viernes, 18 de mayo de 2007

La determinación de la forma de la Tierra: Jorge Juan y Antonio Ulloa


Cualquier alumno de secundaria sabe que la Tierra no es una esfera perfecta, sino que está ligeramente achatada por los Polos. Lo que no les contamos nunca, es que dos científicos españoles participaron en este descubrimiento.

Jorge Juan (1713-1773) nacido en Novelda y Antonio Ulloa (1716-1795), sevillano, tuvieron un papel importante en la determinación de la forma exacta de la Tierra.

Existían dos teorías: científicos como Newton o Maupertius, proponían, que la Tierra era ligeramente achatada por los Polos (forma de naranja) , y, por otro lado, Cassini, astrónomo de Luis XIV, y Descartes postulaban que la forma era achatada en el ecuador (forma de limón). Para zanjar esta polémica científica, los franceses enviaron dos expediciones una a Laponia y otra a Quito, para medir el arco correspondiente a un grado de latitud. Si Newton tenía razón el arco de un grado de meridiano tendría mayor longitud en los Polos que en las proximidades del ecuador terrestre.

La primera expedición se dirigió a Finlandia, en la zona del golfo de Bothnia. Fue dirigida por Maupertius y en un año, entre 1736 y 1737, realizaron sus mediciones.

La expedición al ecuador presentó más dificultades. El territorio pertenecía a la corona española y Luis XIV pidió permiso a su primo Felipe V, para que una expedición de la Academie Royale des Sciences de Paris formada por Louis Godin, Pièrre Bouger y Charles M. de la Condamine, viajase a Quito, a medir un arco de meridiano. Felipe V accedió, con la condición de que participaran también españoles. Sorprendentemente, eligieron a dos jóvenes guardiamarinas: Jorge Juan que contaba con 21 años, y que sus compañeros en la academia militar apodaban “El Euclides”, en calidad de matemático y Antonio Ulloa de 19 como naturalista, que no contaban con el prestigio y con el reconocimiento científico de sus colegas franceses y que, en principio, tenían como misión controlar los movimientos de los franceses en territorio español.
Las mediciones se prolongaron desde 1736 a 1744. Los cinco investigadores se dividieron en grupos. Comisiones posteriores han estudiado los trabajos llegando a la conclusión de que los resultados más precisos fueron los que obtuvo Jorge Juan. Estimó el valor lineal para un grado de meridiano en el Ecuador en 56.767,788 toesas. La toesa era la unidad de longitud que se empleaba en Francia, valor que resultaba superior al medido en Laponia (57.437,9 T), lo que confirmaba la hipótesis de Newton sobre la forma de la Tierra.
Finalmente, decidieron regresar en navíos distintos, con el fin de asegurar que uno de los duplicados de las notas y cálculos llegara a su destino. Embarcaron el 22 de octubre de 1744 en dos fragatas francesas. Jorge Juan llegó a Brest el 31 de octubre de 1745. Desde allí se dirigió a París para cambiar impresiones sobre su obra y contrastar algunas particularidades observadas por él y Godín en sus observaciones astronómicas, conociendo a los célebres astrónomos Marian, Clairaut y La Caille, autores de las fórmulas que tantas veces habían empleado. Conoció a Reaumur, inventor del termómetro, y a otros célebres académicos que, en compañía de La Condamine y Bourguer, reintegrados a sus actividades, le votaron como miembro correspondiente de la Royal Academie des Sciences.
Antonio de Ulloa tuvo más dificultades. Apresada su fragata por los ingleses, que declararon la guerra a Francia durante la travesía, tuvo que arrojar al agua la documentación comprometida, no así lo referente a la medida del grado, observaciones físicas y astronómicas, y noticias históricas, que entregó no sin advertir del interés que todas las naciones de Europa habían mostrado en esta empresa.
Le llevaron preso cerca de Portsmouth, pero el duque de Bedford le concedió la libertad expresando que la guerra no debía ofender a las ciencias, ni a las artes, ni a sus profesores. Pasó a Londres, donde el Ministro de Estado Conde de Harrington, que fue embajador en España, le propuso como Miembro de la Royal Society.
Además de las mediciones, Antonio de Ulloa en sus estudios sobre la minería fue el primero en hablar de la platina o platino, como mineral diferente de la plata y el oro.

http://www.jorgejuan.net/ESPANOL/Marino/0biografia.htm
http://www.mgar.net/exp/ulloa.htm
http://www.mappinginteractivo.com/plantilla-ante.asp?id_articulo=1348

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