The Astronomy Book

23

See also: The Copernican model 32–39 ■ Elliptical orbits 50–55

T

he Chief Astrologer at the

court of Chinese emperor

An-ti, Zhang Heng was

a skilled mathematician and a

careful observer. He cataloged

2,500 “brightly shining” stars and

estimated that there were a further

11,520 “very small” ones.

Also a distinguished poet,

Zhang expressed his astronomical

ideas through simile and metaphor.

In his treatise Ling Xian, or The

Spiritual Constitution of the

Universe, he placed Earth at the

center of the cosmos, stating that

“the sky is like a hen’s egg, and is

as round as a crossbow pellet, and

Earth is the yolk of the egg, lying

alone at the center.”

Shape but no light

Zhang concluded that the moon

had no light of its own, but rather

reflected the sun “like water.” In

this, he embraced the theories of

his compatriot Jing Fang who, a

century earlier, had declared that

“the moon and the planets are Yin;

they have shape but no light.” Zhang

saw that “the side that faces the

sun is fully lit, and the side that

is away from it is dark.” He also

described a lunar eclipse, during

which the sun’s light cannot reach

the moon because Earth is in the

way. He recognized that the planets

were similarly subject to eclipses.

Zhang’s work was developed

further in the 11th century by

another Chinese astronomer,

Shen Kuo. Shen demonstrated

that the waxing and waning of

the moon proved that the moon

and sun were spherical. ■

FROM MYTH TO SCIENCE

THE MOON’S

BRIGHTNESS IS

PRODUCED BY THE

RADIANCE OF THE SUN

THEORIES ABOUT THE MOON

IN CONTEXT

KEY ASTRONOMER
Zhang Heng (78–139 ce)

BEFORE
140 bce Hipparchus discovers
how to predict eclipses.

1st century bce Jing Fang
advances the “radiating
influence” theory, stating that
the light of the moon is the
reflected light of the sun.

AFTER
150 ce Ptolemy produces
tables for calculating the
positions of celestial bodies.

11th century Shen Kuo’s
Dream Pool Essays explains
that heavenly bodies are round
like balls rather than flat.

1543 Nicolaus Copernicus’s
On the Revolutions of the
Celestial Spheres describes
a heliocentric system.

1609 Johannes Kepler
explains the movements of
the planets as free-floating
bodies, describing ellipses.

The sun is like fire

and the moon like water.

The fire gives out light

and the water reflects it.

Zhang Heng