Earth, Sun, Moon and Seasons

The Earth and the Solar System

• Earth spins on its axis, causing day and night.
• A complete rotation takes 24 hours.
• Day occurs on the half of Earth facing the Sun, while night is on the half facing away.

Rising and Setting of the Sun

• Earth's rotation makes the Sun appear to move east to west.
• The Sun rises exactly in the east and sets exactly in the west during equinoxes.
• In the northern hemisphere during summer, the Sun rises north of east and sets north of west.
• In winter, the Sun rises and sets south of these points.

The Seasons

• The seasons result from Earth's motion around the Sun (365 days) and the tilt of Earth's axis.

Motion of the Moon

• The Moon, a satellite of Earth, orbits approximately every month.
• The average distance from Earth is about 400,000 km.
• The Moon revolves on its axis, always showing the same side to Earth.
• The Moon reflects sunlight, has no atmosphere, and has a weaker gravitational field (one-sixth of Earth).

Phases of the Moon

• The Moon's appearance changes during its monthly orbit.
• During the New Moon, the Moon is between the Sun and Earth, with its unlit side facing Earth.
• A crescent appears and increases until the first quarter, where half of the Moon is visible.
• The Full Moon occurs when the Moon is opposite Earth from the Sun and is fully visible.
• Waning phases follow as the moon's illumination decreases.

Orbital Speed

• Average orbital speed (uu) is calculated as 2πr/T, where rr is the average radius of the orbit and TT is the orbital period (time for one orbit).
• The Moon travels in a circular path around the Earth, and the distance traveled in one orbit is the circumference of the circle, 2πr.
• Time taken for one orbit is TT, and speed is distance divided by time.

The Solar System

• It consists of the Sun as a star, eight planets in elliptical orbits, dwarf planets and asteroids orbiting the Sun, moons orbiting planets, and smaller bodies like comets and natural satellites.

Inner Planets

• The inner planets are Mercury, Venus, Earth, and Mars, and they are small and similar in size.
• They are solid and rocky with layered structures and high density.
• Inner planets formed close to the Sun, where it was too hot for gases to condense, allowing only metals and silicates to form solid bodies.
• In the early Solar System, the Sun's heat caused lighter gases to evaporate, leaving only heavy elements like iron and silicon to form solid planets.

Outer Planets

• The outer planets are Jupiter, Saturn, Uranus, and Neptune, and they are much larger and colder than the inner planets.
• They are mainly composed of gases and have low density, with many moons and rings of icy materials.
• Outer planets formed in cooler regions where gases could condense, capturing even the lightest elements.
• In the outer regions of the Solar System, lower temperatures allowed gases like hydrogen and helium to remain in solid or liquid forms, forming gas giants with thick atmospheres.

Asteroids

• Asteroids are pieces of rock of various sizes, mostly located between Mars and Jupiter.
• They orbit around the Sun and have a similar density to inner planets.
• Some asteroids burn up in Earth's atmosphere as meteors.

Comets

• Comets consist of dust embedded in ice made from water and methane.
• They orbit the Sun in highly elliptical paths and develop a bright tail when approaching the Sun due to radiation pressure.

Elliptical Orbits

• Planets, dwarf planets, and comets orbit the Sun in an ellipse.
• The Sun is at one focus of the ellipse, not the center.
• Comets have highly elliptical orbits, while planets' orbits are more circular.

Origin of the Solar System

• The Solar System formed from gravitational attraction pulling together clouds of hydrogen gas and dust (nebulae) about 4.5 billion years ago.
• Planets formed from the disc of matter left over from the nebula that formed the Sun.
• Inner planets were formed from materials with high melting temperatures, such as metals and silicates.
• Outer planets formed from light molecules that existed in solid icy forms, growing large enough to capture hydrogen.

Travel Times

• The distance from the Sun to Earth is approximately 150 million km.
• Light travels at 300,000 kilometers per second.
• Light from the Sun takes about 8 minutes to reach Earth.

