January 7th, 1610: Galileo Observes the Satellites of Jupiter

By Cavendish, Richard | History Today, January 2010 | Go to article overview

January 7th, 1610: Galileo Observes the Satellites of Jupiter


Cavendish, Richard, History Today


The telescope is derived from the pairs of spectacles with glass lenses which were made in medieval Europe to enable people to see further and more clearly than they could without them. By 1608 the first spyglasses had appeared and a Flemish spectacle-maker in Holland who applied to the States General for a patent on an 'instrument for seeing far' was quickly challenged by two other Dutch spectacle-makers, who submitted their own claims. Early in 1609 word of the new devices reached the professor of mathematics at Padua University in the Republic of Venice.

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His name was Galileo Galilei and he was in his mid-forties when he heard from the influential Venetian humanist Paolo Sarpi that a Dutchman was on his way to Venice hoping to sell one of the new instruments to the doge, Leonardo Dona. Galileo was immediately galvanised and asked Sarpi to caution the doge and the Venetians not to accept a device from anyone else until he could show them a better one of his own. Sarpi did as he was asked and Galileo worked frantically trying out different pairs of spectacle lenses fixed into a wooden tube, grinding his own lenses until he worked out an effective arrangement. He quickly improved on his first instrument and the one he and Sarpi demonstrated to the doge in August worked far more effectively than any of its predecessors.

Galileo and Sarpi took Doge Dona and his advisers to the top of the tower of St Mark's and pointed the new telescope towards Padua. The doge was astounded to be able to see the tower at the centre of the city more than 50 kilometres away. After gazing across Venice itself, they looked far out into the Adriatic where the Venetians saw ships that would not be visible to the unassisted eye for another two hours or more. The party returned to the doges palace and, far from demanding a high price for his invention as expected, Galileo shrewdly offered the telescope to Venice as a gift. The present was eagerly accepted and Galileo went away with his salary at Padua almost doubled and a lifetime tenure in his post.

Galileo and Sarpi had taken care to stress the military advantages of the telescope. In his letter to the doge confirming his gift, Galileo drove home the point that the new instrument would enable Venetian ships at sea to discern the strength of an enemy fleet at a far greater distance than before and similarly allow an army on land to look inside distant strongholds and watch the movements of enemy troops from much further away.

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January 7th, 1610: Galileo Observes the Satellites of Jupiter
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