History of Astronomy

Mul Apin tablet

http://www.mesopotamia.co.uk/astronomer/explore/exp_set.html

Time as a parameter

Year: interval between midsummer days

Position of Sunrise and Sunset varies throughout the year

Midsummer day: when the sun rises/sets in most northerly position. Measured at Stonehenge: important do define seasons and hence time to plant crops Probably 2000 BC ± 1000.

>Photograph by Joe McNally/Sygma: http://www.nationalgeographic.com/history/ancient/photo/stonehenge-from-above.

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Alignments let you measure summer soltstice

Midsummer sunrise
Winter moonrise low point
Midwinter Sunrise
Southern moonrise (minimum)
Southern moonrise (maximum)
Midwinter sunset
Northern moonset (minimum)
Northern moonset (maximum)
Winter moonrise high point

http://www.fas.harvard.edu/~scidemos/ AstronomyAstrophysics/Stonehenge/Stonehenge.html

But suppose you lived in Babylon...

Babylon

The questions:

(Easy) Why do you belive the earth is not flat?
(Hard) Why do you believe the earth rotates?
(Very hard) Why do you believe the earth orbits the sun?

Why Babylon?

Enuma Elish tablets : the first (written) Creation Myth From The First Tablet

When in the height heaven was not named,
 And the earth beneath did not yet bear a name,
 And the primeval Apsu, who begat them,
 And chaos, Tiamut, the mother of them both
 Their waters were mingled together,
 And no field was formed, no marsh was to be seen;
 When of the gods none had been called into being,
 And none bore a name, and no destinies were ordained;
 Then were created the gods in the midst of heaven,
 Lahmu and Lahamu were called into being...
 Ages increased,...
 Then Ansar and Kisar were created, and over them....
 Long were the days, then there came forth....
 Anu, their son,...
 Ansar and Anu...
 And the god Anu...
 Nudimmud, whom his fathers, his begetters....
 Abounding in all wisdom,...
 He was exceeding strong...
 He had no rival --

Babylonian Observations

Most detailed ancient observations.

Star Catalogs 1600 BC.
Eclipse Observed 1500 BC.
Continuous Records 900 BC.
Records on stone tablets.

It reads:

On the 1st of Nisannu the Hired Man becomes visible.
On the 20th of Nisannu the Crook becomes visible.
On the 1st of Ayyaru the Stars become visible.
On the 20th of Ayyaru the Jaw of the Bull becomes visible.
On the 10th of Simanu the True Shepherd of Anu and the Great Twins become visible.
On the 5th of Du'uzu the Little Twins and the Crab become visible.
On the 15th of Du'uzu the Arrow, the Snake, and the Lion become visble; 4 minas is a daytime watch, 2 minas is a nighttime watch.
On the 5th of Abu the Bow and the King become visible.
On the 1st of Ululu [. . . .]
On the 10th of Ululu the star of Eridu and the Raven become visible.
On the 15th of Ululu Shu-pa, Enlil, becomes visible.
On the 25th of Ululu the Furrow becomes visible.

http://www.mesopotamia.co.uk/astronomer/explore/exp_set.html

Position of Sunrise and Sunset varies throughout the year.

Position of the noon sun in the sky varies throughout the year:

It moves against the fixed stars because

the earth orbits the sun
the earths axis is tilted

it also moves in the sky at a given time of day: (i.e. the time of noon varies) because the earth moves at varying speeds in its orbit, so we actually need a better clock than the sun to measure this

Eclipses

Tablet with a list of eclipses between 518 and 465, mentioning the death of king Xerxes.

British Museum, London

Why do these matter?

CALPURNIA: When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of princes.

Julius Caesar

(Chinese astronomers Hi and Ho executed for failing to predict eclipse in 300BC).

