The Astronomy Book

22

See also: Gravitational theory 66–73 ■ Halley’s comet 74–77

I

n about 130 bce, the Greek

astronomer and mathematician

Hipparchus of Nicaea noticed

that a star named Spica had moved

2o east of a point on the celestial

sphere, called the fall equinox

point, compared to its position

recorded 150 years earlier. Further

research showed him that the

positions of all stars had shifted.

This shift became known as

“precession of the equinoxes.”

The celestial sphere is an

imaginary sphere surrounding

Earth, in which stars are found at

specific points. Astronomers use

exactly defined points and curves

on the surface of this sphere as

references for describing the

positions of stars and other celestial

objects. The sphere has north and

south poles, and a celestial equator,

which is a circle lying above Earth’s

equator. The ecliptic is another

important circle on the sphere,

which traces the apparent path

of the sun against the background

of stars over the course of the year.

The ecliptic intersects the celestial

equator at two points: the spring

and fall equinox points. These mark

the positions on the celestial sphere

that the sun reaches on the

equinoxes in March and September.

The precession of the equinoxes

refers to the gradual drift of these

two points relative to star positions.

Hipparchus put this precession

down to a “wobble” in the movement

of the celestial sphere, which he

believed to be real and to rotate

around Earth. It is now known

that the wobble is actually in

the orientation of Earth’s spin

axis, caused by the gravitational

influence of the sun and the moon. ■

THE EOUINOXES

OVE OVER TIMEM

SHIFTING STARS

IN CONTEXT

KEY ASTRONOMER

Hipparchus (190 –120 bce)

BEFORE

280 bce Greek astronomer

Timocharis records that the

star Spica is 8° west of the fall

equinox.

AFTER

4th century ce Chinese

astronomer Yu Xi notices

and measures precession.

1543 Nicolaus Copernicus

explains precession as a

motion of Earth’s axis.

1687 Isaac Newton

demonstrates precession to

be a consequence of gravity.

1718 Edmond Halley discovers

that, except for the relative

motion between stars and

reference points on the

celestial sphere, stars have a

gradual motion relative to each

other. This is because they are

moving in different directions

and at different speeds.

Industrious, and a

great lover of the truth.

Ptolemy

describing Hipparchus