The Astronomy Book

78

See also: Shifting stars 22 ■ Stellar parallax 102 ■ Rømer (Directory) 335

I

n the 1720s, while seeking

proof that Earth was moving

by tracking changes in the

apparent positions of stars, Oxford

astronomer James Bradley found

another phenomenon that also

provided proof—stellar aberration.

The aberration of light causes

objects to appear to be angled

toward the direction of a moving

observer (in this case, Earth as it

moves through space). Aberration

angles are tiny: no more than the

speed of Earth perpendicular to the

star’s direction divided by the speed

of light, which is 20 arcseconds

at most. Earth moves at about 20

miles/s (30 km/s), but both its speed

and direction of travel change as it

orbits the sun. As a result, a star’s

observed position follows a small

ellipse around its real position.

Bradley observed this in the case of

the star Gamma Draconis—the first

irrefutable proof that Earth moves.

He also discovered another

periodic variation in star positions,

called nutation. Like aberration,

the effect is small. Earth’s spin axis

gradually changes its orientation

in space. The greatest change is

precession, and a full cycle takes

26,000 years to complete. Nutation

is a small wobble in precession

with an 18.6-year cycle. Precession

and nutation are both caused by

gravitational interactions between

the moon, Earth, and sun. Bradley

made his discovery public in 1748,

after 20 years of observations. ■

THESE DISCOVERIES

ARE THE MOST

BRILLIANT AND USEFUL

F THE CENTURYO

STELLAR ABERRATION

IN CONTEXT

KEY ASTRONOMER

James Bradley (16 9 3 –1762)

BEFORE

17th century The general

acceptance of a sun-centered

cosmos leads astronomers to

search for stellar parallax—the

apparent movement of stars

caused by Earth’s movement.

1676 Danish astronomer Ole

Rømer estimates the speed of

light using observations of the

Jovian satellites.

1748 Swiss mathematician

Leonhard Euler outlines the

physical cause of nutation.

AFTER

1820 German optician Joseph

von Fraunhofer builds a new

type of heliometer (a device for

measuring the sun’s diameter)

for the study of stellar parallax.

1838 Friedrich Bessel

measures the parallax of the

star 61 Cygni. He finds that it

is 600,000 times farther away

from Earth than the sun.

Stellar aberration

is caused by

Earth’s movement.

Changes in

Earth’s velocity

can be detected

through changes

in the position

of the stars.

Earth

Real position

Observed position

Movement of Earth