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moons and claimed to have
discovered them before Galileo.
Galileo would later accuse
Marius of plagiarism, but it is
now generally accepted that he
made his discovery independently
at around the same time. Marius
named the moons Io, Europa,
Ganymede, and Callisto after the
Roman god Jupiter’s love conquests,
and these names are still used.
They are now known collectively
as the Galilean moons.
A Jovian clock
Galileo carefully studied the
changing positions of the Jovian
moons from day to day. He
concluded that, like the planets,
their positions could be calculated
in advance. Galileo saw that, if
this could be done accurately, the
system would act as a universal
clock and could solve the problem
of measuring longitude at sea.
To establish longitude requires
the ability to tell the time, but
in Galileo’s day, there were no
timepieces that would work on
a boat. Because Jupiter is at least
four times farther away from Earth
than the sun, the Jovian system
looks the same from anywhere
on Earth, so a “Jovian clock”
would work from anywhere. The
longitude problem was finally
solved with the introduction of
accurate chronometers by the
English clockmaker John Harrison
around 1740. This was well before
the orbits of Jupiter’s moons had
been worked out in detail.
Galileo’s discovery of four
satellites around Jupiter had
another interesting consequence.GALILEO’S TELESCOPE
When Jonathan Swift published
Gulliver’s Travels in 1726, he
predicted, in the chapter on
Laputa, that Mars would have
two moons simply because Earth
had one and Jupiter four. In 1877,
this prediction was fortuitously
proved to be correct when Asaph
Hall discovered Mars’s two small
moons, Phobos and Deimos, using
a new 26-in (66-cm) refracting
telescope at the US Naval
Observatory in Washington.Support for Copernicus
In Galileo’s time, there was still a
heated debate between believers
of the old biblical theory that Earth
was stationary at the center of the
cosmos and Copernicus’s new
idea that the Earth was in orbit
around the sun. The geocentric
(Earth-centered) idea stressed the
uniqueness of the planet, while the
heliocentric (sun-centered) proposal
made Earth just one of a family
of planets. The assumption that
Earth does not occupy a privileged
place in the cosmos is now known
as the Copernican principle.The Bible shows the way
to go to heaven, not the
way the heavens go.
Galileo Galilei