in retrospect
The celestial message
John Heilbron reflects on the first telescopic survey of 
the sky, published 400 years ago. 

The inventory of the heavens swelled in March 1610 with the publication of Galileo Galilei’s Sidereus nuncius 
(The Celestial Message, or Messenger). 
The first survey of the sky using a telescope 
increased the number of visible stars by 
more than a factor of ten and resolved the 
Milky Way into starlets. The tally of known 
satellites rose five-fold with the discovery of 
the four moons of Jupiter, and the count of 
Earth-like bodies doubled with the detection 
of mountains on our Moon. 

Galileo’s little book — which occupies 
just 60 pages in the 1989 English translation 
by Albert van Helden — caused a sensation 
among the learned of Europe. Galileo ini-
tially aimed his manuscript at a local audi-
ence and began his record of observations in 
Italian. When he found the starlets around 
Jupiter, however, he recognized it as a dis-
covery of world and historical importance, 
and switched to Latin. His plan to reissue the 
book in the vernacular later on, filled with 
commissioned accolades, foundered under 
the pressure of his ongoing discoveries — the 
phases of Venus, bumps on the edges of Sat-
urn (prefiguring its rings), splotches on the 
Sun — and because of career moves abetted 
by his new-found fame. 

Galileo’s matter-of-fact account carried 
conviction of his wonders well before other 
astronomers had telescopes to confirm them. 
That took time because he gave the first 
instruments he made to princes and prelates, 
from whom he anticipated admiration and 
gifts, rather than to his peers, from whom 

he expected the usual academic scepticism. 
Among the first of Galileo’s contemporaries 
to celebrate his marvels were poets. Seizing 
on the analogy closest to hand, they extolled 
Galileo as a greater Christopher Columbus 
— as superior to the original as the heavens 
are to Earth.

The new Columbus was a professor of 
mathematics at the University of Padua, 
a man admired for his wit and literary tal-
ent, fond of wine and women, yet inclined 
to melancholy and hypochondria. He was 
sometimes cutting and querulous, but 

usually cautious and 
circumspect, as was 
fitting for an expatriate 
professor from Flor-
ence without tenure. 

He was further constrained by debt, an age-
ing mistress and three illegitimate children. 
Yet Galileo had many friends and patrons, 
particularly among priests, artists, astrolo-
gers and philosophers. 

In 1609 one of these friends, the notori-
ous anti-Jesuit monk Paolo Sarpi, had drawn 
Galileo’s attention to the military and com-
mercial potential of a gadget invented in the 
Netherlands that brought distant objects 
closer. Galileo maintained a workshop for 
making mathematical instruments and 
assembling eyeglasses for sale. With supplies 
from the shop, some inspired tinkering and 
the expert advice of Sarpi, Galileo turned the 
Dutch gadget, which magnified three-fold, 
into a telescope that magnified by 20 times. 

Galileo was perfectly, perhaps uniquely, 

equipped to be the first to explore the heavens 
by telescope. In addition to his experience 
with instruments, his industry and his open-
mindedness about astronomical systems, he 
had excellent vision, a good imagination and 
an ability to draw in perspective (see Nature 
452, 289; 2008). Galileo’s painterly portrait 
of the mountains and valleys of the Moon 
contrasts with the flatter sketch by Thomas 
Harriot (see Nature 460, 957; 2009), a math-
ematician and surveyor who had mapped 
the New World for Walter Raleigh. Harriot’s 
Moon, as viewed through a telescope he had 
invented independently of Galileo, depicts 
the boundary between the bright and dark 
sides of the Moon as a coastline and the 
rest without relief. Galileo’s cratered land-
scape mirrored the romantic description of 
the Moon in his favourite poem, Ludovico 
Ariosto’s Orlando furioso (1532). 

Galileo’s discoveries and the praises 
heaped on them transformed him from 
an undistinguished mathematician into a 
courtier and then into a missionary. The 
telescope enabled him to realize the profes-
sorial dream of a position free from teach-
ing. In the autumn of 1610, he moved to 
Florence as mathematician and philosopher 
to the Grand Duke of Tuscany, Cosimo II 
de Medici, keeping a post as chief profes-
sor of mathematics at the University of Pisa 
without the obligation to lecture or reside 
there. He could devote his time to railing 
against Aristotelian philosophers and pros-
elytizing for the Copernican cause. 

Others were exploring the night sky 
opened up by Sidereus nuncius. A major 
improvement in instrumentation soon 
replaced Galileo’s telescope design with 
Johannes Kepler’s, which allows a wider field 
of view and the use of a micrometer to meas-
ure angles on the sky. Using this instrument, 
the Jesuit mathematician Christoph Scheiner 
produced a thorough and still useful study of 
sunspots, published in 1630. In the genera-
tion after Galileo’s death in 1642, Christiaan 
Huygens showed that the bumps of Saturn 

Sidereus nuncius 
(The Celestial 
Messenger)
galileo galilei
First published 1610

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Galileo’s sketches (left and centre) conveyed the mountainous lunar landscape; Thomas Harriot’s Moon map (right) lacked relief.

© 20  Macmillan Publishers Limited. All rights reserved10



were incompletely resolved rings and, 
with his discovery of Saturn’s Titan, 
added another moon to the cosmos. The 
number of known satellites grew to eight 
in the late seventeenth century with the 
observations of Giovanni Domenico 
Cassini, another professor of mathemat-
ics and seasoned courtier. Together with 
the six planets then known, Cassini said, 
the eight satellites made a total of 14 — 
honouring his royal patron, the Sun King, 
Louis XIV of France.

