EARLY GREEK ASTRONOMY
• prehistoric monoliths; Stonehenge:
• The Stonehenge in Britain, there are
• Inside the ditch of the stonehenge is called theAubrey holes
• in the centre arrange of large stones, in a form of a horseshoe
• Outside there is the heelstone
• When observe the sunrise on the day of summer solstice, from the point inside of this horseshoe
we see the the sun rising right above the heelstone
• There are some astronomical arrangement with the stones
• We don't know the true pourpose of the stonehenge
• Circle of stones in southern Sahara desert, built 1000 years earlier has allignments of stones in
nouth south and east west directions
• Babylonian astronomy
• how ancient Greek astronomers knew the Earth is spherical
He used observations to form his views on the universe, eg. From the observations that when one
travels south one can see new stars and constellations. He concluded that the Earth must be a sphere.
• Thales of Miletus: the universe is rational
• The universe is rational, ie. The human mind can understand it • Pythagoras: the universe is mathematical and geometrical
• Many things in nature seem to be governed by mathematical and geometrical relations.
• Aristotle: the geocentric universe: Wikipedia
• Believed that the earth was the centre of the universe and that everything else revolved around
• Aristarchus of Samos: the origin of the heliocentric model:
• Proposed a theory that the earth rotated about it's axis and that it revolved around the sun. He
used observations to come up with this view of the universe.
1) He concluded that the sun is at much greater instance from us than the moon.Assume the
opposite, that the sun and the moon are at comparable distance from us. Than the interval of the
first quarter to the third quarter would be much longer than the interval from third quarter to
• He wanted to determine the angle a because than using geometry, he could find the ratio
Earth – Moon Distance
Earth – Sun Distance
• He obtained the value of 87 degrees (the actual value is 89 degrees and 53/60 degress < 59
minutes of arc. Thus he found
Earth – Moon Distance
Earth – Sun Distance
= 1/20 In other words the Earth – Sun distance is 20 times larger than the Earth – Moon
• The actual value is 1/400. The Earth – Sun distance is 400 times larger than the Earth – Moon
• Then he deduced, because the angular size of the sun and the moon is the same, that sun's
diameter must be 20 times larger than the moon's.
• He used observations to deduce the relative size of the earth and the moon.
• Lunar Eclipse: He timed the beginning of the lunar eclipse till the moon disappeared in the
moon's shadow. Than measured the moon to reemerge behind the shadow. The time it takes to
go fromAto B is proportional to the diameter of the moon.
• He found that the diameter of the moon is about one third diameter of the earth
• The earth is about three times according to his measurements bigger than the moon. But the sun
is 20 times more bigger than the moon.
• This is how he reached to the conclusion of the heliocentric system, where the earth is revolving
around the sun, rather than the other way around • Stellar parallax and objections to the heliocentric model
• The argument is if the sun is revolving around the earth at the distance of one astronomical unit
• Aparallax is this apparent change in the position of an object due to the change of the observer.
In this a case a nearby star due to the change in the location of the observer
• Our vision is based on parallax
• Parallax is in general, an apparent change in the position of an object caused by the change in
the position of the observer.
• If the distance the to the star (d) is very large p is very small and hard to detect
• Eratosthenes: measuring the circumference of the Earth
• Eratosthenes (276 BC – 195 BC)
• The first to determine the circumference of the earth or the radius of the earth
• C= 2Pi R, by using geometry and observations
• Geometry: if I have two parallel lines and intersect with any straight line...all equal angles.
• motions of the planets; prograde and retrograde motions:
Motion of the Planets:
Most Planets are visible to the naked eye except for Uranus and Neptune. We can always see them
because they are very close to the ecliptic (because their orbital planes lie in nearly the same plane as
the orbit of the earth).
• Venus was called Hesperus and when they observed it after the sunset and just before the
sunrise they called it Phosphorus. They they realized it's just the same thing. “Hesperus is
• Mars moves west to east most of the time, than periodically stops and reverses the motion east
to west like the sun
• Propgrade motion eg. West to east, Retrograde motion opposite of prograde eg. Now east to
west • Ptolemy: the problem of planetary motion in the geocentric model
EPICYCLES; EXPLAINING RETROGRADE PLANETARY MOTIONS IN THE GEOCENTRIC
AND HELIOCENTRIC MOTIONS:
• How to fit this into theAristotelian model of geocentric universe? So they invented the system
of epicycles to describe the motion of planets
• What are epicycles? Planets are moving along small circles whose centers are moving along
small circles, which are are moving around in a big circle
• If you complicate things to explain things happening around you, that means your starting to
assumption is wrong
• If you have the need to modify the theory, to tweak it all the time to account for larger number
of events, the assumption is wrong
• In Heliocentric model we can understand the the retrograde motion, the closer the planet is to
the sun, the faster it moves, bigger it's orbital speed is. Earth and the Mars, the earth is moving
faster around it's orbit around the sun, than mars.
• To account for the retrograde motion of planets within the geocentric model the Greeks had to
invent the EPICYCLES.
• Epicycles are basically are
• theAristotelian view of the universe:
THEARISTOTELIAN VIEW OF THE UNIVERSE:
• measuring the relative distances of the planets to the Sun:
TRANSITION TO MODERNASTRONOMY :
• Copernicus: a return to the heliocentric model:
• In a book called “On the Revolutions of the Hevanly Bodies”, Copernicus proposed that the sun is in the center of the solar system.
• This model is called the “Heliocentric System”
• In this new system,