3. THE CLASSICAL PERIOD (500BC - 1400AD)

• Dominated by the Greeks who were the first to try and explain the workings of the heavens in a careful, systematic manner using models.
• A period dominated by some extremely brilliant individuals whose quest was aided by logic, mathematics, and aesthetics.

3.1 Shape of the Earth

(a)Pythagoras ($\sim 500$BC) - put forward the idea that the earth was round, but not on the basis of observation; rather he, like many ancient philosophers, believed that a sphere was the perfect shape and the gods would have therefore created the Earth in this form.

[Spheres: a common feature of subsequent models and a common shape in nature]

(b)Aristotle (350BC) - Earth was spherical on the basis of the curved shape of the Earth's shadow projected on the Moon during a lunar eclipse. He also noticed that as he travelled south, he saw stars that were previously hidden below the southern horizon.

3.2 The size of the Earth

(c)Eratosthenes (230BC) - head of the library at Alexandria in Egypt, he was the first to measure the Earth's size, obtaining a value of $\sim 25,000$miles for its diameter, close to the presently known value!

How did he do it? He utilised the Sun when at summer solstice, ie. at its highest point in the noon-day sky:

Having measured the angle of sun to the vertical at Alexandria to be $7\deg$, and knowing the distance from Alexandria to Syene (=5000stadia; 1stadia = 0.1mile), circumference of earth $=360/7\times 5000$stadia =25,700miles.

TECHNIQUE ABSOLUTELY CORRECT!!

3.3 Distance and size of the Sun & Moon

(d)Aristarchus - estimated the relative size of the earth, moon & sun and the relative distances to the moon & sun.

Earth/Moon size: compared size of earth's shadow on the moon during a lunar eclipse to the size of the moon's disk $\rightarrow$

Sun/Moon/Earth distance: calculated sun to be $\sim 20$ times farther away than moon by measuring sun/moon angle $\alpha$ at first quarter $\rightarrow$

From knowing ES/EM$\simeq$20 and d $_{moon}\simeq 1/3$d_earth, Aristarchus deduced that d $_{sun}\simeq 7\times$d_earth. Although ES/EM$\simeq$20 and d_sun/d_earth were underestimates, Aristarchus was the first to establish:

• Sun was very far away
• Sun was much larger than the earth.

The vastness of the Sun led Aristarchus to the revolutionary idea that the Sun and not the Earth was the centre of the heavens.

ARISTARCHUS WAY AHEAD OF HIS TIME (BY 2000 YEARS)!!

But, there was a good reason not to believe Aristarchus' model: if earth was moving around the sun then the positions of stars should change throughout the year - an effect called PARALLAX. But no change in separation of stars was observed.

[Later it was realised that the stars' distances had been underestimated and so this parallax effect was a lot smaller than thought and too small to measure (with the naked eye at least)]

3.4 Motion of the planets

The problem of explaining the retrograde motion of the planets dominated cosmology for more than 2000yrs. The Greeks formulated a basic geocentric model in which the earth was at the centre of a whole series of rotating spheres.

The moon, planets, Sun, stars all moved on separate spheres whose radius was inversely proportional to the speed at which they travelled across the sky. Order of the spheres: Moon (innermost), Venus, Sun, Mars, Jupiter....(spheres slightly tilted with respect to each other to reproduce the observed motions).

This could still not explain the retrograde motions! - the contrived situation of spheres at times stopping, reversing direction, and then resuming original motion was considered too clumsy and unappealing even to the Greeks.

PTOLEMY ($\sim$ AD125) - came up with an explanation for planetary motion via epicyclic (`wheels on wheels') behaviour: planets moved in a small circle as well as a larger one.

While more successful than any previous model, discrepancies remained between predicted & observed positions of planets. Epicyclic models with greater complexity were devised but mistrusted due to being far too contrived and overly complex.

3.5 Contributions by other civilizations in this period

• Islamic - celestial phenomena used to set religious calendar. Arabic scholars made fundamental contributions to mathematics - developed algebra, numeral system and concept of zero.
• Asian - best known for their accurate recordings of transient events such as comets, novae and supernovae.

3.6 Closing note on the Classical Period

After $\sim 300$AD, Greek civilization declined and astronomy/cosmology languished. It was not until the 12th century when Arabic manuscripts of Greek thought were translated into Latin, that the knowledge and interpretation accumulated by the Greeks, became widely known.