Astronomy 115, Spring 2013

Prehistoric Astronomy
 Introduction
 People of antiquity most likely began studying the heavens many thousands of years ago.
 Early astronomical observations certainly revealed the obvious:
 Rising of the Sun in the eastern sky and its setting in the west
 Changing appearance of the Moon
 Eclipses
 Planets as a distinct class of objects different from the stars

Prehistoric Astronomy
 Introduction
 Many astronomical phenomena are cyclic on a day-to-day and year-to-year basis and consequently gave prehistoric people:
 Methods for time keeping
 Ability to predict and plan future events
 Incentive to build monumental structures such as Stonehenge

built roughly 4500 years ago, used as a burial site, gathering area and timekeeping

Prehistoric Astronomy
 The Celestial Sphere
 Vast distances to stars prevents us from sensing their true 3-D arrangement
 Naked eye observations treat all stars at the same distance, on a giant celestial sphere with the Earth at its center

Prehistoric Astronomy
 Constellations
 Constellations are fixed arrangements of stars that resemble animals, objects, and mythological figures
 Stars in a constellation are not physically related
 Positions of stars change very slowly; constellations will look the same for thousands of years
 Origin of the ancient constellations is unknown although they probably served as mnemonic devices for tracking the seasons and navigation

Prehistoric Astronomy
• Motion of the Sun and the Stars
– Daily or Diurnal Motion
• Sun, Moon, planets, and stars rise in the east and set in the west
• Daily motion can be explained by the rotation of the celestial sphere about the north and south celestial poles located directly above the
Earth’s north and south poles
• The celestial poles can act as navigation aides and astronomical reference points
• The celestial equator , which lies directly above the Earth’s equator, provides another astronomical reference marker

built between 1000 to 1500 years ago; used for rituals, timekeeping

built between 800 and 1250 years ago; trade, timekeeping

Introduction
 Western astronomy divides into 4 periods
 Prehistoric (before 500 B.C.E.
)
 Cyclical motions of Sun, Moon and stars observed
 Keeping time and determining directions develops
 Classical (500 B.C.E
. to C.E.
1400)
 Measurements of the heavens
 Geometry and models to explain motions
 Renaissance (1400 to 1650)
 Accumulation of data lead to better models
 Technology (the telescope) enters picture
 Modern (1650 to present)
 Physical laws and mathematical techniques
 Technological advances accelerate

Early Ideas of the Heavens
 Ancient Greek Astronomers
 Through the use of models and observations, they were the first to use a careful and systematic manner to explain the workings of the heavens
 Limited to naked-eye observations, their idea of using logic and mathematics as tools for investigating nature is still with us today
 Their investigative methodology is in many ways as important as the discoveries themselves

Early Ideas of the Heavens
• The Shape of the Earth
– Pythagoras taught as early as 500
B.C.
that the Earth was round, based on the belief that the sphere is the perfect shape used by the gods
– By 300
B.C.
, Aristotle presented naked-eye observations for the Earth’s spherical shape:
• Shape of Earth’s shadow on the Moon during an eclipse
• A traveler moving south will see stars previously hidden by the southern horizon

Early Ideas of the Heavens
• The Size of the Earth
– Eratosthenes (276-195
B.C.
) made the first measurement of the Earth’s size
– He obtained a value of 25,000 miles for the circumference, a value very close to today’s value
– His method entailed measuring the shadow length of a stick set vertically in the ground in the town of
Alexandria on the summer solstice at noon, converting the shadow length to an angle of solar light incidence, and using the distance to Syene, a town where no shadow is cast at noon on the summer solstice

Early Ideas of the Heavens
• Distance and Size of the Sun and Moon
– The sizes and distances of the Sun and Moon relative to Earth were determined by Aristarchus about 75 years before Eratosthenes measured the
Earth’s size
– These relative sizes were based on the angular size of objects and a simple geometry formula relating the object’s diameter, its angular size, and its distance
– Aristarchus realizing the Sun was very large proposed the Sun as center of the Solar System, but the lack of parallax argued against such a model
– Once the actual size of the Earth was determined, the absolute sizes and distances of the Sun and
Moon could be determined

• About 310 to 230 BCE
• We have no writings that have survived but
Aristarchus is referred to in the writings of other philosophers
• Most noted for proposing the idea of a heliocentric universe with the earth as one of the planets moving around the Sun.

