🪐Intro to Astronomy Unit 4 – Earth, Moon, and Sky

Key Concepts and Terminology

• Celestial sphere imaginary sphere surrounding Earth on which celestial objects appear to be fixed
• Zenith point directly overhead an observer on Earth's surface
• Nadir point directly opposite the zenith, below an observer's feet
• Celestial equator projection of Earth's equator onto the celestial sphere
• Celestial poles points where Earth's axis of rotation intersects the celestial sphere (North Celestial Pole and South Celestial Pole)
• Ecliptic apparent path of the Sun on the celestial sphere over the course of a year
  • Caused by Earth's orbit around the Sun
  • Forms a great circle tilted about 23.5° relative to the celestial equator
• Equinoxes points where the ecliptic intersects the celestial equator (vernal equinox and autumnal equinox)
• Solstices points on the ecliptic farthest from the celestial equator (summer solstice and winter solstice)

Earth's Structure and Characteristics

• Earth is the third planet from the Sun in our solar system
• Consists of four main layers: crust, mantle, outer core, and inner core
  • Crust thin, outermost layer composed of solid rock
  • Mantle layer beneath the crust, primarily solid but can flow slowly over time
  • Outer core liquid layer composed mainly of iron and nickel
  • Inner core solid layer at the center, extremely hot and dense
• Earth's atmosphere is composed primarily of nitrogen (78%) and oxygen (21%)
  • Atmosphere protects life on Earth by absorbing harmful solar radiation and regulating temperature
• Earth's magnetic field is generated by the motion of the liquid outer core
  • Magnetic field deflects charged particles from the solar wind, protecting Earth's atmosphere
• Earth's surface is covered by a variety of landforms (mountains, valleys, plains) and water bodies (oceans, lakes, rivers)
• Earth's climate is influenced by factors such as its distance from the Sun, atmospheric composition, and ocean currents

The Moon: Our Celestial Neighbor

• The Moon is Earth's only natural satellite
• Average distance from Earth is about 384,400 km
• Diameter is about 3,474 km, roughly 1/4 the size of Earth
• Moon's surface is heavily cratered due to impacts from asteroids and comets throughout its history
• Moon has a very thin atmosphere (exosphere) composed mainly of helium, neon, and hydrogen
• Moon's surface features include maria (dark, flat plains) and terrae (lighter, mountainous regions)
  • Maria formed by ancient volcanic eruptions that filled large impact basins with basaltic lava
• Moon has no global magnetic field or tectonic activity
• Moon's gravity influences Earth's tides, causing regular rise and fall of sea levels
• Phases of the Moon result from the changing relative positions of the Moon, Earth, and Sun
  • New Moon occurs when the Moon is between Earth and the Sun (unilluminated side facing Earth)
  • Full Moon occurs when Earth is between the Moon and the Sun (fully illuminated side facing Earth)

Earth-Moon Relationship

• Earth and the Moon are tidally locked, meaning the Moon always shows the same face to Earth
  • Moon's rotation period is equal to its orbital period around Earth (~27.3 days)
• Earth's gravitational force is the primary influence on the Moon's orbit
• Moon's gravitational pull on Earth causes ocean tides
  • Tidal bulges occur on opposite sides of Earth due to the Moon's gravitational attraction and Earth's rotation
• Synchronous rotation of the Moon is a result of tidal forces between Earth and the Moon over billions of years
• Lunar eclipses occur when the Moon passes through Earth's shadow
  • Can only happen during a full Moon when the Sun, Earth, and Moon are aligned
• Solar eclipses occur when the Moon passes between the Sun and Earth, casting its shadow on Earth's surface
  • Can only happen during a new Moon when the Sun, Moon, and Earth are aligned
• Study of the Earth-Moon system has provided insights into the formation and evolution of planetary systems

