The Astronomy Book

102

See also: The Tychonic model 44–47 ■ Measuring the universe 130–37 ■

Beyond the Milky Way 172–77

P

arallax is the apparent

movement of a nearby

object against distant

objects due to the changing

position of the observer. According

to this phenomenon, nearby stars

should appear to change position

against the background of more

distant stars as Earth moves

around its orbit. The idea that it

might be possible to measure the

distance to nearby stars using

parallax dates back to ancient

Greece. However, it was not

achieved until the 19th century,

due to the distances involved being

far greater than anyone supposed.

Much of German astronomer

Friedrich Bessel’s career had

been dedicated to the accurate

determination of the positions of

stars and finding their proper motion

(changes in position due to the

motion of the star, rather than

changes in apparent position due

to the time of night or the season).

By the 1830s, with improvements in

the power of telescopes, there was

a race to carry out the first accurate

measurement of stellar parallax. In

1838, Bessel measured parallax with

an angle of 0.314 arc seconds for the

star 61 Cygni, which indicated that

it was 10.3 light-years away. The

current estimate is 11.4 light-years,

giving Bessel’s measurement an

error of just under 10 percent. ■

AN APPARENT

MOVEMENT OF

THE STARS

STELLAR PARALLAX

IN CONTEXT

KEY ASTRONOMER

Friedrich Bessel (1784 –1846)

BEFORE

220 bce Aristarchus suggests

that the stars are very far away

since no parallax can be seen.

1600 Tycho Brahe rejects

the Copernican sun-centered

system partly because he

cannot detect stellar parallax.

AFTER

1912 Henrietta Swan Leavitt

discovers a link between the

period of a type of variable

star and its brightness,

allowing these stars to be

used as “standard candles”

for figuring out distances.

1929 Edwin Hubble discovers

the link between the redshift

of a galaxy’s light and its

distance from Earth.

1938 Friedrich Georg

Wilhelm Struve measures the

parallax of Vega, and Thomas

Henderson measures the

parallax of Alpha Centauri.

Due to the effects of parallax, a nearby

star’s apparent position against distant

background stars moves from b in June

to a in December.

Earth’s

position

in June

a b

Sun

Earth’s

position in

December