Introduction to Astronomy and Ancient Greeks

• The presentation is about introduction to astronomy, and ancient astronomy
• The main goal is to explain how the Greeks knew the Earth is spherical
• The learning objectives includes the thoughts of philosophers about the shape of the Earth, describing the size of the Earth, and realizing the importance of the shape of the Earth

Celestial Sphere

• The Greeks believed there was a celestial sphere
• They thought it was a transparent, hollow sphere located beyond the planets

Geocentric Theory

• Geocentric theory indicates the early Greeks thought the Earth was the center of the universe

Oblate Spheroid

• Oblate spheroid represents the shape of the Earth
• It has a bulging equator and squeezed poles

Heliocentric Theory

• Heliocentric theory represents the astronomical model were the Earth and planets revolve around the Sun

Timeline Terms

• BC/AD or BCE/CE are historical terms used to denote time periods
• BC means Before Christ and AD means Anno Domini ("In the Year of our Lord")
• BCE means Before the Common Era and CE means the Common Era

Astronomy Definition

• Astronomy comes from the Greek words astron, which means "star", and nomos, which means "law"
• Astronomy is the scientific study of motions of the heavenly bodies and the laws that govern those motions
• It encompasses the science of the universe outside of our planet, dealing with heavenly bodies
• Astronomy has resulted in practical inventions, ideas, calendars, navigational techniques, laws of motion, engineering of products, and an increased understanding of energy and weather
• Astronomy is one of the oldest sciences, and should not be confused with astrology
• The UN declared 2009 as the International Year of Astronomy

Universe Definition

• The universe means the vast creation of God
• The universe includes everything that exists anywhere in space and time

Views About the Origin of the Universe

• Creation states God created everything as stated in the book of Genesis in the Bible
• Continuous Creation Theory argues studies reveal that the universe is expanding as time goes by
• Big bang Theory states the origin is due to accident cosmic explosion
• Nebular theory states hot gasses were developed, shed-off, and broke into fragments

Flat vs Spherical Earth

• Babylonians, Chinese, Egyptians, and Hebrews believed in a Flat Earth
• Ferdinand Magellan and Sebastian Elcano circumnavigated the world showing it's not flat
• Plato and Aristotle described Earth and the heavens as spherical
• Eudoxus of Cnidus constructed a planetary model based on this

Aristotle

• (384 BC-322 BC)
• Around 340 B.C., Aristotle listed several arguments for a spherical Earth
• His arguments included the positions of the North Star, the shape of the Moon and the Sun, and the disappearance of ships when they sail over the horizon

Eratosthenes

• (276 BC-194 BC)
• Ancient scholars tried to provide proof of a spherical Earth and its circumference through calculations
• Eratosthenes calculated the most accurate size of the Earth for his time

Aristarchus

• (310 BC- 230 BC)
• Is the very first Greek to profess the heliocentric(sun-centered) view

Hipparchus

• (190 BC – 120 BC)
• Considered as the greatest of the early Greek astronomers
• He observed and compared the brightness of 850 stars, arranging them into order of brightness/magnitude

Anaxagoras

• (499 BC-428 BC)
• Anaxagoras further supported Pythagoras' proposal
• Anaxagoras showed the Earth cast shadows on the Moon during a lunar eclipse

Geocentric System

• Geocentric system represents an Earth-centered system
• In Geocentric system, Earth is in the center and other planets including the sun orbit around it
• The Geocentric System was popularized by Aristotle in 300s B.C.
• Aristarchus proposed the earth orbits the sun but his idea was not accepted around 280B.C.

Claudius Ptolemy

• (100 AD-170AD)
• He believed the Earth was the center of the universe
• His Ptolemic Model claimed that the planets moved in a complicated system of circles
• His geocentric model also became known as the Ptolemic System
• Claudius Ptolemy expanded Aristotle's geocentric view
• Ptolemy wrote Almagest which includes detailed diagrams and mathematical data to explain the universe

Sun-Centered System

• The Sun-Centered System was proposed by Nicolaus Copernicus, and is also known as the heliocentric system
• All planets orbit the sun but only the moon orbits the earth
• The early heliocentric models assumed that the orbits of all the planets had to be perfect circles, which are called epicycles

Galileo Galilei

• He built one of the first telescope used for astronomical investigations
• He discovered four moons orbiting Jupiter, known as Galilean moons
• He observed Venus, realizing it would have phases
• The four moons are Io, Europa, Ganymede, and Callisto

Johannes Kepler Definition

• Further supported Copernicus' Heliocentric System
• Johannes discovered planets' orbits are not circles (epicycles), but instead symmetrical oval or ellipse
• The sum of the distances from the two foci(focus) to any point on the ellipse is always the same

Eccentricity

• Eccentricity can be visualized as the "flattening" of an ellipse
• With e = c / a, it's used measure the degree of elongation of an ellipse
• In the planetary orbits, the orbits of the planets are ellipses with sun at one focus
• Eccentricity may be determined by dividing the distance between the focal points by the longer axis

Kepler's First Law of Planetary Motion

• Every planet orbits the sun in an ellipse
• Sun as one focus of the ellipse and an empty point in space as the other
• The shape of each planet's elliptical orbit is different, therefore, all the planets have orbits that are nearly circles

Kepler's Second Law of Planetary Motion

• The speed at which any planet moves through space is constantly changing
• As planet moves closer to the sun, it travels faster, when it moves farther away, it slows down

Kepler's Third Law of Planetary Motion

• The farther a planet's average distance from the sun, the longer the period

Astronomical Unit

• Distance between Earth and the Sun
• 93 million miles

Issac Newton

• Proposed the Law of Universal Gravitation
• Gravity is responsible in holding planets on its orbit

Interplanetary Space Definition

• Interplanetary Space is the space between planets
• Vacuum represents space that does not contain any matter

Explore the basics of astronomy, focusing on ancient Greek contributions. Understand how early astronomers determined Earth's spherical shape and size. Learn about the celestial sphere, geocentric/heliocentric theories, and timeline terms (BC/AD, BCE/CE).