Comprehensive Study Guide to the Universe, the Solar System, Earth, and the Moon

Origin, Evolution, and Structure of the Universe

Definition of the Universe:
- The universe comprises all existing matter and energy, including the entirety of the space contained within it.
Stages of the Big Bang Theory:
- Stage 0: The Beginning (Hasiera): All matter and energy were concentrated in an infinitely dense and hot state.
- Stage 1: Inflation and Expansion (Puztuketa): The universe underwent a rapid initial expansion and began to broaden outward continuously.
- Stage 2: Appearance of Matter and Light (Materia eta argia agertu): Energy condensed into primordial fundamental particles, giving rise to matter and light.
- Stage 3: Formation of Atoms (Atomoak eratu ziren): Hydrogen ( $\text{H}$ ) and helium ( $\text{He}$ ) atoms formed approximately $300.000\text{ years}$ after the initial explosion.
- Stage 4: Formation of Stars and Galaxies (Izarrak eta galaxiak eratu): Gravitational attraction coalesced dispersed matter into the first stars and organized galactic systems.
Primary Components of the Universe:
- Galaxies (Galaxiak): Massive systems composed of millions to hundreds of billions of stars, interstellar dust, and cosmic gas clouds bound together gravitationally. The local galaxy is the Milky Way (Esne Bidea).
- Nebulae (Nebulosak): Expansive interstellar clouds composed of gas and dust situated between stars, often serving as stellar nurseries.
- Stars (Izarrak): Massive, luminous spheres of plasma composed primarily of hydrogen ( $\text{H}$ ) and helium ( $\text{He}$ ) held together by gravity. They produce their own light and heat through internal nuclear reactions. The central star of the local system is the Sun (Eguzkia).
- Planetary Systems (Planeta Sistemak): Gravitationally bound systems consisting of one or more central stars and all orbiting celestial bodies, including planets, dwarf planets, natural satellites, asteroids, and comets. The local system is the Solar System (Eguzki sistema).
- Planets (Planetak): Non-luminous celestial bodies that orbit a central star and do not generate their own light. The host planet of terrestrial life is Earth (Lurra).
- Satellites (Sateliteak): Natural celestial bodies that orbit a planet and lack intrinsic light emission. Earth's natural satellite is the Moon (Ilargia).
Units for Measuring Distances in the Universe:
- Because the dimensions of cosmic space are vast, conventional terrestrial units such as meters ( $\text{m}$ ) and kilometers ( $\text{km}$ ) are insufficient, necessitating specialized astronomical scales.
- Astronomical Unit ( $\text{AU}$ / Unitate Astronomikoa): Defined as the mean distance from the center of the Earth to the center of the Sun. It serves as the primary standard for measuring distances between objects within planetary systems like the Solar System:
  - $1\,\text{AU} = 150.000.000\,\text{km}$
- Light-Year (Argi Urtea): Defined as the total distance traversed by light across a vacuum in the course of one Earth year. It serves as the primary unit for designating distances across interstellar and intergalactic scales throughout the universe:
  - $1\text{ light-year} = 9.460.000.000.000\,\text{km}$