Saros:

Eclipses repeat after 18 years and 11.3 days.
The .3 days shifts the eclipse about 110º degrees west.
Also some saros sequences start at the south and drift North, others at the North and drift South.
This means that the cycle is very complex: can only see it after many years.
Why is it so complicated? Need to combine
1. Earths rotation
2. Moons orbit (not quite circular)
3. Earth's orbit (ditto)
4. and the plane of the moons orbit precesses

Fascination with eclipses remains:

e.g. An eclipse seen from Mir.

e.g. FEC story

Observed total Eclipse 15 April 136 BC.
and they would even have seen it from the moon !
But they shouldn't have!
Earth's rotation has slowed down, by 1/100 sec/century, because of tidal effects! i.e. earth isn't a very good time-keeper

Babylonian Astronomy?

What was wrong with Babylonian Astronomy?
They had no model (but they were very good at arithmetic!)
e.g. the month Kidinnu arrived at 29.530594 days, which is only 0.432 seconds more than the modern estimate of 29.530589 days.

Dates For Early Astronomy: Chinese

Records from 2000 BC.

Eclipses predicted
Comets (Halley's comet in 467BC).
Measured year to be 365 ¹/₄ days, lunar month to be 28 days.

Dates For Early Astronomy: Hindu

Not very systematic observations, and cosmology mixed in with mythology!

Fortunately now revived:
Great A'tuin the turtle comes, swimming through the interstellar gulf, hydrogen frost on his ponderous limbs, his huge and ancient shell pocked with meteor craters. He thinks only of the weight. Most of the weight is accounted for by Berilia, Tubul, Great T'phon and Jerakeen, the four great elephants upon whose shoulders the world rests.
.......The Colour of Magic, Terry Pratchett

Dates For Early Astronomy: Greek

Pythagoras:

First Cosmos Model: Motions of the Spheres.

Motion of Venus and Mercury.

Hisperus appears as an evening star (very bright) for about 8 months.
Followed by Phosphorus (morning star) by a similar period in the Eastern sky at dawn

become Venus, -497 BC. Note that this is much more difficult to study than it sounds, because the orbit is tilted to the horizon, and the orbit is distorted:

Conclusion: One is seeing a "side on" circle.

Eudoxus: 408-355 BC.
First Mechanical Model, Rotating spheres pivoted on each other.

Aristotle: , 384-322 BC.

Parallax, Phases of the moon

Geocentric Model: Why the earth must be stationary:

Suppose that the earth revolved around the sun. In observing two distant stars, α > β
Since the separation between the stars doesn't change, the earth must be stationary.
What is wrong with this argument?

Aristarchus: 310-230 BC.

Heliocentric Model, (1700 years before Copernicus!)

Eratosthenes: 276-195 BC

: Radius of Earth

Sun is vertically above Syene (Aswan) whereas it is 7° of the vertical at Alexandria, 700 km North. Gives ∼ 5800 km instead of 6400.

Hipparchus: 160-127 BC.

Precession of the Equinoxes

Earth's axis is tilted, but doesn't always point to the same place (i.e. the North Star isn't always!)

March 21st & Sept 21st are special days: Sun is on the equator (now), but where on the equator? In 2000 BC

Aries 2000 BC
Pisces 100 BC (which is why the Christians chose the fish as their symbol)
And now This is the dawning of the age of Aquarius
Except it isn't!

Why the Earth is not flat: the earth's shadow on the moon is always curved

Anthony Ayiomamitis

But maybe the earth is a disc... Even so..!

Earth-Moon Distance:

Radius of the earth ~6500 km (4000 mi).
This gives d ~ 375,000 km
d = 384,400 km by modern methods.

¸ Distance to the sun.

Much Cleverer

If the sun was infinitely far away, then half-moon would occur exactly half way through month.

If the sun is closer, half-moon will occur earlier.

But half moon is difficult to estimate: This yielded an earth-sun distance ~20 times Earth-Moon distance.

