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Giovanni Battista Riccioli
and the New Almagest
The New Almagest was astronomy. What was not in the New Almagest did
not need to be known. England’s top astronomer—the first Astronomer
Royal, John Flamsteed—used the New Almagest as a textbook for public
lectures at Gresham College in 1665. Indeed, anyone carrying a copy of the
New Almagest to a lecture would have looked quite learned, for the book
was printed in two volumes, each the size of one of today’s large coffeetable books. Within those volumes were over fifteen hundred pages, filled
with dense text and diagrams. Clearly, anyone who read the New Almagest,
which was published in 1651 by an Italian astronomer by the name of Giovanni Battista Riccioli (1598–1671), knew something about astronomy.
Information on every conceivable topic in seventeenth-century astronomy could be found in the pages of the New Almagest. There were chapters devoted to the motions of celestial bodies, to discussions of geometry,
and to representations of the appearance of Jupiter, Venus, and other planets as seen through the best telescopes. There were reports on, and tables
of data from, physics experiments of different sorts, such as those involving bodies falling through air and through water. There were lots of tables of astronomical data. There was a highly detailed, full-spread map of
the Moon, which featured an interesting new naming system for lunar features. On the map one lunar crater was named “Copernicus.” Another was
named “Galileo.” A large smooth area was named the “Sea of Tranquility.”
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SET T IN G ASIDE AL L AUT HORIT Y
These names would stick—when the Apollo 11 lander the “Eagle” touched
down on the moon in 1969, it landed in the “Sea of Tranquility,” and Neil
Armstrong would radio, “Houston, Tranquility Base here. The Eagle has
landed.”
Inside each volume of the New Almagest was an impressive frontispiece that represented the state of astronomical knowledge in the midseventeenth century (fig. 1.1). The frontispiece showed Jupiter having four
moons, as Galileo had reported in his groundbreaking Starry Messenger of
1610. But the frontispiece also showed Jupiter having cloud bands1—a discovery made since Galileo. It showed Venus having a crescent phase, as
Galileo had first observed with his telescope. It also showed Mercury with
a crescent phase—another discovery made since Galileo. Of course it featured a cratered moon, Mars and Saturn, and more. The New Almagest
frontispiece presented all the key discoveries that had been revealed by the
telescope since Galileo had turned one to the heavens in 1609 (fig. 1.2).
Yet the main feature of the frontispiece was a set of scales, being held by
Urania, the Greek muse of astronomy, while the many-eyed Argus, holding
a telescope, looked on (fig. 1.3). The modern reader who studies the frontispiece will immediately recognize, on the left side of the scales, a representation of the world system of Nicolaus Copernicus (1473–1543)—the
“heliocentric hypothesis.” There is the Sun, in the middle of everything,
with planets circling around it, and the Moon circling the third planet
from the Sun.
On the right side of the scales is not a representation of the “geocentric
hypothesis,” the ancient world system of Ptolemy (who wrote the original Almagest) and Aristotle, in which Earth is in the center of everything,
and everything circles around Earth. Galileo’s famous Dialogue of 1632
concerned “the two chief world systems: Ptolemaic and Copernican.” But
in the New Almagest, the Ptolemaic system is not what is being weighed
against the Copernican system. In Riccioli’s frontispiece, the Ptolemaic
world system is sitting off to the side, on the ground, looking a little forlorn. Clearly the telescopic discoveries shown above the scales, especially
the phases of Venus and Mercury (which proved those bodies circle the
Sun), have removed that ancient system from Riccioli’s consideration.
Rather, on the right side of the scales is a representation of a world
system that is definitely geocentric. There is the Earth, right in the center,
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Figure 1.1. Frontispiece of Giovanni Battista Riccioli’s 1651 New Almagest. Image
courtesy History of Science Collections, University of Oklahoma Libraries.
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Figure 1.2. Details from the New
Almagest frontispiece, showing Jupiter
with moons and cloud bands (left), and showing Mercury and Venus each with a
crescent phase (right). Images courtesy History of Science Collections, University of
Oklahoma Libraries.
Figure 1.3.
