26
THE SUN IS LIKE
FIRE, THE MOON
IS LIKE WATER
ZHANG HENG (78–139 CE)
I
n about 140 BCE, the Greek
astronomer Hipparchus,
probably the finest astronomer
of the ancient world, compiled a
catalogue of some 850 stars. He
also explained how to predict the
movements of the Sun and Moon
and the dates of eclipses. In his
work Almagest of about 150 CE,
Ptolemy of Alexandria listed
1,000 stars and 48 constellations.
Most of this work was effectively
an updated version of what
Hipparchus had written, but in a
more practical form. In the West,
the Almagest became the standard
astronomy text throughout the
Middle Ages. Its tables included
all the information needed to
calculate the future positions of the
Sun and Moon, the planets and
the major stars, and also eclipses
of the Sun and Moon.
In 120 CE, the Chinese polymath
Zhang Heng produced a work
entitled Ling Xian, or The Spiritual
Constitution of the Universe. In it,
he wrote that “the sky is like a
hen’s egg, and is as round as a
crossbow pellet, and Earth is like
the yolk of the egg, lying alone at
the center. The sky is large and the
Earth small.” This was, following
Hipparchus and Ptolemy, a universeIN CONTEXT
BRANCH
PhysicsBEFORE
140 BCE Hipparchus figures
out how to predict eclipses.150 CE Ptolemy improves
on Hipparchus’s work, and
produces practical tables for
calculating the future positions
of the celestial bodies.AFTER
11th century Shen Kuo
writes the Dream Pool Essays,
in which he uses the waxing
and waning of the Moon to
demonstrate that all heavenly
bodies (though not Earth)
are spherical.1543 Nicolaus Copernicus
publishes On the Revolutions
of the Celestial Spheres,
in which he describes a
heliocentric system.1609 Johannes Kepler
explains the movements of
the planets as free-floating
bodies describing ellipses.The Moon
must be bright
because of sunlight.During the day
Earth is bright, with
shadows, because
of sunlight.The Moon is sometimes
bright, with shadows.Therefore the Sun
is like fire, the Moon
like water.