This work should take LESS THAN ONE DAY . From the velocity data, students extract the systemic velocity and the rotation curve . This work should take ONE DAY . Plotting their rotation curves, students immediately realize that they are flat, rather than falling off as did their analogous curves for planets around stars. This is the first surprise.
Students also determine the galaxies' masses from their rotation curves.
This is the second surprise. This work should take ONE DAY . We are now nearly half-way through the semester, and we have reached a major watershed in the course. Here is a good point for a discussion, summarizing what has happened and placing it in context. Many suggestions come up, and the students' first propose far-out things like axions, cosmic strings and the like.