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Ancient Astronomies
Copyright © Mary Ann Sullivan. I
have photographed (on site),
scanned, and manipulated all the
images on these pages. Please feel
free to use them for personal or
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 English
can easily be identified. 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 Illiad 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, air,
fire and water, plus a fifth element, the
quintessence, that filled the heavens. Earth 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 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.
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.
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.
Connecting Lives of Astronomers
to Historical events
1066 Battle of Hastings (Norman invasion of England)
1450 Guttenburg 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)
Issac Newton 1642-1727
1776 Declaration of Independence (America)