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Ancient Astronomies
Copyright © Mary Ann Sullivan. I
have photographed (on site),
scanned, and manipulated all the
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educational purposes.
Since measuring time and determining
seasons are important to all civilizations,
we find that essentially all civilizations
set up some kind of astronomical system. Some of
these civilizations were in warm climates where the
change of seasons was not so obvious, but they still
needed to determine when the floods or the rains
would come for agricultural effectiveness.
Here are a few examples:
Ancient Astronomies
Egyptians based their agriculture on the
flooding of the Nile. They noticed that the
flooding occurred close to the heliacal
rising of Sirius, the brightest star in the
night sky. The pyramids have structures
that are aligned with astronomical events.
Ancient Astronomies
Stonehenge is an ancient and strange site in Britain
that has massive stones that appear to have a
relation to various astronomical occurrences, such
as summer and winter solstices.
The Mayans had a calendar based on the motions of
Venus.
The Babylonians had a well developed astronomy
that detailed the motions of the moon and planets.
The North American Indians developed a
Medicine Wheel that was aligned with the summer
and winter solstices.
Time
Year: Cycle through the seasons.
Month: Based on cycle of the moon – adjusted to
make exactly 12 months in a year instead of the
actual 12.4 cycles per year.
Week: 7 is chosen since there are 7 astronomical
objects besides the stars: Sunday, Moonday,
Thorsday (Thor = Jupiter), Saturnday. In Spanish
you can easily identify Marsday (Martes =
Tuesday), Mercuryday (Miercoles = Wednesday),
and Venusday (Viernes = Friday).
Time
Day: Time from noon on one day to noon on the next.
Hour: Day is broken into day and night, and the
daytime is broken into 12 units, just like the year, and
the night is broken into 12 units, also. Total is 24
hours in a day.
Minute: A minute (pronounced my-nute’ meaning tiny) part
of an hour – based on Egyptian idea of a minute (tiny)
part = 1/60th .
Second: A minute (tiny) part of a minute (tiny) part of an
hour, or a second minute (tiny) part of an hour, or
simply a second.
The Classical Greeks
Homer: 8th century BC; famous for the Iliad and the
Odyssey : celestial objects are gods, flat earth,
dome for sky.
Thales: ~600 BC; Determined length of year, stars
are not gods (since their motions are too orderly).
Pythagoras: ~500 BC; geometry developed, idea of
spherical objects. Earth is NOT flat!
Plato: ~400 BC; pure form, ideals in the heavens,
abstract, mathematical forms versus real objects.
The Classical Greeks
Aristotle: ~350 BC; placed emphasis on
observations; postulated four elements: earth,
wind (air), fire and water, plus a fifth element, the
quintessence, that filled the heavens. Earth is not
spinning since there is no great wind outside, so
Earth must be stationary, so other things must be
revolving around the earth (geocentric model).
** Alexander the Great lived during this time
Eratosthenes: ~250 BC; used geometry and
observations to determine the circumference of the
spherical earth – accurate to about 1%.
The Classical Greeks
Ptolemy: ~150 AD, in Alexandria, Egypt. Put
together a book, the Almagest. This offered a
theory of the heavens that gave fairly good
predictions. It was based on circular orbits of
objects around the spherical earth
(geocentric), but included circles on circles
(epicycles) that accounted very well for the
retrograde motion of the planets.
Epicycles
planet
earth
center
Beginnings of Modern Theories
** Gutenberg Printing Press invented in 1450
** Columbus discovers America in 1492
Copernicus (1473-1543) Proposed a sun
centered system (heliocentric) to more
easily explain retrograde motion. (We’ll show
this in lab with a computer simulation to see how this
works.)
Book published in 1543.
Beginnings of Modern Theories
Tycho Brahe (1546-1601) He collected great amounts
of very precise data about the positions of the
planets using instrumentation he developed.
Johannes Kepler (1571-1630) He used Brahe’s data
and mathematics to come up with three laws:
1. Planets follow elliptical instead of circular orbits.
2. Planets sweep through equal areas in equal times.
3. The square of the period is proportional to the cube
of the semimajor axis of the ellipse.
Beginnings of Modern Theories
Galileo Galilei (1564-1642) contemporary of Kepler.
He used the telescope to look at astronomical objects.
He discovered craters on the Moon, moons going
around Jupiter, rings around Saturn, and the fact that
Venus has phases that are related to its orbit.
The heliocentric model can explain all of these, whereas
the geocentric model fails.