Hence ~7,500,000 km. We now know the distance ~150,000,000 km

Antikythera Mechanism

An extraodiinary discovery from 1901: probably from late second century BC

National Archaeological Museum in Athens: wikipedia

X-rays show very complex structure

Appears to be a very sophisticated astronomical computer: includes Hipparchos' discovery of irregularities in Moon's orbit

T. Freeth et al, Nature 444, 587-591 (30 November 2006), wikipedia

The front dial has two concentric scales. The inner scale shows the Greek zodiac with 360 divisions.
The Parapegma is a star almanac showing rising and settings at dawn or evening of particular stars or constellations
Consistent with Egyptian calendar of 365 days, with twelve 30-day months and 5 extra
The effect of the extra quarter day in a year could be corrected by turning the scale one day every four years
The spiral upper back dial displays the luni-solar Metonic sequence of 235 lunar months with a subsidiary dial showing the Callippic cycl
spiral lower back dial displays the 223-lunar-month Saros eclipse cycle with a subsidiary dial showing the Exeligmos cycle.

Ptolemy:

Almagest (13 Volumes on Astronomy), Ptolemaic model, ~140 AD.

The simplest model is Geocentric, that is with the earth at the centre of the circular orbits of the Moon, Venus, Mercury, the Sun, Mars, Jupiter, and Saturn in ascending order. Orbits of Mercury and Venus are locked to that of the sun. As well, all planets are given epicyclic orbits, that is they orbit about a point, which revolves about the deferent - or orbital path about the earth. Finally, the earth is removed slightly to one side of the centre of all of these orbital paths.

BUT:

Mercury & Venus never get far from sun.
Retrograde Motion.
Changing brightness during year.

In the Ptolemaic model, the orbits of Mercury and Venus are locked to that of the sun. As well, all planets are given epicyclic orbits, that is they orbit about a point, which revolves about the deferent - or orbital path about the earth. Finally, the earth is removed slightly to one side of the centre of all of these orbital paths.

Retrograde Motion

Apparent path against the fixed stars, viewed from within the orbit:

so need to introduce epicycles

200 B.C. -1400 AD:

And now....Nothing.... In Europe, nothing....

Chinese observed supernova in 1054 as "guest star" in Taurus. Brighter than Sirius, probably brighter than Venus.
No European Records!
Il sole no si muove (The sun does not move) Leonardo da Vinci

COPERNIKUS (1473-1543)

Ptolemy's model by now required 40 epicycles to explain the increasingly accurate observations. Copernicus proposed a Heliocentric solar system model as a device to explain the motions. His model used circular orbits (thus still needing epicycles to achieve good agreement with observations).

Motion of Mercury & Venus "unlocked" from sun.

Lack of parallax because fixed stars are very far away

So why did people at the time believe in Copernicus?

Reasons for asserting the earth is motionless:

David in Psalm 89: God has founded the earth and it shall not be moved.
Joshua bade the sun stand still-which would not be notable were it already at rest.
The earth is the heaviest element, therefore it more probably needs rest.
Everything loose on the earth seeks its rest on the earth, why should not the whole earth itself be at rest?
We always see half of the heavens and the fixed stars also in a great half circle, which we could not see if the earth moved, and especially if it declined to the north and south...
A stone or an arrow shot straight up falls straight down. But if the earth turned under it, from west to east, it must fall west of its starting point.
In such revolutions houses and towers would fall in heaps.
High and low tide could not exist; the flying of birds and the swimming of fish would be hindered and all would be in a state of dizziness.

Reasons for the belief that the earth is moved:

The sun, the most excellent, the greatest and the midmost star, rightly stands still like a king while all the other stars with the earth swing round it.
That you believe that the heavens revolve is due to ocular deception similar to that of a man on a ship leaving shore.
That Joshua bade the sun stand still Moses wrote for the people in accordance with the popular misconception.
As the planets are each a special created thing in the heavens, so the earth is a similar creation and similarly revolves.
The sun fitly rests at the center as the heart does in the middle of the human body.
Since the earth has is itself its especial centrum, a stone or an arrow falls freely out of the air again to its own centrum as do all earthly things.
The earth can move five miles in a second more readily than the sun can go forty miles in the same time.
Voight (1667). Der Kurstgunstein Einfalt Mathematisher Raritaten Erstes Hundert.

