It's 400 years old, and that must be as good a reason for a holiday as any: a telescope made by Galileo Galilei in 1609 has been the centrepiece in a Philadelphia exhibition of Galileo and the Medicis.

It had never left Italy before.

The piece, made from two half-shells of carved wood and completed with carefully coiled copper wire, varnish and paper is a very beautiful item through which Galileo would have had the then rare privilege of being able to see four of Jupiter's moons and our moon's craters.

"To think that Galileo saw the heavens through these same lenses," Andrew Chaiken, a science writer, commented.

"It represents the beginning of an era in which we could explore the universe not only with our minds but with instruments."

Historic scientific instruments are becoming a collecting field in their own right, and various items starting to pick up in value.

There were originally around 100 telescopes made by Galileo, but many were discarded as people lost interest in the first telescopes in favour of newer telescopes which could see further still.

This carelessness is part of the reason this one is so valuable, and is carried in a multi-padded, screwed shut case with its own guards, and insured for $3m.

Only two of the original 100 Galileo telescopes are known to exist.

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