Notes: Ancient to Modern Astronomy

Greeks used basic geometry and trigonometry to estimate sizes/distances of Sun and Moon.
Geocentric view: Earth at the center; celestial bodies (Sun, Moon, planets) revolve around Earth; Earth is sphere.

Anaxagoras (c. 499–428 BC): Moon is a sphere; shines by reflected sunlight; explained phases/eclipses; Sun is a hot rock; taught eclipses caused by Earth’s shadow or Sun–Moon alignment; jailed for arguing the Sun/Moon were not gods.
Aristotle (384–322 BC): Earth is a sphere; argued from gravitational pull toward center; observed circular lunar shadow during eclipses; noted sky star positions vary with latitude; used these as evidence for a relatively small Earth.
Aristarchus (c. 310–230 BC): Proposed heliocentric model; Sun-centered system; suggested relative distances/sizes of Sun and Moon; only one work survives: On the Sizes and Distances of the Sun and Moon.
Hipparchus (190–120 BC): Cataloged ~850 stars; created a brightness (magnitude) scale; measured the year length; discovered the Precession of the Equinoxes; used epicycles for Sun/Moon; considered the first real astronomer due to systematic observations.
Claudius Ptolemy (c. 85–165 AD): Developed the geocentric Ptolemaic system; wrote the Almagest (Syntaxis) — the leading predictive model in Greek astronomy for ~1400 years.

Proponent of heliocentrism; Sun far larger than Earth and at the center of planetary orbits.
Key quantitative results (ancient):
- Distance Sun to Moon is >18× but <20× the Moon’s distance;
- Radius Sun >18× but <20× Moon’s radius;
- Radius Sun > (19/3) to (43/6) × Earth’s radius.
Notes: His numbers were approximate; method was sound but data were limited.

Geocentric: Earth at center; Moon, Sun, planets orbit Earth; epicycles used to fit observed motion.
Heliocentric: Sun-centered model; planets (including Earth) orbit the Sun; stars are distant and fixed; apparent retrograde motion explained by relative motion.

Nicolaus Copernicus (1473–1543): Advocated heliocentrism; published in Commentariolus (~1511).
Tycho Brahe (1546–1601): Precise naked-eye observations; questioned both Copernican and Ptolemaic models; built own instruments to improve data; proposed the Tychonic model.
Johannes Kepler (1571–1630): Laws of planetary motion:
- Law 1 (Ellipses): Planets move in ellipses with the Sun at one focus.
- Law 2 (Equal areas): A line from Sun to planet sweeps out equal areas in equal times; dA/dt = constant.
- Law 3 (Harmonics):
  $\frac{P^2}{a^3} = \text{constant},$
  where P is orbital period and a is the semi-major axis.
Galileo Galilei (1564–1642): First telescope utilizations; 1609–1610 discoveries; Venus exhibits phases, supporting Sun-centered system (contextual with competing models).
Isaac Newton (1643–1727): Gravity explains planetary motions; formulated universal gravitation; reinforced heliocentric view.

Astronomy: Study of celestial bodies (stars, planets, galaxies, etc.) and phenomena outside Earth’s atmosphere; involves physics, chemistry, evolution of these objects.
Astrophysics: Branch focusing on the physical nature and processes of celestial objects; applies physics/chemistry to astronomical phenomena.
- Core idea: use physics to explain how celestial bodies work.
Celestial Mechanics: Application of classical mechanics to the motion of celestial bodies under gravitation; predicts orbits and interactions.
Center of mass concept: Objects orbit around their center of mass within a system (e.g., binary stars, planetary systems).

Branch of astronomy studying the origin, evolution, and fate of the Universe.
Key focus: large-scale structure, expansion, cosmic history, and fundamental physics governing the cosmos.

Big Bang as origin event; subsequent epochs include inflation, formation of fundamental particles, nuclei, atoms, stars, galaxies, and large-scale structure.
Observational pillars: cosmic microwave background, expansion, nucleosynthesis, large-scale surveys.

Telescope: Optical instrument that magnifies distant objects and collects light/other electromagnetic radiation.
Primary goal: produce a clear, magnified image of distant objects by gathering and focusing light.

Objective lens or mirror: Collects and concentrates light to form the first real image.
Eyepiece (Ocular lens): Magnifies the image produced by the objective.

Two basic parts:
1) Objective lens or mirror — collects light and forms the first real image.
2) Eyepiece — magnifies the image for viewing.

Invented by Galileo Galilei.
Light enters through objective lens, refracts to a focal point, then magnified by eyepiece to form an image.

Invented by Isaac Newton.
Uses mirrors (concave primary mirror and secondary mirror) to reflect light to the eyepiece.
Provides images by reflecting light internally rather than through lenses alone.

Light enters, reflects off primary mirror to secondary mirror, then to eyepiece for viewing.

Refracting telescopes rely on lenses to bend light.
Reflecting telescopes rely on mirrors to reflect light.
Each design has advantages/limitations in terms of aberrations, light gathering, and construction.