By associating planets and moons with 
flattery of his patron, Cassini continued a 
game begun by Galileo, who named the 
four companions of Jupiter the Medici 
stars. Galileo’s discovery and identifica-
tion of these satellites, and his deduction 
of accurate values for their periods, were 
great technical achievements. He recog-
nized that a table of the satellites’ eclipses 
could support a method for finding longi-
tude at sea. He offered to sell the method 
to the King of Spain and the Estates of 
Holland, but the technique proved 
impractical and the terms too dear.

Although technical difficulties under-
cut the exploitation of the Medici stars 
for navigation, their application to cos-
mology was easy. Of all the discoveries 
that Galileo announced 400 years ago, 
they provided the strongest argument 
yet against the geocentric world view. 
The Jovian system obviously was not 
centred on the Earth, and Jupiter’s abil-
ity to retain its satellites during its orbit 
showed that Earth might move without 
losing its Moon.

Sidereus nuncius can mean either celes-
tial message or celestial messenger. Gali-
leo intended it to mean ‘message’ when he 
applied to the Venetian censors in 1610 
for permission to publish his book. Most 
translators, however, follow Kepler in ren-
dering the meaning of nuncius as ‘mes-
senger’. In this they have captured the way 
in which Galileo came to regard himself. 
His quixotic self-identification as a mes-
senger from heaven or agent of the stars 
gave him the psychological strength to 
make Sidereus nuncius more than a report 
of marvels. Whereas Columbus added a 
new hemisphere to an existing one, Gali-
leo announced his celestial message to 
his contemporaries as a demand for the 
replacement of the cosy Christian cosmos 
by an uncomfortable new world. ■

John L. Heilbron is professor of history 
emeritus at the University of California, 
Berkeley, USA, and an honorary fellow 
of Worcester College, University of 
Oxford, UK. His biography of Galileo 
will be published in October (UK) and 
December (US). 
e-mail: johnheilbron@berkeley.edu

E a r T h  S C i E N C E

Fire from the depths
powerful volcanoes remind us of the fragile boundary 
between Earth’s crust and mantle, finds Laura Spinney.

The eruption of the Icelandic volcano Eyjafjallajökull this April, and the month-long havoc it caused in the 
skies over Europe, was a salutary lesson in 
how susceptible our global, interconnected 
society is to natural perturbations. This saga 
and other spectacular eruptions are explored 
in the exhibition Supervolcano at the Natural 
History Museum in Geneva, Switzerland.

The highlight is the display of photographs 
by Geneva’s volcanology society. These self-
confessed volcano addicts travel the world to 
record the beauty and devastating impacts 
of the latest eruptions. The images attest to 
the many ways that volcanoes can kill you — 
through flying debris, burns, asphyxiation or 
being struck by the lightning generated within 
the columns of charged particles they eject. 

Interactive installations convey the volcanic 
experience. Visitors can feel seismic tremors, 
listen to a mud volcano and walk through a 
reconstructed lava tunnel. The destructive 
power of volcanic ash is revealed in a mock-
up of a crushed office: it is the weight of settled 
ash that causes buildings to cave in.

Other displays explore the wider societal 
risks of massive eruptions and their histori-
cal influence. Eyjafjallajökull was a relatively 
modest geological event, but volcanologists 
fear that it might trigger a far more danger-
ous neighbouring volcano, Katla, now under 
high surveillance. If Katla blows, the fallout 
could put April’s upset in the shade.

The 1783–84 eruption of Laki, another 
Icelandic volcano, might have contributed 
to the French Revolution a few years later. 

Laki’s toxic cloud polluted the atmosphere, 
lowered temperatures and caused famines 
across the Northern Hemisphere.

Even more destructive are supervol-
canoes — eruptions with the maximum 
score of eight on the Volcanic Explosivity 
Index, measured according to the volume 
of material ejected, among other factors. 
Eyjafjallajökull qualified as a four. The 
eighteenth-century Laki eruption and the 

1 9 9 1  e r u p t i o n  o f 
Mount Pinatubo in the 
Philippines — which 
spewed out 10 cubic 
kilometres of debris 
— both scored a six.

No known supervolcanoes are currently 
active, although the imploded remains of one 
lie in Yellowstone National Park in Wyoming. 
From the size of its caldera, researchers 
think that it dumped ash across much of the 
continent about 640,000 years ago. Toba in 
Sumatra was one of the last super volcanoes 
to erupt, 75,000 years ago. Forty kilometres 
away is Mount Sinabung, which erupted last 
month after a long period of inactivity. It too 
is being monitored closely.

This timely exhibition reminds us of our 
vulnerability to volcanoes; we inhabit Earth’s 
cool, thin crust, but more than 99% of the 
planet smoulders at temperatures above 
1,000 °C. ■

Laura Spinney is a writer based in 
Lausanne, Switzerland.  
e-mail: lfspinney@googlemail.com

Supervolcano
Natural History 
Museum, Geneva, 
Switzerland
Until 4 September 
2011

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Eyjafjallajökull’s billowing ash triggered lightning when particles were charged by friction. 

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	In retrospect: The celestial message