Also, no need for Aristotle’s “quintessence” since what
is up there looks like it is the same as what is down
here.
Modern Theories
Issac Newton (1642-1727) Formulated Newton’s three
laws of motion, and Newton’s law of gravity. He
invented the calculus to work with these laws. Using
these tools and the idea that what is up there obeys
the same laws as what is down here, he explained
Kepler’s three laws in terms of these more
fundamental laws. He worked in optics as well, and
invented the reflecting telescope (more about this in part
2 of the course).
Newton’s Laws of Motion
1st Law of Motion: Objects tend to go in a
straight line with constant speed.
2nd Law of Motion: Forces tend to change an
object’s motion (speed it up, slow it down, or
change its direction). The more mass an object
has, the harder it is to change its motion:
ΣF = ma .
3rd Law of Motion: For every action, there is
an equal and opposite reaction. (You can’t
push yourself. You have to push something else,
and hope it pushes back.)
Newton’s Law of Gravity
Every object that has mass attracts every other
object that has mass with a force that
depends on the masses of both objects, and
falls off with the distance between those
objects. That force is very small unless one
or both objects has a lot of mass (like the
earth). F = G*m1*m2 / r2 .
Planetary Motion
All planets experience the gravitational force of
the sun. They would all fall into the sun,
except they are already moving sideways.
They would normally continue their motion
and move away from the sun, but the sun is
pulling them in.
To orbit the sun, they must move not too fast to
escape, nor too slow so that they fall into the
sun. The result is elliptical orbits. We’ll
simulate this on a computer in one of the labs.
Planetary Orbits
The closer the planet is to the sun, the more force (from
Newton’s Law of Gravity). In order not to fall in, the
closer planets must move faster.
The farther the planet is from the sun, the less force. In
order not to escape from the sun, the farther planets
must move slower.
Not only do the inner planets have shorter orbits, they
have to move faster around those orbits.
Mathematically, this works out to explain Kepler’s 3rd
law that relates the period of orbit to the planet’s
distance from the sun.
Philosophy & Religion I
Newton’s Laws prompted some philosophers and
theologians to propose a mechanistic view of the
universe: God set the universe in motion, and then let it
go. The universe will simply follow those laws. Nothing
anyone can do about it after it is set in motion.
This led to the idea that there is essentially no free will.
This led some to propose the idea of predestination: you
can’t change anything. Some are born and will be
“saved”, others are born and will go to “hell”.
Modern Theories
Albert Einstein (1879 – 1955) Theory of Relativity
(1905 and following) predicted that different observers
will measure different times, distances and masses
based on the observers’ relative speeds. In particular,
lifetimes and masses of particles differ if they are
stationary compared to when they are moving. These
effects are only noticeable as the speeds approach the
speed of light (~670 million miles per hour). This theory
implies that the measurement of time is not absolute,
but rather relative. The basis of the theory is that the
laws of physics are true in all inertial frames (they
are absolute), but the measurements are relative (they
are different in different frames).
Modern Theories
Quantum Theory (20th century) was developed by
many different people in trying to make sense of
measurements on the atomic scale. Basically, some
things (for example, light and electrons) act like particles
sometimes and act like waves at other times. This is
not random – in certain well defined situations, they
always act like particles; in other well defined
situations, they always act like waves.
The wave idea of light has the electric and magnetic
fields waving in space; the particle of light is called
the photon. An electron, usually thought of as a
particle, sometimes behaves light a wave instead.
Philosophy and Religion II
Relativity has implications about space travel: we can’t go
faster than light (670 million mph), but we can travel farther than 100
light years. This doesn’t seem to make sense, but it actually does
when you understand the theory. This theory is necessary in designing an
accurate GPS system. This theory may have influenced some theories
about moral relativism.
The laws of the Quantum Theory say that it will be
theoretically impossible to know the exact initial conditions of the
universe, and so it will be impossible to predict the exact future of
the universe. This goes against the ideas of the mechanistic
universe and predestination.
Connecting Lives of Astronomers
to Historical events
1066 Battle of Hastings (Norman invasion of England)
1450 Guttenberg printing press invented
1492 Columbus “discovers” America Copernicus 1473-1543
1588 Spanish Armada defeated
Tycho Brahe 1546-1601
by Britain
Johannes Kepler 1571-1630
1607 Jamestown established
Galileo Galilei 1564-1642
(1st permanent English settlement)
Isaac Newton 1642-1727
1776 Declaration of Independence (America)