The Milky Way and the Solar System

Structure of the Milky Way Galaxy (Esne Bidea):
- The Milky Way is a barred spiral galaxy containing approximately $200.000\text{ million}$ stars ( $200.000.000.000$ stars). It comprises three primary morphological regions:
  - Core (Gunea): A dense, luminous central spherical structure characterized by an exceptionally compact concentration of stars.
  - Disk (Diskoa): A flattened region consisting of prominent spiral arms that emanate outwards from the central bar. The disk contains an abundance of young stellar populations, vast clouds of gas, and cosmic dust. The Solar System is situated within this disk.
  - Halo (Haloa): A broad spherical envelope completely surrounding the galactic disk and core. It contains ancient stars, dark matter, and dispersed galactic objects.
Architecture of the Solar System (Eguzki Sistema):
- The Solar System consists of a central star, the Sun (Eguzkia), and all celestial objects orbiting within its gravitational influence, including major planets, dwarf planets, satellites, asteroids, comets, and small interplanetary bodies.
- The Sun (Eguzkia): A massive gaseous star composed primarily of hydrogen ( $\text{H}$ ) and helium ( $\text{He}$ ) that generates radiant light and thermal energy via ongoing nuclear reactions in its interior.
Planets of the Solar System:
- Planets orbit the Sun directly and do not produce intrinsic light. The eight planets are categorized into two structural classes:
  - Terrestrial / Rocky Planets (Harrizko planetak): Four inner planets with solid rocky surfaces, listed in order of increasing distance from the Sun: Mercury (Merkurio), Venus (Artizarra), Earth (Lurra), and Mars (Marte).
  - Gas Giants (Gasezko planetak): Four outer giant planets lacking solid surfaces, listed in order of increasing distance from the Sun: Jupiter (Jupiter), Saturn (Saturno), Uranus (Urano), and Neptune (Neptuno).
Dwarf Planets (Planeta Nanoak):
- Celestial objects orbiting the Sun that possess sufficient mass for self-gravity to pull them into a nearly spherical shape, but have not cleared their orbital neighborhood of other debris:
  - Ceres: Located within the main asteroid belt between Mars and Jupiter.
  - Pluto (Pluton): Located in the trans-Neptunian Kuiper belt.
  - Eris: Located in the outer regions of the Kuiper belt.
Natural Satellites (Sateliteak):
- Celestial bodies orbiting major planets that do not emit light of their own. Notable examples include:
  - Earth: The Moon (Ilargia).
  - Mars: Phobos (Fobos) and Deimos (Deimos).
  - Jupiter: Europa (Europa), Ganymede (Ganimedes), and Callisto (Callisto).
  - Saturn: Titan (Titan).
Minor Planetary Bodies and Distant Reservoirs:
- Asteroids (Asteroideak): Small irregular objects composed of rocky and metallic minerals. The majority reside in the Asteroid Belt (Asteroideen gerrikoa) positioned between the orbits of Mars and Jupiter. The dwarf planet Ceres is situated inside this belt.
- Comets (Kometak): Small cosmic bodies composed of volatile ice, cosmic dust, and frozen gases. As they approach the Sun along eccentric orbits, solar radiation vaporizes volatile materials, generating an expansive, glowing gaseous tail.
- Kuiper Belt (Kuiperren gerrikoa): A circumstellar disc located beyond the orbit of Neptune that contains dwarf planets such as Pluto and Eris, alongside numerous frozen icy bodies.
- Oort Cloud (Oorten hodeia): A theoretical vast spherical cloud of predominantly icy planetesimals enveloping the outermost boundaries of the Solar System. It serves as the primary reservoir and origin of long-period comets.

Historical Astronomical Models of the Universe

The Geocentric Model (Eredu Geozentrikoa):
- The dominant astronomical framework accepted throughout Antiquity and the Middle Ages.
- Posited that the Earth is stationary at the absolute center of the universe, with all other celestial spheres and bodies revolving around it.
- Grounded in classical Greek philosophical thought, particularly developed and systematized by Aristotle (Aristoteles) and Ptolemy (Ptolomeo).
- Accounted for basic observational phenomena available to naked-eye observers at the time.
- Arranged celestial bodies in increasing distance from the central Earth: Earth (Lurra), Moon (Ilargia), Mercury (Merkurio), Venus (Artizarra), Sun (Eguzkia), Mars (Marte), Jupiter (Jupiter), and Saturn (Saturno).
The Heliocentric Model (Eredu Heliozentrikoa):
- Formally proposed by Nicolaus Copernicus (Koperniko) and subsequently reinforced, validated, and refined through observational and mathematical analyses conducted by Galileo Galilei (Galileo) and Johannes Kepler (Kepler).
- Establishes that the Sun is situated at the center, with the planets orbiting around it in nearly circular (elliptical) orbits.
- Explicitly recognizes Earth as a planet that also orbits the Sun while rotating on its axis, accompanied by its satellite, the Moon.
- Supported by rigorous astronomical observations, telescopic discoveries, and empirical scientific data.

