Harquahala Peak Observatory, built by the Smithsonian Institution in 1920 on a remote summit in western Arizona near Wenden, served as an early high-altitude solar research station. Chosen for its clear skies and lack of pollution, the adobe observatory operated from 1920 to 1925, using a pyroheliometer to measure the solar constant and explore the Sun’s influence on Earth’s climate. Though the project did not yield conclusive weather forecasting results, work here and at Mt. Montezuma helped support the conclusion that solar energy reaching Earth was constant over the planet’s surface. Its scientific importance later earned National Register recognition.