Belgian Transit of Venus Observation Site

Houzeau's first Texas years were political. Removed from the Belgian Royal Observatory in 1849 for his views, he was in Texas by 1858, sided with Unionists and abolitionists, helped some Unionists escape from San Antonio, and fled to Mexico disguised as a laborer. In New Orleans he worked on Louis Charles Roudanez's bilingual Tribune, which historian Eric Foner credits him with making widely respected, then spent eight years in Jamaica. For the transit he lodged in a house near Fort Sam Houston, in a city then rated the best observing station in North America.

What the plaque says

Belgian Transit of Venus Observation Site 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)

Where it stands

Worth the stop nearby

• The Alamo — 1.7 mi
  Mission San Antonio de Valero before it was a fort: a church its builders never finished, a thirteen-day siege in 1836, and the Army gable of 1850-52 that became its emblem.
• La Villita Historic Arts Village — 2.0 mi
  The little village across the river from the presidio, where Cos surrendered San Antonio in 1835, rebuilt by Depression-era youth crews as a working crafts village.
• San Antonio River Walk — 2.0 mi
  A Depression-era park one level below downtown, built because San Antonio refused to pave its river after the 1921 flood, and designed by an architect the project later fired.
• San Fernando Cathedral — 2.1 mi
  The Canary Islanders' parish church on Main Plaza, where Bowie married and Santa Anna raised a red flag, rebuilt in Gothic Revival around the walls of the 1750s church.

More markers nearby

• Ludwig Mahncke — 1.1 mi
• Acequia de Arriba — 1.2 mi
• Central Catholic High School — 1.2 mi
• Myra Lillian Davis Hemmings — 1.4 mi