Earth: Characteristics, Internal Activity, and Structural Spheres

General Physical Properties of Planet Earth (Lurra):
- A differentiated rocky planet with the geometric shape of an oblate spheroid—a sphere flattened at the poles and bulging at the equator (poloetan zapalduta den esfera).
- The only known planet in the universe that supports life (bizia).
- Maintains a mean global surface temperature of approximately $15\,^\circ\text{C}$ , an optimal thermal regime that preserves water in a liquid state ( $\text{ur likidoa}$ ), which is essential for biological processes.
- Possesses an active global magnetic field (eremu magnetikoa) surrounding the planet, functioning as a defensive shield against harmful ionizing solar and cosmic radiation.
- Exhibits dynamic internal geological activity (geologia-jarduera handikoa), driven by internal thermal processes that continuously modify surface topography through the uplifting of mountain belts and volcanic eruptions.
The Four Terrestrial Spheres (Lurraren Lau Zatiak):
- Atmosphere (Atmosfera): The protective gaseous envelope surrounding the solid planet, known collectively as air. Its primary chemical components are molecular nitrogen ( $\text{N}_2$ ) and molecular oxygen ( $\text{O}_2$ ). Typical phenomena and manifestations include atmospheric air, cloud formations, and wind currents.
- Hydrosphere (Hidrosfera): The combined mass of all water found on, under, and over the surface of the Earth. The vast majority of liquid water resides in saltwater oceans and seas. Manifestations include oceans, seas, river systems, lakes, and glaciers.
- Geosphere (Geosfera): The solid, mineral-based structural component of the Earth. Its outer surface forms the continental landmasses and the oceanic basins. Key components include solid rocks, mineral soils, mountain ranges, and volcanic structures.
- Biosphere (Biosfera): The global ecological shell comprising all living organisms inhabiting the planet. Encompasses all animals, plant life, fungal kingdoms, and single-celled microorganisms.

Earth's Movements and Their Global Effects

Rotational Motion (Errotazio-higidura):
- Earth spins continuously around its internal geographical axis, which is inclined at an angle of $23{,}5^\circ$ .
- Period of Rotation: Completes one full revolution in $24\text{ hours}$ ( $1\text{ day}$ ).
- Direction of Rotation: From west to east (mendebaldetik ekialdera), corresponding to a counter-clockwise rotation when viewed from above the North Pole.
- Consequence: The regular diurnal alternation of day and night (gaua eta eguna).
Translational Motion (Translazio-higidura):
- Earth travels along an orbital path around the Sun that forms a nearly circular ellipse.
- Period of Translation: Requires $365\text{ days and } 6\text{ hours}$ to execute one complete revolution ( $1\text{ year}$ ).
- Axial Invariant Tilt: The rotational axis maintains an invariant tilt of $23{,}5^\circ$ oriented in the same spatial direction throughout the entire translational orbit.
- Consequence: The continuous variation in the angle of incidence of incoming solar radiation, driving the cycle of the four seasons (urtaroak) and producing variations in day length.
The Annual Cycle of Seasons, Equinoxes, and Solstices:
- Spring Equinox (Udaberriko ekinoxioa - March 21 / Martxoak 21):
  - Solar rays hit at an intermediate angle.
  - Day and night are approximately equal in duration across the globe.
  - Surface temperatures progressively rise.
- Summer Solstice (Udako solstizioa - June 21 / Ekainak 21):
  - The Northern Hemisphere is tilted maximally toward the Sun.
  - Solar rays arrive perpendicular and direct (zut), concentrating thermal energy and heating the surface intensely.
  - Days reach their maximum duration, and nights are shortest.
  - High seasonal temperatures prevail.
- Autumn Equinox (Udazkeneko ekinoxioa - September 22 / Irailak 22):
  - Solar rays hit at an intermediate angle.
  - Day and night are approximately equal in duration.
  - Surface temperatures progressively drop.
- Winter Solstice (Neguko solstizioa - December 22 / Abenduak 22):
  - The Northern Hemisphere is tilted away from the Sun.
  - Solar rays arrive at an oblique, highly inclined angle, distributing energy over a broader surface area and heating less effectively.
  - Days reach their shortest duration, and nights are longest.
  - Low seasonal temperatures prevail.
Ecological and Biological Consequences for Living Organisms:
- Regulation of Biorhythms (Ziklo / Erritmo Biologikoak): The regular day-night cycle controls internal circadian biological clocks (bioritmoak / bioerlojuak), regulating plant photosynthetic activity, sleep-wake cycles in animals, and metabolic feeding behaviors.
- Organismal Seasonal Adaptations: Seasonal shifts in temperature, photoperiod (light duration), and climate govern biological life cycles, triggering plant developmental phases, animal migration schedules, and breeding/reproductive periods.
- Plant Phenology: Governs seasonal vegetative growth, active flowering, autumnal leaf shedding, and fruit maturation at specific periods of the year.
- Animal Migration (Migrazioak): Triggers seasonal geographic relocations in numerous animal species (such as migratory birds and butterflies) seeking favorable weather and resource availability.
- Hibernation (Hibernazioa): Enables specific animal taxa to withstand severe winter cold and nutrient scarcity by entering a dormant state characterized by depressed metabolic activity.
- Global Biome Distribution: The complex interaction between axial tilt, differential solar radiation, and seasonal variations produces distinct planetary climatic zones, fostering the establishment of diverse terrestrial biomes:
  - Forest (Basoa)
  - Grassland / Prairie (Pradera)
  - Desert (Desertua)
  - Taiga (Taiga)
  - Tundra (Tundra)
  - Savanna (Sabana)

