Theme 3 – Astronomy of the Ancients ASTR 101 Prof. Dave Hanes.

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Presentation transcript:

Theme 3 – Astronomy of the Ancients ASTR 101 Prof. Dave Hanes

In this unit Stonehenge Stonehenge The Egyptian Pyramids The Egyptian Pyramids Greek Cosmology after Ptolemy Greek Cosmology after Ptolemy its assumptions its assumptions its problems, and their resolution its problems, and their resolution a sophisticated astronomical clockwork device a sophisticated astronomical clockwork device how to test the Ptolemaic model how to test the Ptolemaic model

Stonehenge

Many Stone Circles Some large Others less imposing Others less imposing

Elsewhere in the World SwedenFrance Africa

Stonehenge is Unique: the Stones are Shaped

Sarsens …and Bluestones brought in from a long distance Sarsens …and Bluestones brought in from a long distance

Big Stones!

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 Alignment with midsummer’s day sunrise Alignment with midsummer’s day sunrise Maybe nothing more! Maybe nothing more! (Aubrey Holes = eclipse predictor? Probably not!) (Aubrey Holes = eclipse predictor? Probably not!)

Egyptian Pyramids More than 120 in all!

Elsewhere in the World Chichen Itza (Mayan) Chichen Itza (Mayan)Cambodia

Giza Pyramids in Detail just outside Cairo

How Were They Built? Evidence of development over 2000 years Not slave labour

Elaborate Structure, with Some Astronomical Significance

Changing Pole Stars! Precession

Greek Cosmology Meet Ptolemy and his working assumptions: The Earth is the unique center of all motions The Earth is the unique center of all motions All motion is in circles (= perfect!) All motion is in circles (= perfect!) All motion is at uniform, unchanging speed 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: (but don’t agonize over the details!) (but don’t agonize over the details!)

The Modern Correct Explanation Crucial ingredients: All planets orbit the Sun All planets orbit the Sun The planets farther out move more slowly The planets farther out move more slowly We overtake outer planets ‘ on the inside track ’ We overtake outer planets ‘ on the inside track ’ So they appear to drift ‘ backwards ’for a time Watch

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:

Coincidence? Also found on that ship: a bust of the “Philosopher of Antikythera,” on the left. On the right: myself (Dave Hanes) On the right: myself (Dave Hanes)

The Ultimate Question How could you ever prove Ptolemy’s model wrong? There are two obvious answers: 1. Demonstrate that the other planets (in particular, Venus and Mercury) orbit the Sun rather than orbiting us. (But how? We shall see.) 2. 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.