• Aristarchus is the first to attempt to measure the relative distance between the Earth-Moon and the
Earth-Sun without the aid of trigonometry.
• Actual angle = 89 ° 50” not 87 °

• 190 to 120 BCE
• Based on measurements during an eclipse, he is able to place a range on the distance to the moon.
• He estimated it to be between 59 and 67 Earth diameters. The actual number is 60.
• Writes the first catalog of stars, in which he describes the brightest stars as “stars of the first magnitude”, the next-brightest stars as “second magnitude, and so on, a scale we still use in modified form today.

Early Ideas of the Heavens
 The Motion of the Planets
 Because of the general east to west motion of objects in the sky, geocentric theories were developed to explain the motions
 Eudoxus (400-347 B.C.
) proposed a geocentric model in which each celestial object was mounted on its own revolving transparent sphere with its own separate tilt
 The faster an object moved in the sky, the smaller was its corresponding sphere
 This simple geocentric model could not explain retrograde motion without appealing to clumsy and unappealing contrivances

– 384 to 322 BCE
– Founded his own school called the Lyceum in Athens
– Made contributions to all areas of philosophy but math was his weakness
– He did not believe that empirical evidence was necessary to prove ideas.
Refines the “Geocentric Model” which places the
“imperfect” Earth at the center and all of the
“perfect” celestial objects go around us in perfect circular motion.

Shell of “fixed” stars

• Ptolemy is the most influential astronomer in his day and his models of the universe will prevail for the next 1400 years
• He is a great admirer of
Hipparchus and his rigorous method of verifying ideas with empirical data.

Early Ideas of the Heavens
• Ptolemy (about
C.E.
150)
– Ptolemy of Alexandria improved the geocentric model by assuming each planet moved on a small circle, which in turn had its center move on a much larger circle centered on the Earth
– The small circles were called epicycles and were incorporated so as to explain retrograde motion
– Ptolemy’s model was able to predict planetary motion with fair precision
– Discrepancies remained and this led to the development of very complex Ptolemaic models up until about the 1500s

• Of the early philosophers, only Hipparchus favored the heliocentric model
• Most philosophers thought that the evidence supported the geocentric model more than the heliocentric model
• What was the evidence?

• Everything appears to revolve around the
Earth each day (diurnal motion).
• There is no observable parallax of the stars, planets, Moon, or Sun.
• The motions of the stars and heavens are perfect circles.
• The heavens were unchanging but the
Earth was not.

• The planets appear to change brightness, implying a change of distance.
• The planets undergo retrograde motion (they move backwards compared to the direction the
Sun moves).
• The Sun, Moon, and planets do not move at the same speed all the time.
• Mercury and Venus are never seen at opposition
(they always appear close to the Sun, never in the opposite part of the sky from the Sun).

• The changing brightness of the planets is explained by the Earth getting closer and farther from the planets during our orbit of the Sun.
• Our passing planets explain the retrograde motion of the outer (“superior”) planets.
• Mercury and Venus are not seen at opposition because they orbit the sun, just as we do, but closer to the Sun than us (“inferior” planets).

• The changing speeds of the Sun, Moon, and planets is not explained by simply placing the
Sun at the center of the universe.
• We do not feel the Earth moving or the atmosphere being pulled away as we fly around the Sun.
• If the Earth were spinning, wouldn’t we be thrown off into space?
• Why is there no parallax due to our spin or our orbit?

• The ancients favored the geocentric model because it seemed impossible to believe that the Earth was moving.
• During those times, the mathematical and scientific tools were not available to answer the question without dispute.