THE STORY OF
ASTRONOMY
Around the year A.D. 140, an astronomer named Ptolemy
wrote down his ideas about the motion of bodies in space.
Ptolemy shared the view of many Greek astronomers that
the Sun, the Moon, and the planets orbit Earth in perfect
circles. The Greeks had observed that planets sometimes
seem to reverse direction in their motion across the sky.
Ptolemy explained that the backward movements are smaller
orbits within the larger orbits. For 1400 years, Europeans
accepted this Earth-centered model. In the mid-1500s,
however, astronomers began to challenge and then reject
Ptolemy’s ideas.
1543
Sun Takes Center Stage
Nicolaus Copernicus, a Polish
astronomer, proposes that the planets
orbit the Sun rather than Earth.
His Sun-centered model shocks
many because it conflicts with the
traditional belief that Earth is the
center of the universe.
The timeline shows a few events in the history of astronomy. Scientists have developed special tools and procedures
to study objects in the sky. The boxes below the timeline
show how technology has led to new knowledge about
space and how that knowledge has been applied.
EVENTS
1500
1520
APPLICATIONS AND TECHNOLOGY
APPLICATION
Navigating by Sunlight and Starlight
For thousands of years, sailors studied the sky to find
their way at sea. Because the Sun and stars move in
predictable ways, sailors used them to navigate across
water. During the 1400s, sailors began to use
a device called a mariner’s astrolabe to observe
the positions of the Sun and stars. Later devices
allowed sailors to make more accurate
measurements.
This mariner’s astrolabe
was made in the 1600s.
736 Unit 6: Space Science
1540
1560
1609
Scientist Pinpoints
Planet Paths
German astronomer
Johannes Kepler
concludes that the orbits
of planets are not circles but ellipses, or
flattened circles. Kepler, formerly the
assistant of Tycho Brahe, reached his
conclusion by studying Brahe’s careful
observations of the motions of planets.
1863
Stars and Earth
Share Elements
English astronomer William
Huggins announces that stars are
made of hydrogen and other
elements found on Earth.
Astronomers had traditionally
believed that stars were made of a
unique substance. Huggins identified the elements in stars by studying their spectra.
1687
Laws of Gravity Revealed
English scientist Isaac Newton explains
that gravity causes planets to orbit the
Sun. His three laws of motion explain how
objects interact on Earth as well as in space.
1600
1620
1640
1660
1680\
1860
TECHNOLOGY
Viewing Space
The telescope was probably invented in the early
1600s, when an eyeglass maker attached lenses to
both ends of a tube. Soon afterward, Italian scientist Galileo Galilei copied the invention and used it
to look at objects in space. Galileo’s telescope
allowed him to study features never seen before,
such as mountains on the Moon.
Most astronomers now use
telescopes that gather
visible light with mirrors
rather than lenses.
There are also special
telescopes that gather
other forms of electromagnetic radiation.
Timelines in Science 737
Hubble Space Telescope
image of very distant
galaxies
1912
Cycles of Stars Are Key to Distances
Certain types of stars, called Cepheid variables,
get brighter and then dimmer in a regular cycle.
Astronomer Henrietta Leavitt finds that brighter
stars have longer cycles. This discovery will allow
the distances to these stars to be calculated.
1929
1916
Time, Space, and Mass Are Connected
The general theory of relativity expands
Newton’s theory of gravitation. Albert Einstein
shows that mass affects time and space.
According to this theory, gravity will affect the
light we receive from objects in space.
1880
1900
TECHNOLOGY
Colliding Particles Give Details About the
Start of the Universe
Scientists think that all matter and energy was in an
extremely hot, dense state and then exploded rapidly
in an event called the big bang. Some scientists are
attempting to re-create some of the conditions that
existed during the first billionth of a second after the
big bang. They use devices called particle accelerators
to make tiny particles move almost at the speed of
light. When the particles crash into each other, they
produce different types of particles and radiation.
Scientists use what they learn from the particles and
the radiation to develop models of conditions at the
beginning of the universe.
738 Unit 6: Space Science
Big Is Getting Bigger
Edwin Hubble has already used Cepheid variables
to show that some objects in the sky are actually
distant galaxies. Now he finds that galaxies are
generally moving apart, at rates that increase with
distance. Many astronomers conclude that the
universe is expanding.
1920
1940
1960
1998
Fast Is Getting Faster
Two groups of astronomers studying exploding
stars called supernovae come to the same
remarkable conclusion. Not only is the
universe expanding, but the rate of expansion
is increasing. In the diagram below, the rate of
expansion is shown by the distances
between rings
and between
galaxies.
Throughout history, people have learned about
the universe from visible light and other
radiation. New and better measurements have
been made as technologies improved. Better
and more complex models are filling in details
that cannot be measured directly. In the future,
improvements will continue. Computers,
telescopes in space, and other instruments
will allow astronomers to collect better data
and make better models.
Some matter in the universe does not give
off or reflect any detectable radiation. This is
called dark matter. Astronomers infer its existence from its effects on matter that is detected.
In the future, astronomers hope to determine
what dark matter is, exactly where it is, and
how it moves in the universe. In a similar way,
astronomers will learn more about why the
universe is expanding faster with time and
what energy is involved in this acceleration.
RESOURCE CENTER
CLASSZONE.COM
Learn more about
current advances in
astronomy.
1980
2000
TECHNOLOGY
Measuring the Big Bang
In 1965 two researchers noticed radio waves that came
from all directions instead of from just one direction,
like a signal from a space object. They inferred that
the radiation was left over from the big bang.
In 1989 and again in 2001, NASA launched spacecraft
to study the radiation. Data gathered using these
telescopes in space are still being used to test different
models of the big bang, including the arrangement
of matter in the universe. In this map of the sky, red
and yellow show the areas that were hottest after
the big bang.
Reliving History
Some early astronomers observed
the Moon in order to develop and
test their ideas about space. For two
weeks or more, make frequent
observations of the Moon and keep
your notes, sketches, and thoughts
in a notebook. You might look for
the Moon at a certain time each day
or night or perhaps record the
direction in which the Moon sets.
A newspaper may list the times of
moonrise and moonset for your
location.
Compare your observations and
thoughts with those of other students. You might also find out what
people in other cultures thought of
the patterns of change they saw in
the Moon.
Writing About Science
Choose one of these famous astronomers and research his or her story.
Write a biographical profile or an
imaginary interview with that person.
Timelines in Science 739