Detail from the New
Almagest frontispiece,
showing the world
systems being
weighed in the
balance. Image
courtesy History of
Science Collections,
University of
Oklahoma Libraries.
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Giovanni Battista Riccioli and the New Almagest
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with the Sun and Moon circling it. But this world system is not purely
geocentric. Planets circle the Sun. This world system may be geocentric,
but it has some heliocentric features. It is a “hybrid geocentric” (or “geoheliocentric”) hypothesis.
Moreover, the balance is obviously tipping in favor of this hybrid
geocentric world system. The New Almagest is clearly promoting the view
that, in the age of new telescopic discoveries, this hybrid geocentrism is
superior to Copernican heliocentrism. And yet the hand of God reaches
down from above the page, indicating “Numerus,” “Mensura,” “Pondus”
(“Number,” “Measure,” “Weight”), while verses from the Vulgate Bible2
grace the picture:
“dies diei eructat verbum et nox nocti indicat scientiam”—“Day to day
uttereth speech, and night to night sheweth knowledge” (Psalm 18:3)
“videbo caelos tuos; opera digitorum tuorum”—“I will behold thy
heavens, the works of thy fingers” (Psalm 8:4)
“non inclinabitur in saeculum saeculi”—“[Earth] shall not be moved
for ever and ever” (Psalm 103:5)
What is Riccioli saying, anyway, by representing the latest discoveries with
all this?
Riccioli, a Jesuit priest who was originally trained as a theologian, before he discovered and fell in love with astronomy,3 appears to be saying
that, in light of telescopic discoveries, in light of a serious scientific analysis (numbering, measuring, weighing), Copernicus is wrong. The Earth
does not move. Indeed, beyond the frontispiece, a very large part of the
New Almagest consists of an analysis of the world system debate, and this
analysis concludes with a question and a main conclusion. The question is
this: according purely to reason, and setting aside all authority, which hypothesis may be asserted as true—that which supposes the motion of the
Earth, or that which supposes the immobility of the Earth?4 And what was
the conclusion?
Reasoning and intrinsic arguments alone considered, and every authority set aside; the hypothesis supposing the immobility or quiet of
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SET T IN G ASIDE AL L AUT HORIT Y
the Earth absolutely must be asserted as true; and the hypothesis that
bestows to the Earth motion (either solely diurnal, or diurnal and annual) absolutely must be asserted as false and disagreeing with physical and indeed physico-mathematical demonstrations.5
In other words, in the age of the telescope, science shows the Copernican
hypothesis to be wrong. And after this main conclusion, Riccioli includes
copies of the condemnation of the Copernican system by church authorities in Rome, of the judgment against Galileo, and of Galileo’s abjuration.6
In his analysis of the heliocentrism vs. geocentrism (of the hybrid
sort) world-system debate in the New Almagest, Riccioli reviewed 126 arguments put forward by both sides. Edward Grant has described this as
“probably the lengthiest, most penetrating, and authoritative analysis
made by any author of the sixteenth and seventeenth centuries.”7 Fortynine of the arguments Riccioli reviewed favored heliocentrism; seventyseven favored geocentrism.8 But to Riccioli, this was not merely a counting of arguments, for numbers of arguments were not what tipped the
balance of Urania’s scales.
The vast majority of arguments on both sides, he said, were unconvincing. Some were unconvincing because they were so bad (Riccioli says
he is ashamed to bring up some, but does so because they have been used
by various people).9 Others were reasonable, but still unconvincing. For
example, consider pro-heliocentrism argument number 22:
The sun is the center of the Planetary System—as is demonstrated in
the case of Mercury and Venus [by telescopic observations of their
phases, illustrated in the frontispiece], and conjectured in the case of
the others—so it ought to be the center of the Universe.10
This seems like a very reasonable argument. Riccioli notes, however, that
the geocentrists have a valid answer to it. The answer is that the Sun is neither the center of the Moon’s orbit, nor the center of motion for the fall of
heavy bodies and the rise of fire, nor the center of the stars.11 As another
example, consider pro-geocentrism argument number 42, which says that
the weighty Earth must be at the center of the universe because there is no
explanation as to what would keep it in any other position. Here Riccioli
notes that the heliocentrists can answer that the entire Earth has a natural
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circular motion about the center of the universe, in which its weight is not
a factor.12 Then there is pro-geocentrism argument number 53 (one of
only two among the 126 that involve religious questions), which says that
if Earth is not the center of the universe, then Hell is not at the lowest
place, and someone going to Hell could conceivably ascend in doing so.
