AstronomyThe Original Science
Imagine that it is 5,000 years ago. Clocks and
modern calendars have not been invented. How
would you tell time or know what day it is?
One way to tell the time is to study the movement of
stars, planets and the moon. Studying the ancient
skies was so important that ancient people built
Over time, the study of the night sky became the
science of Astronomy. Today Astronomy is known as
the study of the universe.
Who’s Who of Early Astronomy
• The careful work of early
astronomers helped people
understand their place in the
universe. Almost everything early
astronomers knew about the
universe came from what they could
discover with their eyes and minds.
• Not surprisingly, most early
astronomers thought that the
universe consisted of the sun, the
moon and the planets. They thought
that the stars were at the edge of
the universe.
An Earth-Centered Universe
• A Greek Astronomer-around 100 AD
• Ptolemaic Theory-he wrote a book that combined all of the
ancient knowledge of astronomy that he could find. He
then expanded on it with careful mathematical
• Ptolemy thought that the Earth was at the center of the
universe and that the other planets and the sun revolved
around the Earth.
• Although his theory was incorrect, it predicted planetary
motion better than any other theory at the time.
• His theory was the most popular for the next 1,500 years.
(Geocentric Theory)
Nicholas Copernicus:
A Sun-Centered Universe
•A Polish astronomer (1543)
•Revolutionized astronomy with his new theory
•Heliocentric theory-the sun is at the center of the
universe, and all of the planets, including the Earth,
orbit the sun.
•The theory correctly explained the movement of the
planets around the sun but it did not replace Ptolemy’s
theory immediately.
•When Copernicus’s theory was accepted, major
changes in science and society were taking place.
Tycho Brahe: A Wealth of Data
•Danish astronomer, late 1500’s
•Used several tools to make the most detailed
astronomical observations that had been recorded to
•Brahe favored a modified version of Ptolemy’s
theory; the sun and the moon revolved around the
earth and that other planets revolve around the sun.
•While his theory was not correct, Brahe recorded
very precise observations of the planets and stars
that helped future astronomers.
Johannes Kepler:
Laws of Planetary Motion
•Was Brahe’s assistant-continued the work after Brahe’s death
•1609-after much analysis of the Brahe’s data, Kepler concluded
that all of the planets revolve around the sun in elliptical orbits
and that the sun in not the exact center of the orbits.
•Stated his ideas in three laws of planetary motion:
1-the sun is the center of universe and the planets revolve
around it in elliptical orbits.
2-the planets move faster when their orbits bring them closer
to the sun.
3-a mathematical formula used to determine the distance of a
planet from the sun.
•These laws are still used today.
Galileo: Turning a
Telescope to the Sky
•In 1609, Galileo Galilei became one of the first
people to use a telescope to observe objects in space.
•He discovered craters and mountains on the Earth’s
moon, four of Jupiter's moons, sunspots on the sun,
and the phases of Venus.
•These discoveries showed that the planets are not
“wandering stars” but are physical bodies like the
Earth and it gave him proof that the planets did indeed
revolve around the sun, as Copernicus had stated.
Isaac Newton: The Laws of Gravity
•In 1687, Isaac Newton showed that all objects in the
universe attract each other through gravitational force.
•The force of gravity depends on the mass of the objects
and the distance between them.
•Newton’s law of gravity explained why all of the planets
orbit the most massive object in the solar system---the
•Newton once said that “I could see so far because I
stood on the shoulders of giants.” He gave credit the
observations and ideas of all the scientists who came
before him.
Modern Astronomy
• The invention of the telescope and the
description of gravity were two milestones
in the development of modern astronomy.
• In the 200 years following Newton’s
discoveries, scientists made many
discoveries about our solar system. But
they did not learn that our galaxy has
cosmic neighbors until the 1920’s.
Edwin Hubble: Beyond the Edge of
the Milky Way
•In 1924, Edwin Hubble proved that other galaxies existed
beyond the edge of the Milky Way.
•His data confirmed the beliefs of some astronomers that the
universe is much larger than our galaxy.
•Today, larger and better telescopes on the Earth and in space,
new models of the universe, and spacecraft help astronomers
study space.
•Computers help process data and control the movement of
•These tools have helped answer many questions about the
universe, yet new technology has presented questions that were
unthinkable even 10 years ago.
Ptolemy: Geocentric
Earth-Centered Universe
Copernicus: Heliocentric
Sun-Centered Universe
Kepler: Heliocentric with Elliptical
Theories of the Universe
Galileo: Telescope

Astronomy- The Original Science