La Lettre de l'Académie des sciences n°37-38

350

YEARS

SCIENCE

An image from the Hubble Ultra Deep Field

The Century of the Enlightenment would resolutely

feed a taste for scientific exploration, whether on

Earth, sea or sky, and theories would follow to

explain the observations of the great naturalists

(Buffon, Lamarck), geographers (Von Humboldt),

geologists (Hutton, Cuvier and, later, Brongniart)

and astronomers (Cassini, Lalande, La Caille)

of the time. It is established that Earth revolves

around the sun or that it was not formed in six days

some thousand years ago. Drawing on the study

of the cooling time of metal spheres from various

diameters, Buffon calculated that 10 million years

was the age of our planet.

Astronomers built telescopes to probe the sky

with ever-growing precision and sensitivity. The

catalogues of the stars and nebulae, much in the

way of the observation reports that naturalists

compiled, became more and more detailed. They

allowed Messier and Herschel to classify the stars

with ever-increasing precision, which was a very

useful preamble to physically understand them.

In the 19

Century, the theoretical and experimental

works of Young, Fresnel, Foucault and Maxwell

provided support for the wave-like nature of light

that Huygens had put forward for consideration in

1670, and allowed for its speed to be measured.

Fraunhofer invented the spectroscope and it

became possible to study the chemical composition

of the atmospheres of the sun, planets, solar system

and, later, stars and galaxies. The Doppler-Fizeau

effect made it possible to measure the speed of stars

with reference to an observer located on Earth. This

effect, indeed, would be decisive in showing that

the galaxies moved away from each other and in

establishing the expansion of the Universe during the

1920s as Lemaître and Hubble did, which Einstein’s

theory of general Relativity accounted for.

Two systems describing the world : the

geocentric system (Ptolemy, on the left)

and the heliocentric system (Copernicus,

on the right). The Harmonia Macrocosmica

of Andreas Cellarius (1660).