736 Unit 6: Space Science

Around the year A.D. 140, an astronomer named Ptolemywrote down his ideas about the motion of bodies in space.Ptolemy shared the view of many Greek astronomers thatthe Sun, the Moon, and the planets orbit Earth in perfect circles. The Greeks had observed that planets sometimesseem to reverse direction in their motion across the sky.Ptolemy explained that the backward movements are smallerorbits within the larger orbits. For 1400 years, Europeansaccepted this Earth-centered model. In the mid-1500s,however, astronomers began to challenge and then rejectPtolemy’s ideas.

The timeline shows a few events in the history of astron-omy. Scientists have developed special tools and proceduresto study objects in the sky. The boxes below the timelineshow how technology has led to new knowledge aboutspace and how that knowledge has been applied.

ASTRONOMYTHE STORY OF

1500 1520 1540 1560

EVENTS

APPLICATIONS AND TECHNOLOGY

APPLICATION

Navigating by Sunlight and Starlight

For thousands of years, sailors studied the sky to findtheir way at sea. Because the Sun and stars move inpredictable ways, sailors used them to navigate acrosswater. During the 1400s, sailors began to use a device called a mariner’s astrolabe to observe the positions of the Sun and stars. Later devicesallowed sailors to make more accurate measurements.

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.

This mariner’s astrolabe

was made in the 1600s.

Timelines in Science 737

TECHNOLOGY

Viewing Space

The telescope was probably invented in the early1600s, when an eyeglass maker attached lenses toboth ends of a tube. Soon afterward, Italian scien-tist Galileo Galilei copied the invention and used itto look at objects in space. Galileo’s telescopeallowed him to study features never seen before,such as mountains on the Moon. Most astronomers now use telescopes that gather visible light with mirrorsrather than lenses.There are also specialtelescopes that gatherother forms of electro-magnetic radiation.

1860

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 identi-

fied the elements in stars by study-

ing their spectra.

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.

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\

Hubble Space Telescope

image of very distant

galaxies

738 Unit 6: Space Science

1880 1900 1920 1940 1960

TECHNOLOGY

Colliding Particles Give Details About the Start of the Universe

Scientists think that all matter and energy was in anextremely hot, dense state and then exploded rapidlyin an event called the big bang. Some scientists areattempting to re-create some of the conditions thatexisted during the first billionth of a second after thebig bang. They use devices called particle acceleratorsto make tiny particles move almost at the speed oflight. When the particles crash into each other, theyproduce different types of particles and radiation.Scientists use what they learn from the particles andthe radiation to develop models of conditions at thebeginning of the universe.

1929

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.

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.

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.

Timelines in Science 739

Reliving HistorySome early astronomers observedthe Moon in order to develop andtest their ideas about space. For twoweeks or more, make frequentobservations of the Moon and keepyour notes, sketches, and thoughtsin a notebook. You might look forthe Moon at a certain time each dayor night or perhaps record the direction in which the Moon sets. A newspaper may list the times ofmoonrise and moonset for yourlocation.

Compare your observations andthoughts with those of other stu-dents. You might also find out whatpeople in other cultures thought ofthe patterns of change they saw inthe Moon.

Writing About ScienceChoose one of these famous astron-omers and research his or her story.Write a biographical profile or animaginary interview with that person.

1980 2000

Learn more aboutcurrent advances inastronomy.

RESOURCE CENTERCLASSZONE.COM

Throughout history, people have learned aboutthe universe from visible light and other radiation. New and better measurements havebeen made as technologies improved. Betterand more complex models are filling in detailsthat cannot be measured directly. In the future,improvements will continue. Computers,telescopes in space, and other instruments will allow astronomers to collect better dataand make better models.

Some matter in the universe does not giveoff or reflect any detectable radiation. This iscalled dark matter. Astronomers infer its exis-tence from its effects on matter that is detected.In the future, astronomers hope to determinewhat dark matter is, exactly where it is, andhow it moves in the universe. In a similar way,astronomers will learn more about why the universe is expanding faster with time andwhat energy is involved in this acceleration.

TECHNOLOGY

Measuring the Big Bang

In 1965 two researchers noticed radio waves that camefrom all directions instead of from just one direction,like a signal from a space object. They inferred thatthe radiation was left over from the big bang. In 1989 and again in 2001, NASA launched spacecraftto study the radiation. Data gathered using these telescopes in space are still being used to test differentmodels of the big bang, including the arrangementof matter in the universe. In this map of the sky, redand yellow show the areas that were hottest afterthe big bang.

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.