94 PART 1^ |^ EXPLORING THE SKY
Africa. Months before the eclipse, they photographed the part of
the sky where the sun would be located during the eclipse and
measured the positions of the stars on the photographic plates.
Th en, during the eclipse, they photographed the same star fi eld
with the eclipsed sun located in the middle. After measuring the
plates, they found slight changes in the positions of the stars.
During the eclipse, the positions of the stars on the plates were
shifted outward, away from the sun (■ Figure 5-16). If a star had
been located at the edge of the solar disk, it would have been
shifted outward by about 1.8 arc seconds. Th is represents good
agreement with the theory’s prediction.
Because the angles are so small, this is a very delicate obser-
vation, and it has been repeated at many total solar eclipses since
1919, with similar results. Th e most accurate results were
obtained in 1973 when a Texas–Princeton team measured a
defl ection of 1.66 ± 0.18 arc seconds—good agreement with
Einstein’s theory.
Th e general theory of relativity is critically important in
modern astronomy. You will meet the theory again in the discus-
sions of black holes, distant galaxies, and the big bang universe.
Einstein revolutionized modern physics by providing an explana-
tion of gravity based on the geometry of curved space-time.
Galileo’s inertia and Newton’s mutual gravitation are shown to be
not just descriptive rules but fundamental properties of space
and time.even happier with modern studies that have shown that Mercury,
Venus, Earth, and even Icarus, an asteroid that comes close to the
sun, have orbits observed to be slipping forward due to the cur-
vature of space-time near the sun. Th is same eff ect has been
detected in pairs of stars that orbit each other.
A second test of general relativity was directly related to the
motion of light through the curved space-time near the sun.
Because light has a limited speed, Newton’s laws predict that the
gravity of an object should slightly bend the paths of light beams
passing nearby. Th e equations of general relativity indicated that
light would have an extra defl ection caused by curved space-time,
just as a rolling golf ball is defl ected by undulations in a putting
green. Einstein predicted that starlight grazing the sun’s surface
would be defl ected by 1.75 arc seconds, twice the defl ection that
Newton’s law of gravity would predict (■ Figure 5-15). Starlight
passing near the sun is normally lost in the sun’s glare, but during
a total solar eclipse stars beyond the sun can be seen. As soon as
Einstein published his theory, astronomers rushed to observe
such stars and test the curvature of space-time.
Th e fi rst solar eclipse following Einstein’s announcement in
1916 occurred on June 8, 1918. It was cloudy at some observing
sites, and results from other sites were inconclusive. Th e next
occurred on May 29, 1919, only months after the end of World
War I, and was visible from Africa and South America. British
teams went to both Brazil and Príncipe, an island off the coast of
Apparent
position of starTrue position
of starSunEartha b■ Figure 5-15
Like a depression in a putting green, the curved space-time near the sun
defl ects light from distant stars and makes them appear to lie slightly farther
from the sun than their true positions.
■ Figure 5-16
(a) Schematic drawing of the defl ection of starlight by the sun’s gravity.
Dots show the true positions of the stars as photographed months before
the eclipse. Lines point toward the positions of the stars during the eclipse.
(b) Actual data from the eclipse of 1922. Random uncertainties of observa-
tion cause some scatter in the data, but in general the stars appear to move
away from the sun by 1.77 arc seconds at the edge of the sun’s disk. The
defl ection of stars is magnifi ed by a factor of 2300 in both (a) and (b).