Early AstronomersPhysics 113 Goderya

Chapter(s): 4Learning Outcomes:

The Roots of Astronomy

• Already in the stone and bronze ages, human cultures realized the cyclic nature of motions in the sky.

• Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance.

• Those monuments were probably used as calendars or even to predict eclipses.

Stonehenge

• Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices

• Probably used as calendar.

Summer solstice

Heelstone

• Constructed: 3000 – 1800 B.C.

Pythagoras

580BC - 500 BC• Calls the heavens

“cosmos”.• Suggest that Earth

is spherical. The First Scientific

Theory• The daily motion of the

sun and the starry heavens is due to the earth rotating in a circle

Eudoxus

408BC - 355 BC• Homo centric

model of the Universe

• What is wrong with the model?Observation of

Mars

Homocentric hollow Spheres

Aristotle

• 348 BC• Lunar Eclipses• Shape of Earth

– Shadow during Lunar eclipse

• Earth is round

– Different sky for A and B.

– The ocean liner in the sea

A

B

Aristarchus

• 300 BC - 200 BC– From earth shadow in Lunar

eclipse:determines Moon-Earth diameter

– From Trigonometry and phases of the Moon:Sun-Earth-Moon distance

– Proposes Heliocentric Universe1

20

1/3

Moon

SunEarth

Earth Shadow

Moon

Eratosthenes

• 276 BC - 195 BC– Size of the earth.

• when the sun is directly overhead at noon on some summer day in Syene the sunray made a 7-degree angle for an obelisk in Alexandria.

Eratosthenes’s Experiment

(SLIDESHOW MODE ONLY)

Calculations

– Eratosthenes calculation

Distance between Syene and Alexandria = 500 milesOne full circle = 360 degreesObelisk angle difference = 7 degrees7/360 = 500/CC = (500 . 360 ) / 7 = 26,000 miles

Hiparchus

• 200 BC - 100 BC– Catalogs the

positions of stars– Brightness scale

for stars:Magnitude scale

– Motion of Sun:Tropical yearSidereal year

Ptolemy

• 100 AD -200 AD– Wrote Astronomy text

Almagest.

– Geocentric model

• Earth center of the universe and the moon, mercury and Venus are in between the earth and sun.

Later refinements (2nd century B.C.) • Hipparchus: Placing the Earth away from the centers of the

“perfect spheres”

• Ptolemy: Further refinements, including epicycles

Epicycles

The Ptolemaic system was considered the “standard model” of the Universe until the Copernican Revolution.

Introduced to explain retrograde (westward) motion of planets

Epicycles

(SLIDESHOW MODE ONLY)

Copernican Revolution

• Copernicus (1473-1543)• Supported heliocentric model of the

Universe.• Explained Retrograde Motion.

– The apparent retrograde motion of Mars is created by the fact that the earth passes Mars. This occurs every 26 months.

• Determined distances to the planets.

Copernicus’ new (and correct) explanation for retrograde motion of the planets

This made Ptolemy’s epicycles unnecessary.

Retrograde (westward) motion of a planet occurs when the Earth passes the planet.

Tycho Brahe (1546 - 1601)• He proposed a model of the

solar system. – The Sun and the Moon orbit the

earth while the other planets orbit the Sun.

• Invented instruments for planetary motion studies.

• Discovered exploding stars and comets.

• Debated the validity of the heliocentric model.

• Realized that the Universe is changing and is complex.

Johannes Kepler (1571 – 1630)

• Used the precise observational tables of Tycho Brahe (1546 – 1601) to study planetary motion mathematically.

1.Circular motion and

• Planets move around the sun on elliptical paths, with non-uniform velocities.

• Found a consistent description by abandoning both

2.Uniform motion.

Galileo Galilei (1564 - 1642 )• Discovered the law of falling

bodies. – All objects fall to earth at the

same rate in the absences of air resistance. In other words all falling objects experience the same acceleration due to gravity in vacuums.

• First person to use the telescope for Astronomy.– Studied in detail the moon

surface, sunspots, suns rotation, Jupiter’s satellite, Saturn (not including the rings) the Milky Way and the phases of Venus.

Issac Newton (1642 - 1727)

• Discovered the laws of Motion (Details to come later)

• Discovered the law of Gravity (Details to come later)

The Modern Ideas –Einstein and Weird physics

• Weird Physics - Einstein• Theory of Special Relativity• General Theory of Relativity• Space-Time continuum

Source: Wikipedia