Benefit of astronomy to ancient cultures

• Usefulness as a tool to predict the weather

(seasons)

• Usefulness as a tool to tell time (sundials)

Central Africa (6500 B.C.)

Alignments

• Many ancient

cultures built

structures to mark

the seasons

• The structures

were often aligned

North-South, East-

West

• Purpose -

astronomical and

social (rituals)?

Sunlight pierces the center of carved spiral

only at noon on summer solstice

Stonehenge

Modern Science and the Greeks

• The Greeks are credited for developing the scientific

method

• Instead of using superstition, they were the first to

use logic and geometry to explain the motions of

astronomical objects

• Thales: the first astronomer

• First to ask “What is the universe made of?”

• Model: Universe is mostly water, and the Earth is a flat

disk floating on the infinite ocean

• Even wrong ideas are good - they get people thinking

and coming up with (hopefully!) better models

How do we know the Earth is round?• Ships sailing out to sea

disappear from the bottomup. Were the Earth flat theywould just get smaller

• The edge of the Earth’sshadow on the Moon isalways part of a circulararc. Only a sphere alwayscasts a circular shadow

• The altitude of theconstellations changes asone moves north-south.This cannot happen if theEarth is flat(Anaximander)

• Pythagoras: the Earth is a sphere within a celestial

sphere. The reasoning was that the sphere is

geometrically perfect

• Aristotle: the Earth’s curved shadow during lunar

eclipse proves that the Earth is spherical

• Aristarchus: the Earth orbits the Sun

1) Distance to the Moon

2) The moon is closer to us than the Sun

3) The Size of the Earth

4) The distance of the Sun from the Earth

Brief trigonometry review -

1) Distance to the Moon

• Parallax - the apparent displacement of an

object caused by the motion of the observer

2) How do we know the Moon is

closer to us than the Sun?

• Solar eclipses

• I.e., the moon, being

closer, blocks the

light from the Sun

3) Size of the Earth

• Because the Sun is so far away, the shadow of the

Earth is more-or-less cylindrical. Thus, the size of the

Earth can be estimated from the size of its shadow

• The Earth’s radius is 3 times that of the Moon’s

4) Size of the Sun

• Aristarchus (280 b.c.) used

geometry

• When the moon is half

illuminated, the Earth-Moon-

Sun angle is 90o

• By measuring then the Moon-

Earth-Sun angle, the relative

distances between the Moon

and the Sun, and thus relative

size can be calculated.

Size of the Sun (cont’)

• Aristarchus’ answers: the Sun

is 20 times farther away than

the Moon, and thus 20 times

its size.

• Since the Earth is 3 times the

size of the Moon, the Sun

must be 7 times the Earth’s

size

• Real numbers: The Sun is

400 times the size of the

Moon and 100 times the size

of the Earth

• The Moon-Earth-Sun angle is

extremely difficult to measure!

Size of the Sun (cont’)

• Punchline: Aristarchus was

right in principle, but wrong in

detail.

• He did, however, conclude

that the Sun, the largest (and

brightest) object in the known

universe, must be at the

center, and that the Earth

must orbit the Sun

Eratosthenes Measured Earth’s Size• Syene: Sun passes directly

overhead at summer solstice

• Alexander: Sun comes within

7o of zenith at summer solstice

• Thus, Alexandria is 7o in latitude

to the north of Syene

• Syene-Alexander distance =

5,000 stadia

• Thus,

The equivalent value of his estimate in km is 42,000 km. The

actual circumference of the Earth is 40,000 km.

• Apollonius and Hipparchus laid the

foundation for the Ptolemaic, geocentric

model of the Universe

• Old belief – Geocentric: Earth-centered (Ptolemy, 2nd

century A.D.)

• New belief – Heliocentric: Sun-centered (Copernicus,1473-1543 A.D.)

Ptolemaic System

Modeling of observational data helped to

confirm the Heliocentric Model

• Tycho Brahe (1546-1601): Made accurate

measurements of the positions of stars &

planets

• Johannes Kepler (1571-1630): interpreted

Tycho’s data

Phases of Venus

Old model.

Problem – Venus

would always be in

a crescent phase

Galileo (1564-1642):

Made use of a telescope

To discover:

• Phases of Venus

• 4 brightest moons of

Jupiter