Space Physics: Earth, Moon & Sun

Space Physics

Contents Overview

• Space Physics includes Earth & The Solar System, which covers the Earth, Moon & Sun and calculating Orbital Speeds.
• Further topics are Orbiting Bodies, Gravitational Effects on Orbits, Stars & The Universe.
• In Stars & The Universe you will learn about The Sun as a Star, Stars, The Expanding Universe, The Big Bang Theory and Hubble & The Age of the Universe.

Earth & The Solar System

• The Earth is a rocky orbiting planet, that rotates around the Sun
• It rotates via its axis which travels through the north and south poles
• The axis is tilted at an angle of 23.4° from the vertical
• It completes one rotation (revolution) in 24 hours (1 day)
• This rotation gives the appearance the Sun creating daily motions such as the Sun rising and setting
• Rotation of the Earth on its axis is responsible for the cycle of day and night
• Earth's rotation creates day and night
• Day is experienced by the half of the Earth facing the Sun
• Night is the other half of Earth facing from away the Sun
• Earth's rotation makes the Sun move from east to west
• At the equinoxes the Sun rises exactly in the east and sets exactly in the west
• Equinox (meaning 'equal night') is when day and night are about the same length
• The location of where the Sun rises and sets changes throughout the seasons
• The locations of the northern hemisphere (above the equator) includes the sun rising north of east and sets north of west, and in winter, the sun rises south of east and sets south of west
• In Northern hemisphere, daylight hours are longest up until roughly June 21st
• This day is the Summer Solstice, with the Sun at its highest point in the sky
• Daylight hours then decrease to their lowest around December 21st
• This is Winter Solstice, when the Sun is at its lowest point in the sky
• The Earth's orbit is around the Sun in 365 days, or 1 year
• The combination of the Earth orbiting and its axis creates seasons
• Over parts B, C and D of the orbit, the northern hemisphere is tilted towards the Sun which is known as spring and summer with longer daylight hours
• Southern hemisphere is tilted away for the opposite effect which is autumn and winter
• Over parts F, G and H of the orbit, the northern hemisphere is tilted away from the Sun that indicates It is autumn and winter in the northern hemisphere, but spring and summer in the southern hemisphere
• At C in the orbit is the summer solstice where the northern hemisphere has the longest day and the southern hemisphere has its shortest day
• At G in the orbit is the the winter solstice which presents the northern hemisphere having its shortest day, and the southern hemisphere having its longest day
• At A and D on the orbit presents day and night which are equal in both hemispheres - these are the equinoxes
• The Moon is a satellite that travels around the Earth once a month in a near circular orbit
• The orbit takes around 27-28 days
• The Moon revolves around its axis so it always presents the same side to Earth
• The unseen side of the Moon has been orbited/photographed by astronauts/satellites
• The Moon shines with reflected light only
• Lunar phases depend on the way the Moon's appearance changes across a month period
• In the image, outer circle shows how the Moon looks like from the Earth
• Moon is always half illuminated by the Sun
• In the New Moon phase, the Moon is between the Earth and the Sun, so sunlight is only on the opposite face, meaning the Moon to be unlit for Earth
• At the Full Moon phase, the Earth is between the Moon and the Sun, so the side of the Moon facing Earth is fully lit
• A crescent can be seen when the Moon is only partially sunlit

Calculating Orbital Speeds

• Planets move around the Sun and moons orbit planets via circular motion
• In one orbit, a planet travels a distance equal to a circle's circumference
• The circumference is 2πr, where r is the radius
• average orbital speed of an object can be with the following equation:
  • v = 2πr / T
  • where orbital speed is in metres per second (m/s), r is the average orbit radius in metres (m), and T = orbital period in seconds (s)
• The orbital or time period is defined as the time taken for an object to complete one orbit
• The orbital radius r is always the distance from the centre of the object being orbited to the object orbiting

The Solar System

• Solar System constituents are the Sun, eight planets, natural and artificial satellites, dwarf planets and asteroids and comets.
• The Sun lies at the centre of the system and makes up over 99% of its mass
• Eight planets orbits the Sun with a gravitational field strong enough to pull nearby objects with the exception of natural satellites
• Dwarf planet gravitational field is not strong enough to pull in nearby objects
• Planets in ascending order of distance from the Sun:
  • Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• Satellites:
  • Natural: Moons that orbit some planets
  • Artificial: Man-made and orbit objects in space (e.g. International Space Station aka ISS orbits the Earth)
• Asteroids and comets orbit the Sun with asteroids being a small rocky object
  • The asteroid belt is located between Mars and Jupiter
  • Comets made of dust and ice, orbit the Sun with melting ice as it approaches it causing a tail

Accretion Model

• There are four rocky and small planets which are Mercury, Venus, Earth and Mars, that are nearest to the Sun.
• Jupiter, Saturn, Uranus and Neptune gaseous and large planets are located furthest from the sun.
• Differences in the types of planets are defined by the accretion model
• The Sun was formed when gravitational attraction pulled together hydrogen dust and gas clouds (nebulae) which formed the Solar System 4.5 billion years ago
• Planets were formed from the remnants of the nebula matter forming the Sun
• Interstellar clouds of gas and dust included elements that were created during the final stages of a star's lifecycle, that collapsed inwards spinning around the Sun
• Gravitational attraction joined small particles together and grew into an accretion process forming a rotating accretion disc as the planets emerged

