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
observatories.
Over time, the study of the night sky became the
science of Astronomy. Today Astronomy is known
as the study of the universe.
The Roots of Astronomy
• Already in the stone and bronze ages,
human cultures realized the cyclic
nature of motions in the sky.
• Monuments dating back to ~ 3000
B.C.E show alignments with
astronomical significance.
• Those monuments were probably used
as calendars or even to predict eclipses.
Stonehenge
Summer solstice
Heelstone
• Alignments with locations of
sunset, sunrise, moonset and
moonrise at summer and winter
solstices
• Probably used as calendar.
•https://www.englishheritage.org.uk/daysout/properti
es/stonehenge/
• Constructed: 3000 – 1800 B.C.E.
Other Examples All Over the World
http://solar-center.stanford.edu/AO/bighorn.html
Big Horn Medicine Wheel (Wyoming)
Built between 300-800 years ago (1200 C.E.)
Other Examples All Over the World (2)
http://www.caracol.org/
Caracol (Maya culture, approx. 1000 B.C.E.)
Ancient Greek Astronomers (1)
• Unfortunately, there are no written
documents about the significance of
stone and bronze age monuments.
• First preserved written documents
about ancient astronomy are from
ancient Greek philosophy.
• Greeks tried to understand the motions
of the sky and describe them in terms
of mathematical (not physical!) models.
Ancient Greek Astronomers (2)
Their models were generally wrong because
they were based on wrong “first principles”,
believed to be “obvious” and not questioned:
1. Geocentric Universe: Earth at the Center of
the Universe.
2. “Perfect Heavens”: Motions of all celestial
bodies described by motions involving
objects of “perfect” shape, i.e., spheres or
circles.
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.
Ptolemy:
An Earth-Centered Universe
• A Greek Astronomer-around 140 C.E.
• 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
calculations.
• 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)
Ptolemy: Geocentric
Earth-Centered Universe
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.
Copernicus: Heliocentric
Sun-Centered Universe
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
date.
•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.
Kepler: Heliocentric with Elliptical
orbits
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
and record his findings. He DID NOT invent the
telescope!
•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.
Galileo: Telescope
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
sun.
•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 1920s.
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
telescopes.
•These tools have helped answer many questions about the
universe, yet new technology has presented questions that were
unthinkable even 10 years ago.
Historical Overview