Sky & Telescope - May 2020 - 63

ity of what appeared to be a miniature analog to the solar system. Others struggled to find reasons to deny
what was clearly obvious, appealing
to the word of Scripture, which
espoused a centralized, motionless
Earth. Still others conjectured that
the instrument was faulty and didn't
portray reality.
However, Jesuit priest and astronomer Christopher Clavius was an
early endorser of Galileo's assertions. Clavius was also responsible
for much of the analysis behind the
1582 reform of the calendar, which
more accurately synchronized it with
the length of the year, thus keeping
holy days (particularly Easter) in step
with the seasons. Ironically, Clavius
adhered to the geocentric view, yet he
didn't turn a blind eye to the reality
presented by Galileo's telescope.
Galileo's observations and scientific musings triggered
years of dispute with the Church, resulting in his eventual
trial and subsequent lifetime house arrest. Multiple factors
played a part in this outcome, including the overt bluntness
of the books Galileo authored, coupled with his sometimes
abrasive personality. Teaching and writing about Copernicanism was forbidden, and Revolutions was placed on the "Index"
- a list of books Catholics were prohibited from reading.

PUBLIC DOMAIN / WIKIMEDIA COMMONS (2)

Down-to-Earth Matters
While Galileo's observations ultimately validated a Suncentered view, observations of Jupiter's moons contributed to
other advancements of the era, particularly for determining
longitude, essential for navigation and cartography.
Although latitude is easy to reckon by measuring the
altitude of stars above the horizon, no equivalent technique
existed for determining longitude. In the 1670s Danish
astronomer Ole Rømer devised a method of calculating longitude by timing the eclipses of Jupiter's satellites. He made his
observations from Uraniborg, Tycho Brahe's observatory near
Copenhagen, in concert with Giovanni Domenico Cassini,
who documented the same events from Paris. By comparing
their results the two could calculate the difference in longitude between the two locations. Although a valid procedure,
it proved unworkable from the deck of a ship navigating
turbulent oceans. Many other scientists and inventors toiled
over the puzzle using various means, but it wasn't until the
late 1700s when John Harrison and others built accurate

t GALILEO'S NEMESIS Simon Marius not
only gave the Galilean moons the names
they now bear, but he also proved to be a
thorn in the great scientist's side by claiming
to have discovered the satellites first and
publishing a book to bolster his assertion.

chronometers that a truly reliable
method for determining longitude at
sea existed.
After relocating to Paris in 1672
to become Cassini's assistant, Rømer
continued to observe Jupiter's satellites, which led him to conjecture
that the speed of light is finite. This
position was counter to the prevailing
view that light propagated instantaneously from a source. Both he and
Cassini recognized that as Earth and
Jupiter move apart in their respective
orbits, the interval between eclipses
of the satellites increases, and when
they approach each other, it decreases. Rømer primarily monitored Io, which, being closest to Jupiter, orbits more frequently
and therefore provides more data points. His conclusion was
that a finite speed of light accounted for the observed differences, although at the time methods sufficiently accurate for
determining the speed of light were unavailable.

More and More Moons
In 1892, American astronomer E. E. Barnard discovered Jupiter's first post-Galilean satellite. Barnard detected the dim
little moon (listed at magnitude 14.1) with the newly commissioned 36-inch refractor at Lick Observatory on Mount
Hamilton in California. This was to be the last visual discovery of a planetary satellite. At 167 km in diameter, Amalthea
(as it was formally named in 1976) is only 3% percent the size

u COPERNICAN REVOLUTION Polish astronomer Nicolaus Copernicus is credited with having first described a heliocentric solar
system in detail in his book De revolutionibus orbium coelestium (On
the Revolutions of the Heavenly Spheres), published in 1543, the year
of his death.
s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 0

63


http://www.skyandtelescope.org

Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020

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