Looking at a graph of sunspot data, I independently discovered the well-known Gleissberg Cycle, a 78 year cycle of sunspots.
If you look at the following graph, you can easily spot the 11 year cycle of sunspots. However, if you look more distantly, you can also spot the longer Gleissberg cycle, with lows in the 1800s and 1900s, and highs in the 1850s and 1950s.
I was a bit sad to find that I had discovered something that was already known. An interesting discovery, nevertheless.
If you want to play with the numbers and plots, here is the source of the data I used to generate the plots.