Theme 3 – Astronomy of the Ancients
ASTR 101
Prof. Dave Hanes
In this unit
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Stonehenge
The Egyptian Pyramids
Greek Cosmology after Ptolemy
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its assumptions
its problems, and their resolution
a sophisticated astronomical clockwork device
how to test the Ptolemaic model
Stonehenge
Many Stone
Circles
Some large
Others less imposing
Elsewhere in the World
Sweden
Africa
France
Stonehenge is Unique:
the Stones are Shaped
Sarsens
…and Bluestones
brought in from a long distance
Big Stones!
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Drag-Stones.mp4
The History
of Construction
Various phases,
spread over many
centuries
It’s larger than just
the obvious stones
56 ‘Aubrey Holes’
Not Connected to Druids
Some Astronomical Connection
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Alignment with midsummer’s day sunrise
Maybe nothing more!
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(Aubrey Holes = eclipse predictor? Probably not!)
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Egyptian Pyramids
More than 120 in all!
Elsewhere in the World
Chichen Itza (Mayan)
Cambodia
Giza Pyramids in Detail
just outside Cairo
How Were They Built?
Evidence of development
over 2000 years
Not slave labour
http://www.harvard-magazine.com/on-line/070391.html/
Elaborate Structure, with
Some Astronomical Significance
Changing Pole Stars! Precession
Greek Cosmology
Meet Ptolemy
and his working assumptions:
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The Earth is the unique center of all motions
All motion is in circles (= perfect!)
All motion is at uniform, unchanging speed
The Inferred Sequence of Planets
Immediate Problems!
How do we explain the retrograde motion of the outer
planets?
If Mercury and Venus orbit the Earth, why do they never
stray far from the Sun from our point of view?
Eclipses are sometimes total, at other times annular, so
the moon’s distance can’t be constant (i.e. not a simple
circle!)
The moon and sun move at varying speeds across the
starry background
Retrograde Loops
The odd behaviour of Jupiter, Dec 2004 – June 2005
Ptolemy’s Explanation for Retrograde Motion:
Circles within Circles!
Watch:
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Epi-Ride.mp4
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Epicycles.mp4
(but don’t agonize over the details!)
The Modern Correct Explanation
Crucial ingredients:
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All planets orbit the Sun
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The planets farther out move more slowly
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We overtake outer planets ‘on the inside track’
So they appear to drift ‘backwards’for a time
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Retro-Why.mp4
Venus and Mercury
Ptolemy gave each of them an epicycle too, but had to assume
that the centres of their epicycles were always directly between
the Earth and the Sun, for some reason
Predictive Power!
One interesting question: did Ptolemy and those who followed him
really believe that the Solar System had this complicated
structure? Or was it simply a useful working tool for predicting
positions and motions? We may never be sure.
It is certainly contrived! Indeed, after hearing an explanation about
Ptolemy’s theory of epicycles, King Alfonso X said (~1260 AD):
"If the Lord Almighty had consulted me before embarking on
creation I should have recommended something simpler.”
But it worked!
A Real Clockwork
In 1900, pearl divers
found a shipwreck in
the Mediterranean,
near Anitkythera.
A mysterious mechanism
was found on board.
A Reconstruction of the
Antikythera Mechanism
It appears to have been a mechanical machine for
predicting the motions of the Sun, Moon and
planets -- very sophisticated for the times. (It
dates back to the first century BC.)
Watch this realization:
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Antik.mp4
Coincidence?
Also found on that ship: a bust of the “Philosopher of
Antikythera,” on the left.
On the right: myself (Dave Hanes)
The Ultimate Question
How could you ever prove Ptolemy’s model wrong?
There are two obvious answers:
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2.
Demonstrate that the other planets (in particular,
Venus and Mercury) orbit the Sun rather than orbiting
us. (But how? We shall see.)
Detect the parallax of nearby stars, which would prove
that the Earth is itself moving through space.
Both of these needed the development of telescopes.