Tycho Brahe 1546-1601

Tycho Brahe: Note the tin nose ...

Ruler of island of Hveen (of coast of Denmark). Constructed Uraniborg (first observatory) to measure position of planets and stars

Now it is quite clear to me that there are no solid sphere's in the heavens, and those devised by the authors to save the appearances exist only in the imaginations for the purpose of permitting the mind to conceive the motion which the heavenly bodies trace in their courses. Brahe

Kepler 1571-1627

Corresponded with Brahe and acquired records after his death. (i.e. refused to give them up to his heirs)

Keplers laws :

Planets move in ellipses, with one focus at the sun

Keplers laws :

Ellipses: a circle is a point moving so that its distance from one point is constant. An ellipse is a point which moves so that the sum of its distance from two points is constant

Relations for ellipse

\color{red}{ r_1 + r_2 = 2a}
a is semi-major axis
b is semi-minor axis
(why?)
Eccentricity e measures deviation from a circle
\color{red}{ b = a\left( {1 - e^2 } \right)^{1/2} }
(e = 0 means a circle)
Equation is $$\color{red}{ r = \frac{{a\left( {1 - e^2 } \right)}}{{1 + e\cos \left( \theta \right)}} }$$

Keplers laws :

Second Law

A vector drawn from the planet to the sun will sweep out equal areas in equal times
A = B = C Means "Planet moves faster closer to sun"

Third Law

The period (P) and the semi-major axis (a) are related by

P² = constant
a³

Note that these are empirical laws: there is no indication of where they come from...

GALILEO

GALILEO (1564-1642)

Developed telescope

Sunspots showed sun "imperfect" (and allowed him to measure rotation rate)

Phases of Venus.Probably most important, since it showed planets shone by reflected light: phases did not agree with Ptolemy

Moons of Jupiter. This is his original notebook: showed conclusively that objects other than earth could act as centre of orbit

and this is a translation.
The moons were simultaneously discovered by a German astronomer Marius (or Mayr). These moons are usually known as the Galilean Moons, although Marius gave them their names Io, Europa, Ganymede and Callisto.
The probe to Jupiter which arrived Dec 7th 1995 was known as "Galileo".

Mountains of Moon. Showed heavenly bodies "Imperfect": can see shadows changing on edge of crates

Rings of Saturn: He also saw these, but he could never figure out what they were! He thought that the planet had "handles".

The greatest discovery Galileo never made!

For a week in 1612, Neptune was in the field of his telescope when he was observing Jupiter, and he even notes that it seemed to be a moving star

A final note: the most famous quote

Epur si muove (And yet it does move)

was almost certainly made by Giardano Bruno before he was burnt at the stake in 1600

Newton

1642-1727 (born the day of Galileo's death)

Newton (in Principia) showed that the motion of the planets can be undestood in terms of the gravitational force at the earth's surface. And now we will go to 1960

History of Astronomy

Time as a parameter

Babylon

Why Babylon?

Babylonian Observations

Eclipses

Babylonian Astronomy?

Dates For Early Astronomy: Chinese

Dates For Early Astronomy: Hindu

Dates For Early Astronomy: Greek

Pythagoras:

Motion of Venus and Mercury.

Eudoxus: 408-355 BC.

Aristotle: , 384-322 BC.

Aristarchus: 310-230 BC.

Eratosthenes: 276-195 BC

Hipparchus: 160-127 BC.

Precession of the Equinoxes

Earth-Moon Distance:

¸ Distance to the sun.

Antikythera Mechanism

Ptolemy:

BUT:

Retrograde Motion

200 B.C. -1400 AD:

COPERNIKUS (1473-1543)

Tycho Brahe 1546-1601

Kepler 1571-1627

Keplers laws :

Keplers laws :

Keplers laws :

Second Law

Third Law

GALILEO

GALILEO (1564-1642)

Newton