The motions of Earth affect our daily lives.
Earth is always moving, which is integral to understanding celestial phenomena.
The Ancient Greeks conceptualized the Earth as being within a hollow sphere called the celestial sphere, encompassing all celestial bodies (stars, sun, etc.).
The celestial sphere is an imaginary sphere centered on Earth, representing the positions of celestial bodies on its inner surface as observed in the sky.
Definition of Ecliptic: The path that the sun appears to take around the celestial sphere.
Despite Earth's orbiting the Sun, it seems that the Sun moves around the celestial sphere, completing this motion in one year.
Imaginary line from Earth's center to the Sun marks where it meets the celestial sphere: the North Celestial Pole (NCP) and South Celestial Pole (SCP).
Definition: Points on the ecliptic at maximum distance from the celestial equator.
Summer Solstice: Sun’s highest position in the sky, occurs around June 21; longest day, shortest night.
Winter Solstice: Sun’s lowest position in the sky, occurs around December 21; shortest day, longest night.
Points where the ecliptic intersects the celestial equator, the Sun's declination is zero.
Autumnal Equinox: Occurs around September 22.
Vernal (Spring) Equinox: Occurs around March 21.
Refers to 12 constellations through which the sun passes, forming a sequence along the ecliptic known as the zodiac.
Definition: Change in the orientation of the Earth’s rotational axis.
Historically known as precession of the equinoxes, affecting equinox positions concerning background stars.
Discoverer: Hipparchus of Nicaea credited with its discovery.
The apparent daily motion of stars and other celestial bodies across the sky due to Earth's rotation.
Responsible for the daily rise and set of the sun and stars.
Accounts for the visibility of zodiac constellations throughout the year and influences seasonal changes.
Early humans used the skies for timekeeping, navigation, and agricultural cycles.
Egyptians created a 365-day calendar, following the star Sirius's cycle correlating with the Nile's flooding.
Sirius (Alpha Canis Majoris): Brightest star, associated with the term "dog days" (July 3 - Aug 11).
Babylonian and Assyrian calendars similarly aided in agricultural planning.
Pyramids of Giza aligned with cardinal directions and represented Orion's belt stars.
One of the most recognizable constellations, linked to mythology and lies on the celestial equator.
Orion's Belt: Composed of three bright stars: Alnitak, Alnilam, and Mintaka.
Ancient models based on philosophical and religious symbolism.
Proposed Earth as a disk floating on water.
Suggested Earth is a cylinder with a curved surface.
The study of the universe's history, formation of stars, and galaxies based on early models.
Astronomer: Conducts research about outer space, requires knowledge in math, physics, and a doctorate in astronomy.
Astronaut: Professional space traveler, requires engineering or physics degree, bachelor’s level education.
Two primary models: Geocentric (Earth-centered) and Heliocentric (Sun-centered).
Pythagorean Model
Eudoxus’ Model
Aristotle’s Model
Ptolemy’s Model
First to argue Earth is round with celestial bodies moving in circles.
Associated planetary motions with mathematical relations (Music of the Spheres).
Developed concentric sphere model with celestial bodies in circular motions; did not include Earth in motion.
Extended Eudoxus' model with additional buffer spheres, assigning the outermost to a Prime Mover.
Natural motion of terrestrial objects related to their element’s tendency to return to a natural state.
Natural Motion: Objects seeking natural placement (heavy elements down, lighter up).
Violent Motion: Caused by external force, differing from natural motion.
Alteration: Change in an object's state or quality.
Introduced epicycles to explain the retrograde motion of planets, with a deferent circle.
Refined epicycles; composed eccentric deferent orbits, lasting over 14 centuries.
Introduced central fire concept, placing sun at universe's center.
Challenged by Ptolemaic model's acceptance despite initial propositions.
Proposed Earth spins daily and revolves around the sun, suggesting uniform circular motion.
Faced criticism due to inconsistencies in observational mechanics and failure to explain stellar parallax.