Models Of The Universe: Greeks Knew Earth Was Round

• This presentation discusses models of the universe and covers Greek astronomy up to Kepler.
• Physical Science for SHS covers Chemistry in the 1st quarter and Physics in the 2nd quarter.

Shape of the Earth

• Earth's true shape is an oblate spheroid, bulging at the equator and squeezed at the poles.

How Greeks Knew the Earth Was Round

• The flat Earth model is an old belief of a plane or discoidal Earth, contrasting with the 6th-century BC idea of a round Earth.
• Egyptians and Mesopotamians believed in a disk-shaped world in the ocean, like Homer's idea in the 8th century BC.
• Pythagoras introduced the round model of Earth's shape in the 6th century BC.
• Anaxagoras, around 430 BCE, made observations supporting Pythagoras's proposal and noting the circular shape of Earth's shadow during lunar eclipses.
• Aristotle argued that on a flat Earth, ships would simply shrink until disappearing; Greeks observed the hull disappearing first, indicating a curved surface.
• Around 240 BCE, Eratosthenes attempted to measure Earth's circumference.
• Eratosthenes learned from Syene that vertical objects cast no shadow at noon on the summer solstice, while in Alexandria, they did.
• He measured the shadow angle in Alexandria as 7.2°, hypothesizing parallel sun rays and a curved Earth.
• On a flat Earth, there would be not a difference in shadow lengths between Syene and Alexandria.
• Since Alexandria had a shadow while Syene did not, this suggested that the Earth's surface is curved.

Modern Evidence

• Satellites provide photographs confirming Earth's spherical shape.
• The presence of different time zones demonstrates the rotation of the Earth on its axis.

Astronomical Observations Before Telescopes

• Before telescopes, observations relied on human senses.

Diurnal Motion

• Diurnal motion is the apparent daily motion of stars and celestial bodies due to Earth's rotation.
• Humans observed the sun rising in the east and setting in the west.

Annual Motion

• Annual motion is the apparent yearly motion of stars and celestial bodies due to Earth's revolution.

Zodiac and the Ecliptic

• A band of thirteen constellations, called the zodiac, can be seen in the ecliptic.
• Ancient civilizations observed these constellations changing through the months.
• Constellations marked the time for planting and developed a chart called a horoscope.
• The Zodiac signs and approximate dates when the Sun appears in them:
  • Aries: March 21 - April 19
  • Taurus: April 20 - May 20
  • Gemini: May 21 - June 20
  • Cancer: June 21 - July 22
  • Leo: July 23 - August 22
  • Virgo: August 23 - September 22
  • Libra: September 23 - October 22
  • Scorpio: October 23 - November 21
  • Sagittarius: November 22 - December 21
  • Capricorn: December 22 - January 19
  • Aquarius: January 20 - February 18
  • Pisces: February 19 - March 20

Equinox and Solstice

• Equinox refers to a day with an equal duration of day and night. There are two equinoxes in a year:
  • Spring/Vernal equinox: March 20 or 21
  • Autumnal/Fall equinox: September 22 or 23
• Solstice refers to a day with longest day or shortest day. The solstices in a year:
  • Winter solstice: December 21 or 22
  • Summer solstice: June 21 or 22
• On March 20-21, the Spring Equinox:
  • The sun is directly over the Equator
  • In the Northern Hemisphere, there is start of Spring
  • In the Southern Hemisphere, there is a start of Autumn
• On June 21-22, the Summer Solstice:
  • The sun is directly over the Tropic of Cancer (23.5°N)
  • In the Northern Hemisphere, there is start of Summer
  • In the Southern Hemisphere, there is a start of Winter
• On September 22-23, the Autumn Equinox:
  • The sun is directly over the Equator
  • In the Northern Hemisphere, there is start of Autumn
  • In the Southern Hemisphere, there is a start of Spring
• On December 21-22, the Winter Solstice:
  • The sun is directly over the Tropic of Capricorn (23.5°S)
  • In the Northern Hemisphere, there is start of Winter
  • In the Southern Hemisphere, there is a start of Summer

Precession

• Precession is the slow 'wobbling' of Earth's axis of rotation due to gravitational pull of the Moon and Sun.
• One cycle of precession takes approximately 26,000 years.
• Approximately 5,000 years ago, the north celestial pole was at the star Thuban.
• Currently, the north celestial pole is located near Polaris, but in 14,000 years it will be near Vega.
• Precession is caused by the gravitational pull of the Sun and Moon on Earth's equatorial bulge.
• The effects of precession include changes to the North Star, shifts in zodiac signs, and alterations in equinox timing.

Eclipse

• Eclipses occur when the Earth or Moon casts a shadow on the other.
• Solar eclipse: The Moon moves between Earth and Sun, blocking Sun's light and casting a shadow on Earth.
• Lunar eclipse: The Earth is directly aligned between Sun and Moon, with Earth casting a shadow on the Moon.

Models of the Universe

• Classical astronomers developed models to explain the positioning of bodies in the universe.

Ptolemaic Model

• The Ptolemaic system is also called the geocentric system or geocentric model.
• Mathematical model of the universe formulated by Claudius Ptolemy about 150 CE.
• It assumes Earth is stationary and at the center of the universe.

Copernican Model

• The Copernican model is also known as the heliocentric model.
• The heliocentric model was developed by the Polish mathematician Nicolaus Copernicus.
• Heliocentric' comes from the Greek words helios (sun) and centric (center).

Tycho Brahe

• Tycho Brahe (1546–1601) was a Danish astronomer.
• Tycho Brahe collected precise observational data on planetary motions, primarily Mars.
• Brahe invented astronomical instruments and extensively studied the solar system.
• He determined the positions of 777 fixed stars with high accuracy.
• Johannes Kepler (1571–1630) was Brahe's assistant and used Brahe's data to develop his three laws of planetary motion.

Johannes Kepler

• Kepler was a German mathematician, astronomer, and physicist.
• He is best known for his three laws of planetary motion describing how planets orbit the Sun.
• Kepler's work was based on Tycho Brahe's astronomical data.
• Kepler laid the foundation for Newton's Law of Universal Gravitation.

Kepler's First Law

• Planets move in elliptical orbits around the Sun, with the Sun at one focus.
• Disproved the old belief that planets move in perfect circles.

Kepler's Second Law

• A planet moves faster when closer to the Sun and slower when farther away.
• Explained why planets do not move at a constant speed.

Kepler's Third Law

• The square of a planet's orbital period (T²) is proportional to the cube of its average distance from the Sun (R³).
• Expressed as T1²/T2² = R1³/R2³, where T is the period and R is the average distance.