How could someone ascend into Hell? Riccioli says that the answer to this
argument is that Hell is a place defined by comparison to this world on
which men13 live and to God’s Heaven; the relationship between Heaven,
Hell, and the world of men is not affected by whether Earth moves.14
Riccioli did, however, find a select few arguments to be convincing—
all of them anti-Copernican arguments for which the heliocentrists had
no good answer. One of these concerned the question of detecting Earth’s
rotation. According to Riccioli, a rotating Earth must produce certain
observable phenomena. These phenomena were, in fact, not observed.
Thus the Earth must not rotate. These are the physical and “physicomathematical” demonstrations Riccioli mentions in his main conclusion
about which “hypothesis” can be asserted as being true.
A second anti-Copernican argument—one that Riccioli thought was
stronger, even though it was technically answerable by the heliocentrists—
concerned the sizes of stars. According to Riccioli, telescopic observations
of the stars showed that, were the Copernican hypothesis correct, the stars
would have to be huge. Indeed, he said, one single star in the Copernican
world system could conceivably be larger than the entire universe in the hybrid geocentric world system. By contrast, in the hybrid geocentric system
the stars would be reasonably sized. When the sizes of stars are taken into
consideration, Copernicus’s heliocentric world system looks absurd.
Riccioli devotes at least two chapters in the New Almagest to the star
sizes issue, discusses the issue in a line of pro-geocentrism arguments
(numbers 67– 70),15 and uses it to counter a pro-heliocentrism argument
(number 9).16 And while he grants to the heliocentrists the possibility that
the above-mentioned demonstrations to detect Earth’s rotation might
not work because experiments could be insufficiently precise,17 he grants
no such escape to them on the problem of star sizes. Their only answer to
this argument he says, is an appeal to Divine Omnipotence.18 Technically,
since he cannot deny the power of God, he cannot fully refute the Copernican answer on this matter. But, he says, “even if this falsehood cannot be
refuted, nevertheless it cannot satisfy more prudent men.”19
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What is remarkable about Riccioli’s analysis is that he was right. His
analysis made sense, granted the knowledge available in his time. Today, in
our time, you the reader have doubtlessly heard that Galileo used science
to prove that the Earth circles the Sun—indeed, the back cover of the standard modern translation of Galileo’s 1632 Dialogue explicitly states just
that.20 But I believe that by the time you finish the pages of this book, you
will agree with Riccioli’s New Almagest frontispiece. You will conclude that
an objective and rational analysis of the best data on hand in the midseventeenth century would lead one to concur with Riccioli that the Copernican hypothesis, while certainly an improvement over the ideas of
Ptolemy, did not compare favorably against hybrid geocentrism. You will
find that some very strange ideas sprouted under Copernicanism—ideas
about giant stars pointing to the nature of God, or indeed being the “warriors” of God. You will find that Riccioli’s ideas can be traced back into the
sixteenth century, to the great Danish astronomer Tycho Brahe, who promoted hybrid geocentrism. And you will find that between Brahe and Riccioli these ideas even played a role in the condemnation of the Copernican
hypothesis by church authorities in Rome. Riccioli causes us to rethink our
ideas of “heliocentrism vs. geocentrism” as being perhaps not so much
about “science (and heliocentrism) vs. religion (and geocentrism)” as perhaps “science vs. science,” or “religion vs. religion.” Riccioli may even cause
us to invert our view of the issue, wondering if at times the debate was
actually a matter of “religion (and heliocentrism) vs. science (and geocentrism).” Giovanni Battista Riccioli invites a reexamination of what we
believe we know about the Copernican Revolution.
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