With your group, travel to a dark spot on a clear, moonless night. Trace the path of the Milky Way across
the sky. Have members of your group point out dark rifts in the Milky Way to one another. (You should do
this in summer or autumn when the summer Milky Way is high in the sky in the evening.)
Take a drive with members of your group on a busy highway. See if members of the group (not the driver)
can spot moving knots of traffic that are analogous to density waves. Note the speed of the car while outside
the knot and while in the knot. See if you can estimate how much denser traffic is in the knot than outside.
Afterward, discuss how your observations are related to the density wave theory of spiral structure.
Using Figure 22.36 as a guide, have members of your group line up and hold a long piece of string. Then
have the group walk in circles of different sizes, all moving at about the same speed. Notice how the string
(the spiral arm) gets thoroughly wound up before even a couple of orbits have been completed.
