Introduction to Astronomy

Astronomy

• Astronomy involves studying celestial objects, space, and the universe
• Astronomers are scientists who study astronomy

Historical Development of Astronomy

• The geocentric model proposes Earth sits at the center of the solar system, with planets orbiting it
• The heliocentric model posits the Sun at the center, with Earth and other planets orbiting it

Early Ideas on Earth's Shape

• Mesopotamians believed Earth was a flat disk floating in an ocean, bounded by a spherical sky
• Aristotle claimed Earth was spherical and perfectly solid, with the heavens being a region of perfection
• Eratosthenes is known for calculating Earth's circumference using eclipse data

Evidence for a Spherical Earth

• Earth's components naturally fall toward the center, forming a round shape due to gravity
• The Earth's shadow during a lunar eclipse is always circular
• The Pole Star appears higher in the sky when traveling north

Astronomical Phenomena Before Telescopes

Phases of the Moon

• Ancient people observed the moon's appearance and path changing within 29.5 days
• The moon's appearance transitions from a thin semicircular disk to a full circular disk
• The periodic changes formed the basis for ancient calendars

Lunar Eclipse

• A lunar eclipse occurs when Earth is positioned between the Sun and the Moon, casting a shadow on the Moon

Solar Eclipse

• A solar eclipse happens when the Moon passes between the Sun and Earth, partially or fully blocking the Sun
• Solar eclipses caused temporary darkness, leading ancient people to associate them with wrath
• The sun is 400x wider than the moon, but is also 400x farther away - thus looking the same size

Four Types of Solar Eclipses

• Total Eclipse: The moon completely covers the sun, visible only in the umbral shadow and creates a path of totality
• Partial Eclipse: The moon passes the sun off-center and is visible from the penumbral shadow
• Annular Solar Eclipse: The moon passes directly across the sun, but appears too small to fully cover it
• Hybrid Eclipse: This may result in an annular or total eclipse depending on the Earth's curvature

Bailey's Beads and Diamond Ring

• Bailey's Beads: Remaining spots of sunlight pass through as the moon covers the sun
• Diamond Ring: A single bead of light appears just before and after totality
• Solar eclipses last for less than 3 minutes

Motion of the Stars

• Constellations' positions in the night sky vary depending on the time of year
• Stars seem attached to a celestial sphere rotating around an axis in one day

Visibility of Planets

• Greeks noticed some brighter stars changing positions periodically which they named "wanderers" or planets
• These planets include Mercury, Venus, Mars, Jupiter, and Saturn which are in fact planets

Tychonic Model

• Tycho Brahe created this model
• This is a geo-heliocentric model of the universe, and is a hybrid between the geocentric model of Ptolemy and the heliocentric model of Copernicus
• The Sun orbits Earth, while other planets orbit the Sun

Johannes Kepler

• He was a mathematician hired by Brahe to prove the geo heliocentric model
• He failed to reconcile existing data, especially the stationary earth notion of the Earth
• Another mind proved that Copernicus was correct using the telescope

Three Laws of Planetary Motion

First Law: Law of Ellipses

• Planets orbit the Sun in an ellipse, with the Sun at one focus of the ellipse
• Planetary orbits are not perfect circles, but are slightly stretched ovals (ellipses)

Second Law: Law of Equal Areas

• An imaginary line connecting a planet and the Sun sweeps out equal areas during equal time intervals
• A planet moves faster when closer to the Sun (perihelion) and slower when farther (aphelion)

Third Law: Law of Harmonies

• The square of a planet's orbital period (T²) is proportional to the cube of the semi-major axis (a³) of its orbit

The Concept of Motion

Aristotle vs. Galileo

• Aristotle believed heavier objects fall faster and require continuous force to maintain horizontal motion
• Galileo argued motion depends on weight and objects in motion requires no force to keep moving

Physical Quantities

• Distance: Length of the part traveled from starting to final point (Symbol: Δd, Equation: Δd = d1 - d2)
• Displacement: Length of the part travelled from the starting point to the final point with consideration to the direction
• Speed: The rate at which distance is covered
• Velocity: Displacement in a given time interval
• Acceleration: Measure of how fast velocity changes with respect to time

Projectile Motion

• Aristotle believed projectile motion is parallel to the ground until it falls
• Galileo determined projectiles move in two dimensions (parabolic path)
• Free fall means a body with only gravity acting on it and includes the force of gravity

Uniform Acceleration

• Neglecting air resistance, bodies have equal acceleration due to gravity (approximately 9.8 m/s²)

Motion and Forces

• Galileo: Constant acceleration means velocity increases proportionately, using inclined planes to study motion
• Aristotle: Object would stop because its natural rest state
• Galileo: The motion requires no force, except to lessen friction
• Friction opposes motion, requiring no force in absence of motion
• Inertia resists change

Sir Isaac Newton

• Newton formulated laws of motion and universal gravitation that formed a dominant viewpoint
• First Law of Motion (Law of Inertia): Objects remain at rest or in motion unless acted upon

The Nature and Energy of Light

Wave Theory

• Christian Huygens proposed that light propagates as a longitudinal wave
• Proposed space was filled with Ether
• Each color of light corresponds to a different wavelength
• Wavelength (λ) is measured between two points in phase (crests or troughs)
• Light is reflected has no speed change
• Light that is refracted will have a change of speed

Corpuscular Theory

• Isaac Newton proposed that light consists of tiny particles or corpuscles
• The different refrangibility of the particles is accountable for different light colors
• Reflection bounces light
• Denser mediums create more power on lighter particles which increases light speed
• Discarded in favor of wave theory because emissions cause reduction in lightness
• Particles making up light have different sizes and masses

Quantum Theory

• Light is emitted in discrete packets of energy called quanta which Albert Einstein referred to as a photon
• If a certain light energy hits a metal surface, the outer electrons are loosened which is called the photoelectric effect
• The EM spectrum is the range of radiation that travels and spreads out
• Photons have energy equal to oscillation frequency times Planck’s constant (E=hf)