According to Official Guinness Records,
The largest astronomy lesson consists of 1,580 people, and was achieved by the Fairbanks Museum and Planetarium (USA) in St. Johnsbury, Vermont, USA, on 10 August 2018. The event was organized to bring attention to the Fairbanks Museum and Planetarium, celebrate that it is the only public planetarium in the state, and provide an exciting experience for the community. Plus, the record attempt matched the museum's mission of "inspiring wonder, curiosity, and responsibility for the natural world.
For a complete list of 2018 records, please visit 2018 Guinness Records in United States.