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Reconstruction & Gilded Age

Belgian Transit of Venus Observation Site

San Antonio, Bexar County, Texas

Marker Inscription

The year 1639 marked the first recorded observation of Venus crossing the sun. Sir Edmund Halley (1656-1742) later predicted that using data from such crossings, known as transits, scientists could precisely quantify the astronomical unit of distance and measure the size of the solar system. The transits of Venus predicatably come in pairs, eight years apart, approximately every 120 years. Scientists from around the world traveled to observation sites during the next events, in 1761 and 1769, but timing of the transits was difficult due to the "black drop effect," in which two objects against a bright background appear to blend. For more than a century, astronomers prepared for the next opportunity to record Venus' transit. Jean-Charles Houzeau (1820-1888) was a Belgian astronomer with an eclectic history. From 1859 to 1861, he worked as a surveyor in Uvalde, Texas. An abolitionist, he left Texas at the start of the Civil War. In the late 1870s, he returned to Belgium, where he became director of the Royal Observatory and planned the Belgian teams that would go to the western hemisphere to observe the December 1882 transit of Venus. He chose one site in Chile and one (22 feet east) in San Antonio. Good observations required clear skies, and San Antonio offered the chance of favorable weather, as well as good logistics for communication and transportation. A U.S. Naval Observatory team observed from a site on the grounds of nearby Fort Sam Houston. Using a heliometer, a device he had developed for the observation, Houzeau obtained 124 photographic plates of Venus silhouetted against the sun. Because of clearer conditions, the team in Chile obtained 606. The Belgian findings equaled those of larger nations, and Houzeau's decision to bring an international team to San Antonio provided the city recognition as part of astronomical history. (2005)

Erected by Texas Historical Commission

The Story

The rare passage of Venus across the face of the sun, which Edmond Halley realized could help measure the size of the solar system, drew astronomers around the world when the paired transits came due. For the December 1882 event, Belgian astronomer Jean-Charles Houzeau—an abolitionist who had once surveyed in Uvalde and left Texas at the outbreak of the Civil War—chose San Antonio as one of his two observing stations for its clear skies and good communications. Using a heliometer of his own design, his team captured 124 photographic plates, matching the work of far larger nations.

Why it matters

The observation placed San Antonio on the map of international science, a reminder that a Texas town once helped humanity take the measure of the heavens.

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Related people

  • · Jean-Charles Houzeau
  • · Edmond Halley

Related events

  • · 1882 Transit of Venus

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