RM

TOPIC 1- Motions in the Sky

Motions in the Sky

Effects of Earth's Motion

  • The motions of Earth affect our daily lives.

Earth in Motion

  • Earth is always moving, which is integral to understanding celestial phenomena.

The Celestial Sphere

  • 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.

Ecliptic

  • 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.

Ecliptic Poles

  • 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).

Solstices

  • 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.

Equinoxes

  • 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.

Zodiac Signs

  • Refers to 12 constellations through which the sun passes, forming a sequence along the ecliptic known as the zodiac.

Precession of the Equinox

  • 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.

Diurnal Motion

  • 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.

Annual Motion

  • Accounts for the visibility of zodiac constellations throughout the year and influences seasonal changes.

Historical Calendars and Astronomy

  • 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.

Architecture Influenced by Astronomy

  • Pyramids of Giza aligned with cardinal directions and represented Orion's belt stars.

Constellation Orion

  • 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.

Early Models of the Universe

  • Ancient models based on philosophical and religious symbolism.

Thales of Miletus’ Model (circa 600 BCE)

  • Proposed Earth as a disk floating on water.

Anaximander's Model (circa 520 BCE)

  • Suggested Earth is a cylinder with a curved surface.

Cosmology

  • The study of the universe's history, formation of stars, and galaxies based on early models.

Astronomy Careers

  • 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.

Models of the Universe

  • Two primary models: Geocentric (Earth-centered) and Heliocentric (Sun-centered).

Geocentric Models Include:

  • Pythagorean Model

  • Eudoxus’ Model

  • Aristotle’s Model

  • Ptolemy’s Model

Pythagoras' Contribution

  • First to argue Earth is round with celestial bodies moving in circles.

  • Associated planetary motions with mathematical relations (Music of the Spheres).

Eudoxus of Cnidus

  • Developed concentric sphere model with celestial bodies in circular motions; did not include Earth in motion.

Aristotle’s Model

  • 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.

Motion Categories

  • 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.

Apollonius’ Concept of Epicycles

  • Introduced epicycles to explain the retrograde motion of planets, with a deferent circle.

Ptolemy’s Complex Models

  • Refined epicycles; composed eccentric deferent orbits, lasting over 14 centuries.

Heliocentric Model by Aristarchus

  • Introduced central fire concept, placing sun at universe's center.

  • Challenged by Ptolemaic model's acceptance despite initial propositions.

Copernicus’ Heliocentric Theory

  • 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.