12 AUSTRALIAN SKY & TELESCOPE JULY 2016
Discoveries
Tabulating the celestial sphere
For hundreds of years, one set of tables was the backbone of astronomy.
T
his month’s focus reminds us how
much the modern discipline of
astronomy (‘the measurement of
the stars’) is rooted in the older practice
of astrology (‘the knowledge of the stars’).
It seems that as long as we have been
aware of the movements of the night sky,
most notably the changing positions of
the Sun, Moon and planets against the
background of the ‘fixed stars,’ we have
looked to them to give guidance as to our
daily lives and our fates. For a great many
ancient scholars, casting ‘horoscopes’
to forecast the future for their
patrons or other significant people
was a prominent item on their duty
statements.
It was therefore important to
be able to predict where the seven
classical celestial objects would lie
among the stars in years to come.
This required a technique for
calculating those positions, based
on some ‘model’ of how the visible
universe moved. For a thousand
years or more the system of choice
was that devised by the ancient
Greeks and perfected by Ptolemy.
In it, the Earth was unmoving at
the centre of all things, and around
it the rest of the universe (which, apart
from the fixed stars, contained no more
than the Solar System, and not even all of
that) moved around the Earth in circles
that turned on circles that turned on still
other circles. It was complex and time
consuming but it seemed to work.
Using this scheme, scholars could
compile tables of planetary positions
for years ahead and use them in their
horoscopes. One of the most famous
was the Alfonsine Tables, named after
the enlightened Spanish king who paid
for their production. We celebrate the
achievement this month by recalling
that on July 4, 1483 (nine years before
Columbus reached the ‘new world,’ if you
need a historical peg) these tables were
published in printed form, the first such
to be so published.
The tables themselves had been
compiled more than 200 years earlier in
the city of Toledo in southern Spain. They
had opened with data for January 1, 1272,
the date of the coronation of Alfonso X;
this indicates how much he planned to
rely on them for astrological predictions
throughout his reign. (The king’s
patronage of the tables is also recalled by
the naming of the lunar crater Alfonso.)al-Zarqali, using data and calculations
from even earlier Arab scholars. This
is another reminder that a great deal
of the stimulus of the development of
astronomy in Europe came from scholars
from the Islamic world, which at the
time stretched from the Middle East to
southern Spain.
Routinely updated, the Alfonsine Tables
remained in common use in Europe for
three hundred years. Nicolas Copernicus,
whose Sun-centred system would in time
replace Ptolemy’s Earth-centred one, had
a copy. (He treasured it so much
that he had it bound professionally
in leather and wood.) To be credible
as an alternative to Ptolemy, his new
system had to match the predictions
in the Tabl es. This it struggled to do
— Copernicus had to add more circles
to his initially much simpler system
to “save the appearances” — that is, to
match the predictions.
The problem, of course, was that
Copernicus had his planets orbiting
the Earth in circles, following the
belief expounded by Ptolemy (and
even Galileo) that circular motion
was perfect and most suited for the
motion of celestial bodies. Once
Johannes Kepler found that orbits were
in fact ellipses, the added complexities
were no longer needed. In 1627 Kepler
published his own set of planetary tables
(called the Rudolphine Tables after his
patron, the king of Bohemia) based on his
refinement of Copernicus’. Thus, having
aided the transition from medieval to
modern astronomy, the Alfonsine Tables
passed into history. ✦The Alfonsine Tables were based on Ptolemy’s
celestial model, as depicted by Andreas
Cellarius in 1660.The complexity of the Ptolemaic
system, with its dozens of interlocking
circles, and the resulting intricacy of
the calculations based on it, gave rise
to this probably apocryphal quotation
attributed to Alfonso: “If the Lord
Almighty had consulted me before
embarking on creation thus, I should
have recommended something simpler”.
The Alfonsine Tables in turn were
based on another set of tables compiled
two hundred years earlier still, also in
Toledo (and thus known as the Tabl es of
Tol e d o), by the Arabic mathematician AbuDAVID ELLYARD
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