sábado, 15 de septiembre de 2007

La medición de la tierra entre Pitágoras y la era espacial


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En la antigua Grecia ya se suponía que la Tierra era esférica, llegando a calcularse la longitud de la circunferencia máxima en varias ocasiones, antes de la conocida medición de Eratóstenes. Los geógrafos árabes continuaron con esa tradición llegando inclusive a idear nuevos métodos, como sucedería con al Biruni en el Califato de Bagdad. Tras el paréntesis de la Edad Media, en la que llegó a cuestionarse la esfericidad terrestre, se realizaron varios intentos de medir el radio de la esfera, con resultados demasiado dispares.

La creación de la Academia de Ciencias francesa trató de evitar esa ambigüedad, encargándole a Piccard la primera determinación rigurosa del radio de la Tierra. La prolongación del arco de meridiano que empleó el abad francés (Amiens-Malvoisine) evidenció la variabilidad de su curvatura, comprobándose tras las expediciones científicas a Laponia y al Virreinato de Perú que el modelo esférico debía sustituirse por otro elipsoidal con achatamiento polar, tal como había preconizado Newton. Sucesivas mediciones del desarrollo de un grado en diferentes latitudes y la comparación entre todas ellas confirmaron que el modelo matemático de la Tierra era un elipsoide de revolución, calculándose su semieje mayor y el aplastamiento, aunque este no se pudiese evaluar entonces con la debida exactitud.

En el siglo XIX se comienza a cuestionar el modelo anterior, defendiendo tanto Gauss, como Helmert, que la forma real de la Tierra era la de una de la superficies equipotenciales de su campo gravitatorio, precisamente la más próxima al nivel medio del mar. Poco tiempo después se acuñaría el término geoide para referirse a dicha superficie equipotencial. Posteriores medidas gravimétricas permitieron mejorar los modelos elipsoidales previos, así sucedió con el de Hayford, adoptado como internacional en Madrid (1924), o con el de Krasovsky.

La llegada de la geodesia espacial, confirmó la bondad de las determinaciones del semieje ecuatorial, precisó el valor del aplastamiento y las discrepancias entre el geoide, como superficie física real, y el elipsoide, como superdicie matemática ideal, además de permitir obtener con gran exactitud todos los téminos que se deseen del potencial de la gravedad.

Ruiz Bustos, M. Ruiz Morales, M. Ministerio de Fomento. Instituto geográfico Nacional.

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