Astro Unit 2
Unit 2: History of Astronomy
2.2 Ancient Astronomy
Ancient Babylonian, Assyrian, and Egyptian astronomers (3000yrs ago)
Developed a 365-day calendar year.
Ancient Egyptians:
Observed the rising time of Sirius, which corresponded with the flooding of the Nile River.
Ancient Chinese astronomers:
Created a working calendar and recorded various celestial events:
Comets
Bright meteors
Dark spots on the Sun.
Maintained records of “guest stars” that flared up to become visible to the unaided eye for weeks or months. These records are still valuable in studying ancient star explosions.
The Mayan culture in Mexico and Central America:
Developed a sophisticated calendar based on the planet Venus and made astronomical observations from dedicated sites a thousand years ago.
Polynesians:
Navigated by the stars over vast stretches of open ocean, enabling them to colonize new islands far from their original locations.
In Britain:
Used stones to track the motions of the Sun and Moon.
2.3 Astrology and Astronomy
Ancient people of Central Africa (6500 B.C.):
Predicted seasons based on the orientation of the crescent moon.
Key Structures:
Egyptian obelisk: Utilized shadows to tell the time of day.
Stonehenge: Completed around 1550 B.C.
Templo Mayor: Related to significant astronomical events such as the Summer Solstice and Equinoxes:
Summer Solstice: June 21
Equinoxes: September 21/March 21
Winter Solstice: December 21
2.4 Early Greek and Roman Cosmology
Cosmology:
The study of the basic structure and origin of the universe, derived from Greek roots.
The ancients developed cosmologies that connected observations of the heavens with rich philosophical and religious symbolism.
Knowledge of Earth’s Shape:
Educated individuals in the eastern Mediterranean recognized Earth was round at least 2000 years before Columbus.
Pythagoras (2500 years ago):
Suggested Earth’s shape should be spherical.
Aristotle (384–322 BCE):
Provided convincing arguments for a round Earth:
Observed Moon’s phases.
Noted that during a lunar eclipse, the Earth’s shadow on the Moon is always round.
The Greeks and the Concept of Perfection
Beliefs about Form:
Ancient Greeks viewed the circle as the perfect form, believing the heavens to be composed of perfect spheres.
Geocentric Model:
Earth is positioned at the center of the universe, with heavenly bodies moving in perfect circular motions around it.
Motion of Planets:
Greeks introduced a basic geocentric model to explain planetary motions through epicycles.
Aristarchus of Samos (310-230BC):
Proposed a sun-centered model, though not favored during his time.
Observations and Measurements
Parallax Knowledge:
Ancient Greeks understood parallax but noted a lack of it when observing stars.
The absence of observable parallax indicated either a stationary Earth or that stars were exceedingly distant.
Eratosthenes (200 BCE):
Achieved the measurement of Earth’s size using geometric methods based on solar observations.
Ptolemy's Model
Ptolemy’s Geocentric Model (A.D. 100–170):
The most sophisticated geocentric model, accurate enough to dominate astronomy for 1500 years.
The Arabic translation of his work is called the Almagest (meaning 'the greatest compilation').
Required many epicycles to describe planetary motions and retrograde motions.
A deferent is a larger circle upon which an epicycle rotates.
Understanding Retrograde Motion:
Retrograde motion occurs when a planet appears to move backward against the background of fixed stars as Earth moves between the planet and the Sun.
This motion complicates accurate descriptions using the geocentric model.
The Birth of Modern Astronomy
Copernicus (1473–1543):
Proposed a sun-centered model of the solar system (published in 1543, the year of his death).
Although it introduced a heliocentric perspective, it still employed perfect circles for orbital paths.
Copernican Revolution:
The critical realization that Earth is not at the center of the universe signifies this revolution.
Tycho Brahe and Johannes Kepler
Tycho Brahe (1546–1601):
Compiled the most accurate naked eye measurements of planetary positions up until that time.
Maintained the belief in an Earth-centered system while acknowledging that other planets orbited the Sun.
Developed large, precise instruments to enhance the accuracy of cosmic observations.
Johannes Kepler (1571–1630):
Initially attempted to match Tycho's observations with circular orbits but was led to ellipses due to an 8-arcminute discrepancy.
Noted that ignoring this discrepancy was not permissible, leading to a complete reformation in astronomy.
Formulated three laws of planetary motion based on his observations:
Planetary orbits are ellipses, with the Sun at one focus.
The line connecting the Sun and a planet sweeps out equal areas in equal times.
The square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit.
Galileo’s Observations:
Found imperfections on celestial bodies, contradicting the belief that the heavens were unchanging:
The Moon has mountains and craters.
The Sun has imperfections and rotates on its axis.
Jupiter possesses its own moons.
Venus exhibits phases similar to the Moon.
Implications of Galileo’s Work
Geocentric vs. Heliocentric:
The phases of Venus supported the heliocentric model, as they could not be explained by the geocentric model:
The geocentric model posits that Venus's phases do not change, as it always presents its sunlit side.
The heliocentric model accounts for the variation in observed sizes and angles of Venus, depending on its position relative to Earth and the Sun.
Challenges to Beliefs:
Galileo faced substantial resistance, particularly from the Roman Catholic Church, which deemed his ideas heretical.
He was subjected to the Inquisition and placed under house arrest for the remainder of his life. His works remained on the Church’s forbidden list until 1836, and the Church only publicly acknowledged its errors concerning Galileo in 1992.
Newton’s Laws of Motion
Newton’s First Law (Law of Inertia):
An object at rest will remain at rest, and an object in motion will continue moving at a constant speed in a straight line unless acted upon by an external force.
Newton’s Second Law:
Acceleration ($a$) of an object is directly proportional to the net force ($F$) acting upon it and inversely proportional to its mass ($m$):
Newton’s Third Law (Action-Reaction Law):
For every action, there is an equal and opposite reaction. If one object exerts a force on another, the second object exerts a force of equal magnitude but in the opposite direction.
Gravity:
Gravitational forces exist between any two masses. The Earth’s gravity keeps objects grounded:
This gravitational force, denoted by $F$, is proportional to the product of the masses ($m1$ and $m2$) divided by the square of the distance ($r$) between those masses:
where $G$ is the gravitational constant.
Gravitational Dynamics
The gravitational pull of the Sun maintains the orbital motions of the planets around it.
In a two-body system, both masses orbit their common center of mass, which lies closer to the more massive object.
Summary of Concepts
The transition from geocentric to heliocentric models laid the foundation for modern astronomy.
Kepler’s laws and Newton's interpretations of gravitational forces validated the heliocentric model and enhanced our understanding of planetary motions.
The journey to modern astronomy involved significant cultural, philosophical, and empirical re-evaluations of our place in the universe.