Celestial Mechanics and Orbits

• Celestial mechanics is the study of the motions of celestial bodies under the influence of gravitational forces
• Kepler's laws of planetary motion describe the orbits of planets around the Sun
  • First law: Planets orbit the Sun in ellipses with the Sun at one focus
  • Second law: A line segment joining a planet and the Sun sweeps out equal areas in equal intervals of time
  • Third law: The square of a planet's orbital period is proportional to the cube of its semi-major axis
• Newton's law of universal gravitation states that every particle attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them
  • $F = G \frac{m_1 m_2}{r^2}$, where $F$ is the gravitational force, $G$ is the gravitational constant, $m_1$ and $m_2$ are the masses of the objects, and $r$ is the distance between them
• Orbits can be classified as circular, elliptical, parabolic, or hyperbolic depending on their eccentricity
  • Eccentricity is a measure of how much an orbit deviates from a perfect circle
• Escape velocity is the minimum speed an object needs to escape a celestial body's gravitational field
  • For Earth, escape velocity is about 11.2 km/s

Sky Observations and Phenomena

• Diurnal motion apparent daily motion of celestial objects across the sky due to Earth's rotation
  • Celestial objects appear to rise in the east, move across the sky, and set in the west
• Seasonal changes in the night sky result from Earth's orbit around the Sun and the tilt of Earth's axis
  • Different constellations are visible at different times of the year
• Meteor showers occur when Earth passes through the debris trail left by a comet
  • Meteors are commonly known as "shooting stars"
• Comets are icy bodies that develop a coma and tail when they approach the Sun
  • Coma is a fuzzy atmosphere that forms around the comet's nucleus as it is heated by the Sun
  • Tail is an extension of the coma that points away from the Sun due to solar wind
• Aurorae (northern and southern lights) are colorful displays caused by charged particles from the Sun interacting with Earth's magnetic field
  • Occur in the upper atmosphere near Earth's magnetic poles
• Zodiacal light faint, triangular glow visible along the ecliptic just after sunset or before sunrise
  • Caused by sunlight scattering off dust particles in the solar system
• Milky Way the band of light that stretches across the night sky, composed of countless stars in our galaxy

Tools and Techniques for Observation

• Naked-eye observations can reveal basic sky phenomena (constellations, planets, meteors)
• Binoculars are a simple and portable tool for observing celestial objects in more detail
  • Provide a wider field of view compared to telescopes
• Telescopes are instruments that collect and focus light, allowing observers to see fainter and more distant objects
  • Refracting telescopes use lenses to focus light
  • Reflecting telescopes use mirrors to focus light
• Astronomical cameras are used to capture images of celestial objects
  • Can be attached to telescopes for astrophotography
  • CCD (charge-coupled device) cameras are commonly used in professional astronomy
• Spectrographs are instruments that split light into its component colors (spectrum)
  • Used to study the composition, temperature, and motion of celestial objects
• Adaptive optics systems correct for distortions caused by Earth's atmosphere in real-time
  • Improve the clarity and resolution of ground-based telescope observations
• Space-based telescopes (Hubble, James Webb) observe the universe without the interference of Earth's atmosphere
  • Provide clearer images and can detect wavelengths of light blocked by Earth's atmosphere

Impact on Human Culture and Science

• Ancient civilizations used celestial objects for navigation (stars, Sun, Moon)
  • Polaris (North Star) used as a reference for latitude and direction
• Celestial events (eclipses, comets) were often seen as omens or signs from deities
  • Many cultures developed myths and legends around celestial objects and phenomena
• Development of calendars based on the motion of the Sun, Moon, and stars
  • Stonehenge (England) and Chichen Itza (Mexico) are examples of ancient astronomical alignments
• Greek philosophers (Aristotle, Ptolemy) developed early models of the universe
  • Geocentric model placed Earth at the center of the universe
• Copernican revolution (16th century) introduced the heliocentric model, with the Sun at the center of the solar system
  • Challenged prevailing religious and philosophical beliefs
• Galileo's observations with the telescope (17th century) provided evidence supporting the heliocentric model
  • Discovered Jupiter's moons, phases of Venus, and sunspots
• Newton's laws of motion and universal gravitation (17th century) provided a mathematical foundation for understanding celestial mechanics
• Modern astronomy has expanded our understanding of the universe (galaxies, dark matter, exoplanets)
  • Hubble Space Telescope has provided detailed images of distant galaxies and cosmic phenomena
  • Detection of gravitational waves (2015) confirmed predictions of Einstein's general theory of relativity