distance to your ¿ nger using basic trigonometry. The same principle applies
to stars, for as the Earth orbits the Sun, the closest stars should appear to
move against the background stars. The Greeks understood these principles,
but even the nearest stars are so remote that detecting their movements
requires very delicate observation. Not until 1838 were the ¿ rst accurate
measurements made.
In the ¿ rst decade of the 20th century, American astronomer Henrietta Leavitt
(1868–1921) found that variations in the light from stars called Cepheid
variables could be used to calculate their true brightness. Comparing this
with their apparent brightness on Earth made it possible to estimate their true
distance even if they were well beyond the range of parallax measurements.
In 1924, Hubble showed that at least some Cepheids existed outside our
galaxy, the Milky Way—proving for the ¿ rst time that the Universe consisted
of many galaxies, not just one.
Second, astronomers tried to determine the motions of the stars. In 1814,
German glassmaker Joseph von Fraunhofer (1787–1826) invented the
spectroscope, a device like a prism, which splits light into its component
wavelengths. Fraunhofer identi¿ ed dark “absorption lines” in the spectra of
starlight. These correspond to particular elements in the stars themselves,
because each element absorbs light energy at different frequencies.
In the late 19th century, Vesto Slipher, at the Lowell Observatory in Flagstaff,
Arizona, showed that some stellar absorption lines were shifted away from
their expected frequencies. Slipher interpreted these shifts as the results of
a Doppler effect, an apparent change in wavelengths caused by the relative
movements of the two bodies. (We experience the Doppler effect when the
pitch of an ambulance siren appears to change as it passes us.) As Slipher
showed, this meant that changes in absorption lines could tell us whether
distant objects were moving toward us or away from us, and at what speed.
Using these ¿ ndings, Hubble showed that all remote objects are shifted to
the red end of the spectrum, which meant they were moving away from us.
Furthermore, the more remote they were, the greater was the red shift, or the
rate at which they were moving away from the Earth.