The Sun

• The sun is a medium-sized star composed mainly of hydrogen and helium.
• It emits energy in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum.
• Energy comes from nuclear reactions in the core, where hydrogen undergoes nuclear fusion to form helium, releasing energy.
• Energy from the core heats outer layers, causing them to glow and emit radiation.

Nuclear Reactions in Stars

• Stars like the Sun are powered by nuclear fusion.
• Core needs to be hot and dense enough for hydrogen to fuse into helium.
• Fusion releases energy, maintaining high core temperatures.
• Some core energy moves to outer layers, and emits electromagnetic radiation.

Light Years

• A light-year is the distance light travels in a vacuum in one year, equivalent to 9.5×10^12 km.

Galaxies

• Galaxies are large collections of stars, gas, and dust.

Origin and Lifecycle of Stars

• Stars form when interstellar clouds of dust and gas collapse under gravitational attraction.
• A protostar forms as mass increases and core temperature rises.
• Hydrogen fuses into helium when the core becomes hot enough, resulting in a star.

Star Types

• Large mass stars are blue or white.
• Smaller mass stars are yellow or red dwarfs.

Life Cycle of Stars - Stable Phase

• Forces of gravity inward balance with thermal pressure outward.
• This lasts up to 10 billion years where hydrogen converts to helium in the core.

Red Giant/Red Supergiant

• As hydrogen depletes, the star becomes unstable.
• The core collapses, and outer layers expand and cool.
• The star turns into a red giant (or red supergiant if massive).
• Helium fuses into carbon in the core.

Low Mass Stars - End Stage

• The core collapses into a white dwarf after all helium is used.
• Outer layers are expelled, forming a planetary nebula.
• White dwarf cools into a black dwarf over about a billion years.

High Mass Stars - End Stage

• High mass stars, more than 8 times the Sun's mass, use hydrogen rapidly, with a shorter stable phase (about 100 million years).
• After helium fusion, the core collapses into a red supergiant.
• Fusion of carbon into heavier elements occurs until iron forms.
• A supernova explosion releases energy and heavy elements into space.
  • Neutron Star: Dense core that may act as a pulsar.
  • Black Hole: Extremely dense core with a gravitational field so strong that even light cannot escape, identified by X-ray radiation from nearby material.

The Universe

• Milky Way: Approximately 100,000 light-years in diameter and contains around 800 billion or more stars, presenting as a spiral galaxy with a central bulge and spiral arms.

Redshift

• Light from distant galaxies shifts towards the red end of the spectrum (longer wavelength).
• Light emitted from stars in distant galaxies appears redder compared to light from closer galaxies.

Doppler Effect

• It occurs when a source of waves, such as sound or light, moves relative to an observer.
• Approaching Source: Waves are compressed, and the frequency and pitch are higher (blue shift for light).
• Receding Source: Waves are stretched, and the frequency and pitch are lower (red shift for light).

Speed of Recession

• The speed at which distant galaxies are moving away can be calculated from the amount of redshift observed.
• Some of the most distant galaxies are receding at speeds up to one-third the speed of light.
• Observed redshift supports the Big Bang theory, because the Universe is expanding.

Big Bang Theory

• The Universe began from an extremely hot and dense state around 14 billion years ago.
• The expansion of the universe has been ongoing ever since.

Microwave Background Radiation

• Radiation is remnant from the Big Bang and fills the entire Universe.
• The radiation has been redshifted into the microwave region due to the expansion of the Universe.
  • It provides strong evidence for the Big Bang theory.

Age of the Universe

• Use Hubble’s Law: v=H0×d, where v is the speed of recession and d is the distance of galaxies
• Hubble Constant (H0) measures the rate of the Universe's expansion, where a higher value indicates faster expansion.
• H0 is estimated to be approximately 2.2×10−18 s−1.
• The estimated age of the Universe is approximately 1/H0.