High on Palomar Mountain, at an elevation of 5500ft to avoid light pollution, the Palomar Observatory is simply spectacular – as large as Rome’s Pantheon, with a classic design dating from the 1930s. Run by Pasadena’s prestigious California Institute of Technology, it houses five telescopes including the 200-inch Hale Telescope, once the world's largest.

　　On weekends between April and October, visitors can take guided tours, usually given at 11am and 1:30pm. Tickets are first-come, first-served. Bring a fleece jacket – temperatures inside the observatory hover just above freezing during cooler months.

　　Call ahead to check road conditions and opening hours before making the long, winding drive here.