The cross staff is an ancient instrument to measure angular size or angular separation. We will use it to determine the angular separation of the stars in the Big Dipper. You will need the template on the following page, file folder or poster board, glue, a scissors or knife, and a yard stick. Mount the template onto the file folder or poster board and carefully cut it out, making sure that the observing jaws are accurately cut to one, two, and four inches (see the drawing below). The little rectangle labeled CUTOUT is where the yardstick fits and should be cut to the size of your yardstick. Cut on the solid lines and fold away from you on the dotted lines. After folding along the long dotted line, the bottom section should be at right angles to the top section. Insert your yardstick through the cutout hole and fold the "T" section down so that the "A's" and "B's" come together. Staple or glue the "A's" and "B's" together.
The drawings also indicate how the cross staff is used. Resting the zero end of your yardstick lightly against your cheek, you can sight down the stick and line up objects with the edges of the observing jaws. Slide the jaws toward you or away from you in order to achieve the proper alignment. Notice that you have three jaws that can be used. They are one inch, two inches, and four inches. Choose the set that is most convenient for the angle you want to measure. For measuring angular separations, it is best to align the objects so that they just barely disappear behind the jaws you choose. When you have achieved a good setting from your cross staff, record the position of the slide on your yardstick. Use the graph to obtain the angular separation.
| Bear | ||
| Flank of the Bear | ||
| Thigh | ||
| Root of the Tail | ||
| unknown | ||
| Horse[and Rider] | ||
| Leader of the Mourning Daughters |
Use your cross staff to determine the angular separations listed below. Note that during the winter months you may have to use Orion or Cassiopeia instead of the Big Dipper for this exercise.