The Moon: Physical Nature, Motions, and Gravitational Phenomena

Physical Characteristics and Surface Topography:
- Earth's only natural satellite, revolving in an orbital trajectory around the planet.
- Surface Terrain Divisions:
  - Lunar Maria / Seas (Itsasoak): Expansive, dark, flat basaltic plains containing relatively few impact craters.
  - Lunar Highlands (Goi lurrak): Highly elevated, bright, mountainous, and rugged terrains characterized by a dense saturation of impact craters.
- Atmosphere: Extremely tenuous and practically non-existent, lacking the substantial protective gas envelope found on Earth.
- Surface Geology: Displays minimal geological activity, meaning that lunar landforms and impact structures remain virtually unaltered over vast spans of time. Lunar rocks share basic mineralogical properties with rocks of Earth's outer crust.
- Surface Gravity: The Moon's gravitational acceleration is six times weaker than that of Earth ( $\frac{1}{6}$ of Earth's gravity).
- Thermal Extremes: In the absence of an insulating atmospheric layer, surface temperatures fluctuate dramatically between $-173\,^\circ\text{C}$ during the lunar night and $127\,^\circ\text{C}$ during the lunar day.
Lunar Motions and Synchronous Rotation:
- Translational Motion (Translazio-higidura):
  - The Moon revolves around Earth along a circular to slightly elliptical orbit.
  - Duration: The true sidereal period (izarren benetako periodoa) lasts $27{,}3\text{ days}$ (approximated as $28\text{ days}$ in broad orbital diagrams).
  - Direction: From west to east (mendebaldetik ekialdera).
- Rotational Motion (Errotazio-higidura):
  - The Moon spins on its own internal axis.
  - Duration: Lasts exactly $27{,}3\text{ days}$ (or $28\text{ days}$ ).
  - Direction: From west to east.
- Synchronous Rotation Effect: Because the rotational period precisely matches the translational period ( $27{,}3\text{ days}$ ), the Moon exhibits synchronous rotation relative to Earth. As a direct consequence, the exact same lunar hemisphere always faces Earth, while the opposite side remains perpetually hidden from terrestrial view as the lunar far side (Ilargiaren aurpegi ezkutua).
The Lunar Phase Cycle (Ilargiaren Faseak):
- As the Moon translates around Earth, observers on the surface view varying proportions of its sunlit hemisphere.
- Synodic Period: The complete phase cycle requires $29{,}5\text{ days}$ (the lunar month of phases / faseen hileko periodoa).
- Primary Phases in Sequence:
  - New Moon (Ilberria): The Moon is positioned between the Sun and Earth; the illuminated face is directed away from Earth, rendering it invisible.
  - Waxing Crescent (Ilgora): A thin crescent of light appears on the right edge and gradually widens.
  - First Quarter (Lehen laurdena): Half of the visible lunar disk is illuminated by sunlight.
  - Full Moon (Ilbetea): The entire terrestrial-facing side of the Moon is illuminated by direct sunlight.
  - Waning Crescent / Third Quarter (Ilbehera): The illuminated area progressively diminishes back to darkness.
Astronomical Eclipses (Eklipseak):
- Occur when the Sun, Earth, and Moon align along an identical orbital line in space (3 astroak lerro berean).
- Solar Eclipse (Eguzki-eklipsea):
  - Orbital Alignment: The Moon moves directly between the Sun and Earth (Ilargia erdian).
  - Phenomenon: The Moon blocks the direct rays of the Sun, casting its shadow upon the surface of the Earth.
- Lunar Eclipse (Ilargi-eklipsea):
  - Orbital Alignment: The Earth moves directly between the Sun and the Moon (Lurra erdian).
  - Phenomenon: The Earth projects its shadow into space, fully or partially obscuring the Moon from sunlight.
Oceanic Tides (Mareak):
- Tides are periodic, rhythmic rises and falls in global ocean water levels caused by the gravitational attractions exerted by the Sun and, predominantly, by the closer gravitational pull of the Moon.
- High Tide (Itsasgora): The maximum periodic elevation of the sea surface (uraren maila altua), occurring on the sides of Earth aligned with the gravitational vector of the Moon.
- Low Tide (Itsasbehera): The minimum periodic depression of the sea surface (uraren maila baxua), occurring at points situated at right angles to the Earth-Moon gravitational axis.