astronomical observatory located 16 km (10 miles) south of the town of Arecibo in Puerto Rico; it is the site of the world's largest single-unit radio telescope (telescope). This instrument, built in the early 1960s, employs a 305-metre (1,000-foot) spherical reflector consisting of perforated aluminum panels that focus incoming radio waves on movable antenna structures positioned about 168 metres (550 feet) above the reflector surface. The antenna structures can be moved in any direction, making it possible to track a celestial object in different regions of the sky. The observatory also has an auxiliary 30-metre (100-foot) telescope that serves as a radio interferometer and a high-power transmitting facility used to study Earth's atmosphere.

Scientists using the Arecibo Observatory discovered the first extrasolar planets (extrasolar planet) around the pulsar B1257+12 in 1992. The observatory also produced detailed radar maps of the surface of Venus and Mercury and discovered that Mercury rotated every 59 days instead of 88 days and so did not always show the same face to the Sun. American astronomers Russell Hulse (Hulse, Russell Alan) and Joseph H. Taylor, Jr. (Taylor, Joseph H., Jr.), used Arecibo to discover the first binary pulsar. They showed that it was losing energy through gravitational radiation (gravitation) at the rate predicted by physicist Albert Einstein (Einstein, Albert)'s theory of general relativity (relativity), and they won the Nobel Prize for Physics in 1993 for their discovery.