Relive the Discoveries of Galileo September 2014 | Page 12

Relive the Discoveries of Galileo

Discovery No. 4
Star Fields of the Milky Way
Ptolemy’s explanation of an Earth-centred of their number, Galileo reasoned that they
universe involved crystalline spheres - one each for
the Moon, Sun, Mercury, Venus, Mars, Jupiter, and
Saturn, and an outermost sphere for the “fixed stars”.
These spheres surrounded earth and turned around
once each day, carrying the heavenly bodies across
the sky.
Of course, the only stars known to exist before

Ptolemy’s Earth-centred Universe in which the moon,
sun, planets and stars rode on crystalline spheres.

Galileo’s time were those visible to the naked eye.
Although impressive in number in the days before
light pollution, Ptolemy reasoned that all those stars
lay an equal distance from Earth and were affixed to
the outermost sphere.
In the autumn of 1609, and into early 1610, Galileo
trained his small 2-inch telescope at the band of soft
light that stretches across the sky – the Milky Way –
and at star clusters like the Pleiades. What he saw
challenged Ptolemy’s sphere of fixed stars.
The soft light of the Milky Way was resolved into an
uncountable density of tiny stars. It wasn’t as though
these additional stars doubled or even tripled the
previously known number of stars. They were so
incredibly numerous as to boggle the mind. Because
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represented the majority of stars, and were probably
no different than the naked-eye stars,