  Slide these two pieces of straw over each end of the dowel.  (Figure 4) 

 

11.     With a thumb tack, make a small hole through the center of both hemispheres (where the ridges cross).  Slide the two star hemispheres onto the dowel, with the southern hemisphere of the small Earth globe facing into the southern bowl of stars and the northern Earth globe hemisphere facing into the northern bowl of stars.  (Figure 5)

 

12.     Rotate the hemispheres until the points where the ecliptic touches the equator match on both hemispheres.  (The ecliptic should completely encircle the sphere and should pass both above and below the equator. )  The dimples on the northern hemisphere should match those on the southern hemisphere.  (Figure 6)

 

Adapted from Project STAR, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

13.     The clear plastic sphere should rotate freely on the dowel; the Earth globe should be at the center of the sphere with North America facing up; and the northern hemisphere should be on the upper half of the sphere.

14.     Tape the edges of your two spheres together.  Trim the rim of the plastic sphere leaving the plastic bumps. With the hemispheres aligned as in Step 12, tape the spheres together.

15.     Use the foam block to hold the sphere at an angle appropriate to your latitude.  (The latitude of Colorado Springs is about 40o)  See figure 7.   

 

 

                                     

 

 

                            

 

 

                                                                                                                                        

 

 

 

 

16.     Measure the height of the center of the Earth globe above the table top.  Cut two strips of construction paper, each with a width equal to the height of the Earth globe above the table.  Tape the pieces together to make a strip of paper 80 cm to 90 cm long.  Cut these strips evenly!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Adapted from Project STAR, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138

17.     Refer to Figure 8. Connect the ends of the long strip together with tape to form a loop.  The top edge of the construction paper loop represents the HORIZON, the imaginary circle one sees where the sky "touches" the earth.  The Celestial Sphere should be able to sit inside of the paper loop.  Make any necessary adjustments of the paper clips to tighten the horizon around the sphere.  The top edge of the loop, which represents the horizon, should be at the same height above the table as the center of the Earth globe.

18.     On the upper, outside edge of the paper collar, at the point directly beneath where the wire touches the sphere, place an N, for North.  On the paper horizon opposite N, place an S for South.  Looking down on the horizon, go clockwise halfway from N to S. Mark this point E for East.  Opposite E, place a W for West.

19.     Save the red pin, paper horizon, and Celestial Sphere for later activities.

 

This Celestial Sphere will be used in this and a later activity to help you describe, explain and predict the motions of the Sun and the stars in the sky.  Identify the following reference points and lines on your Celestial Sphere. 

 

20.  Refer to Figure 9. There are imaginary points such as the North and South Celestial Poles (N.C.P. and S.C.P.) and an imaginary line such as the Celestial Equator positioned on this sphere.  The Earth is located at the sphere's center.  Earth's Equator Currently, the North Celestial Pole is very close to a star called Polaris.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

21.  Refer to Figure 10. The Ecliptic is the apparent annual path of the Sun on the Celestial Sphere.  Notice that the ecliptic is tilted relative to the celestial equator.  The highest point on the Ecliptic is called the Summer Solstice, and the lowest point is the Winter Solstice.  As the sun appears to move across the sky, it passes through a band of constellations known as the Zodiac

 

 

 

 

 

 

 

 

Adapted from Project STAR, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138

 

 

 

 

 

 

22.  Refer to Figure 11. The point directly over the head of the observer is called the Zenith.  The Horizon is located 90o  in every direction away from the observer's zenith. If the observer were standing on a flat desert plain or in a boat on a calm sea, the horizon would be the circle where the sky meets" the land or water.

 

 

 

Figures 9, 10, and 11 are related to the celestial sphere that you have just built.  The dowel that passes through the globe is the vertical dashed line in Figures 9, 10, and 11. The point where the dowel comes out of the northern hemisphere is the North Celestial Pole; the point where the dowel goes into the southern hemisphere is the South Celestial Pole.  The Celestial Equator is where the two hemispheres are fastened.  The Horizon is the top edge of the paper collar.  The Ecliptic is the circle made by the arcs drawn on the inside of each hemisphere.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Adapted from Project STAR, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138