January 1, 2018
Nearly every intro astro class in the previous century probably involved the instructor bringing out the overhead projector and placing two transparencies with some galaxies drawn on them, one of which was slightly larger than the other. The instructor would then overlay them on the projector platform, and slowly move one of them around. The result was a very convincing demonstration that in an expanding universe, every place has the same right (or lack there of) to call itself the center.
Here is the modernized digital version - part of the collection of digital simulation for math and science at ScienceSims.com.