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S
ome of the most important,
but often most challenging,
measurements for astronomers
to make have been the distances to
extremely remote objects—which
includes most celestial objects
aside from the moon, sun, and other
planets of the inner solar system.
Nothing in the light coming from
distant stars and galaxies gives
any direct indication of how far
that light has traveled through
space to reach Earth.
For several hundred years,
scientists realized that it should be
possible to measure the distances
to relatively nearby stars by a
method called parallax. This is
based on comparing the position
of a nearby star against the
background of more distant stars
from two perspectives—usually
Earth’s different positions in space
six months apart in its orbit around
the sun. Although many others
had tried (and failed) before him,
the first astronomer to measure a
star’s distance accurately using
this method was Friedrich Bessel,
in 1838. However, even with
increasingly powerful telescopes,
measuring star distances by
parallax proved difficult and,by the year 1900, the distances
to only about 60 stars had been
measured. Furthermore, the parallax
method could be applied only to
nearby stars. The difference in
perspective for more distant stars
over the course of a year was too
small to be accurately determined.
New methods were therefore needed
to measure large distances in space.Measuring brightness
In the 1890s and early 1900s, the
Harvard College Observatory in
Massachusetts was one of the
world’s leading astronomical researchMEASURING THE UNIVERSE
IN CONTEXT
KEY ASTRONOMER
Henrietta Swan Leavitt
(1868–1921)BEFORE
1609 German pastor David
Fabricius discovers the
periodically variable star Mira.1638 Dutch astronomer
Johannes Holwarda observes
Mira’s variation in brightness
over a regular 11-month cycle.1784 John Goodricke discovers
a periodic variation in the star
Delta Cephei: the prototypic
example of a Cepheid variable.1838 Friedrich Bessel measures
the distance to the star 61 Cygni
using the parallax method.AFTER
1916 Arthur Eddington
studies why Cepheids pulsate.1924 Edwin Hubble uses
observations of a Cepheid
in the Andromeda nebula
to calculate its distance.A remarkable relation
between the brightness
of these (Cepheid) variables
and the length of their
periods will be noticed.
Henrietta Swan LeavittHenrietta Swan Leavitt Henrietta Swan Leavitt developed
an interest in astronomy while
studying at Radcliffe College,
Cambridge, Massachusetts. After
graduation, she suffered a serious
illness that caused her to become
increasingly deaf for the rest of
her life. From 1894 to 1896 and
then again from 1902, she worked
at Harvard College Observatory.
Leavitt discovered more than
2,400 variable stars and four
novae. In addition to her work
on Cepheid variables, Leavitt
also developed a standard of
photographic measurements,
now called the Harvard Standard.Due to the prejudices of the
day, Leavitt did not have the
chance to use her intellect
to the fullest, but she was
described by a colleague as
“possessing the best mind
at the Observatory.” She was
remembered as hardworking
and serious-minded, “little given
to frivolous pursuits.” Leavitt
worked at the Observatory until
her death from cancer in 1921.Key work1908 1777 Variables in the
Magellanic Clouds