Ancient cultures studied the sky and developed early calendars and astronomy by observing cyclic patterns of celestial objects like the sun, moon, and stars. They used structures like Stonehenge and sites in Egypt to track solstices and help form early understandings of days, months and years. Over millennia, models evolved from geocentric views to the eventual heliocentric model established by Copernicus, Kepler, Galileo and Newton, through precise observations, mathematical laws, and the discovery of gravity.

1. How has the
amount of daylight
we are receiving
changed over the
last two weeks?

2. What can you tell
me about how the
stars move in the
sky during an
evening?

3. What is the current
phase of the moon?
How has this
changed over the
last three weeks?

4. What can you tell
me about how the
stars move in the
sky over the period
of a year?

5. Ancient cultures studied the sky and
developed calendars based upon the
patterns they saw
 Motion of the sun
 Motion of the moon
 Motion of the stars
Ancient culture based their calendars on
repeating cycles they observed in the
sky.

6. What is a day?
What they saw:
 The time it takes the sun to cross the
sky and return to the same point
What we know:
 Time required for the Earth to rotate
once on its axis

7. What is a month?
What they saw:
 The amount of time required for the
moon to move through a complete set
of phases
What we know:
 The amount of time it takes the moon
to revolve around the Earth

8. What is a year?
What they saw
 The time it takes for a group of stars
(constellation) to return to the same
part of the sky at a certain time of day
What we know:
 Time required for the Earth to revolve
around the sun.

9. Early Observers
The Beginnings of Astronomy
Scientists have found evidence for
ancient astronomical activities
from all over the world.

10. Early Humans recognized
the cycles of celestial
objects in the sky

11. Nabta
 Southern Egypt – Some stones are
positioned such that they would have
lined up with the sun during the
summer solstice 6,000 years ago.
 The summer solstice occurs on the
longest day of the year. (Day with most
amount of sunlight)

12. Stonehenge
 Another ancient site that was probably
used to make observations of the sky.
 Stones are arranged primarily in circles
which are aligned with the sunrise
during the summer and winter solstices.
 Built over a period of 1,500 years.
 Built for ceremony and ritual.

13. Babylonians
 Ancient empire located in present-day
Iraq
 700 B.C. to A.D. 50, Babylonians
precisely tracked the positions of the
planets and moon.
 Skilled at forecasting the movements of
these celestial bodies, which enabled
them to make an accurate calendar.

14. Ancient Chinese
 1,000 B.C. could predict eclipses.
 The Chinese named 800 stars by 350
B.C.
 The Chinese skillfully tracked and
predicted the same motions in the sky
as the civilizations that influenced
Western astronomy.

15. Ancient Greeks
 Learned to observe the sky to keep track of
time.
 Helped make astronomy a true science.
 Greek philosophers had tried to understand
the place of Earth and humans in the
universe (Logic & Geometry)
 Aristotle (Greek philosopher) successfully
explained the phases of the moon
 Correctly stated that Earth was a sphere (an
idea that was not popular at the time)

16. Native Americans
 Skilled in observing the sky
 Maya – 1,000 years ago in the Yucatan
 Had complex systems of mathematics
and astronomy.
 Many buildings were aligned with
celestial bodies during certain
astronomical events

17. Ancient Arabs
 Followed Greeks, Romans, early Christians.
Used Greeks’ knowledge of astronomy to
develop astronomy into a science while
Europe fell into the “Dark Ages”
 Many stars have Arabic names because of
this period.
 Invented Astrolabe, algebra, modern number
system.

18. Earliest Astronomers
Summary
 The ancient people had no history to learn
from.
 Almost everything they knew about the
universe came from what they could discover
with their own eyes and minds.
 They thought that the universe was made up
of the sun, moon, planets with all the stars
somewhere towards the edge of the universe.

19. Early Models of the Universe

20. Pythagoreans (500 B.C.)
 Believed the stars, planets, sun,
and moon were attached to
crystalline spheres which
rotated around the earth.
 (geocentric)

21. Aristotle (350 B.C.)
 Believed the Earth is motionless at
the center of the universe and all
the stars and planets rotated
around it.
 Stars and planets moved in circular
paths.
(geocentric)

22. Ptolemy (140 A.D.)
 Expanded Aristotle’s theories to try to
account for “retrograde motion” of the
planets (Ptolemaic Theory)
 Planets traveled in smaller circular
paths as they traveled around the Earth
(epicycles and deferents)
 Popular model of universe for 1,500 years.

23. Copernicus (1543)
 Sun at center of universe and the
planets orbit the sun
(Heliocentric)
 Solved the problem of “retrograde
motion”
 Theory did not immediately replace
Ptolemy’s theory.

24. Tycho Brahe (1600)
 Favored an Earth-centered universe
different from Ptolemy’s theory
 Thought that other planets
revolved around the sun, and that
the sun and moon revolved around
the Earth.
 Theory incorrect, but made many
precise observations of planets and
stars.

25. Johannes Kepler (1609)
 Mathematician
 Tycho’s assistant
 Used Tycho’s observational data to develop
laws of planetary motion
 Kepler’s 3 Laws of Planetary Motion
 Law of Ellipses
 Law of Equal Areas
 Law of Periods

26. Galileo Galilei (1609)
First person to use telescope for
astronomical observations
 Discoveries
* Galilean Satellites (Jupiter’s moons)
* Craters on the moon
* Sunspots on the Sun
* Phases of Venus
Favored Copernicus’s theory over Ptolemy’s

27. Isaac Newton
 1687
 Explained why planets orbit the Sun,
and why moons orbit planets
 Newton’s Law of Motion
 Newton’s Law of Gravitation
 Completed the work of Copernicus,
Tycho, Kepler, and Galileo