Orbiting Bodies

• Planets and moons are visible from Earth because reflected light from the sun with the light taking time to travel these distances
• Outer regions of the Solar System are around 5 × 10^12 m from the Sun
• Earth travels for eight minutes before seeing the Sun
• It takes four years to reach the nearest star
• Milky Way galaxy contains billions of stars, with the light taking even longer to be seen from Earth
• The speed of light which is equal to speed = distance/time is a constant 3 x 10^8 m/s:

Elliptical Orbits

• Planets, minor planets and comets orbits are elliptical and not at the elliptical orbit's center
• An ellipse is just a 'squashed' circle

Analysing Orbits

• Data is useful to indicate on the surface of the planets like gravitational field strength and environmental problems
• Uniform Surface Gravitational Field Strength decreases from the Sun outwards

Gravitational Field Strength

• The strength of gravity on planets after measuring an object's weight
• Weight is defined as: The force acting on an object due to gravitational attraction
• Planets have strong gravitational attracting nearby mass to stay firmly on the ground or kept in orbit
• The weight of a body and strength g differs due to the planet or moon's mass, and its surface
• Greater planet mass causes greater gravitational field strength with a larger attractive force
• On a planets surface g varies with the distance from it, which means it decreases as the distance increases
• Earths g is approximately 10 N/kg with the Moon being smaller
• Means it is easier to lift a mass on the Moon than the Earth
• gas giants have more which means it would be harder to lift a mass than on Earth
• However, mass remains the same during weight variance, like an unable human to standup on a planet like Jupiter

Gravitational Attraction

• There are many orbiting objects around different planetary bodies in our solar system.
• Smaller bodies orbit a larger body such as a planet orbit the Sun
• Gravity provides force pulling the object towards larger bodies, which is known as gravitational attraction
• The gravitation is always acting towards the centre

Sun's Gravitational Field & Distance

• Moving away from the Sun increases
  • The strength of planets gravitational field decreases
  • Planets orbital speed decreases
• Objects must have a centripetal force to keep in circular motion
• Strength decreases the further away and weaker the centripetal force which is proportional to the orbital speed
• Furthest from the Sun takes the longest and Travels at the slowest
• Neptume, for example, travels slower than mercury

Conservation of Energy

• Objects in elliptical orbits or comets must conserve energy
• Throughout the orbit, gravitational potential and kinetic energy changes
• As the comet approaches the Sun:
  • Loses gravitational potential energy and gains kinetic energy
  • Increasing speed causes a slingshot effect flying back out once it passes from the sun
• As the comet moves away from the Sun:
  • Gravitational potential energy while losing kinetic energy to eventually causing it slow down and fall back towards the Sun

The Sun as a Star

• Sun is centrally located with over 99% of the Solar System's mass which keeps planets in orbit by gravitational pull
• The sun is a medium sized star with mainly hydrogen and helium
• It radiates energy in the infrared, visible and ultraviolet ranges
• Stars vary in color that comes in sizes and colors ranging from yellow stars to red dwarfs and from blue giants to red supergiants by light
• Stars can be classified by emitted color indicates surface temperatures such as A red star is the coolest (at around 3000 K) and A blue star is the hottest (at around 30 000 K)
• 21H + 31H → 42He + 10n
• A lot of energy can be realesed to a stable star

The Milky Way

• Distances of galaxies separated has astronomical distances that physicist measure with the light-year
• One light-year is the distance traveled by light through (the vacuum of) space in one year
• However, the speed of light is the universal maximum, with the diameter of the Milky Way being approximately 100000 light-years
• One light year = 9.5 x 10^12 km = 9.5 x 10^15 m

Life Cycle of Stars

• Nebula is the origin/ giant interstellar cloud of hydrogen gas and dust
• Gravity then in a nebula pulls the particles together to form a hot ball of gas, known as a protostar
• As particles get pulled, the protostar increases in density, results in frequent collisions increasing the stars temperature/
• Once a protostar gets hot enough, nuclear fusion reactions occur in it's core
• The hydrogen nuclei will fuse to form helium nuclei that release heat and energy to keep the core hot
• The different life cycle depends on the mass to know if its is a main-sequence star if it is a size of a sun or much more massive , for example the the lifecycle of a star which is the same size as the Sun (solar mass)
• When the star is born, it is a main-sequence star
• the star is in equilibrium where the inward force from gravity is equal to outward pressure fusion

Red Giant or Supergiant

• After several years the hydrogen reactions will run out and the fusion will dye, causing the core to shrink and heat up
• This is due to stronger gravity because the fusion dies, however a new series will then occur for core reactions like helium creating to beryllium increasing the outer expansion
• Smaller stars (up to 8 suns) the red giant transforms into a larger red super giant if it has over 8 suns to cool it's red surface
• Once fusion reactions are done, the star will become unstable and eject the dust layer (planetary nebula)
• The core will collapse (known as white dwarf), due to pull and energy will dissipate with cooling core leading to a black dwarf
• Once star has lost a lot of energy it will disappear

Galaxies & Redshift

• Stars emit waves causing wavefronts to symmetrically
• However a moving sources causes waves to squash/stretch out increasing and decreasing wave length in and behind the target
• If the object moves away, the light wave length increases which is known as redshift, otherwise known as:
• Increase of the observed wavelength radiation from the light emitting from receding stars and galaxies
• light moving towards object is blueshift if object is moving further away the color moves towards the other end which is redshift from observation by distance

The Expanding Universe

• Distant galaxies are redshift
• The lines shift indicating that all moving galaxies shows the universe is expanding
• The greater the distance, the greater the redhift
• Further objects are faster at distancing

The Big Bang Theory

• 14 billion years ago, the Universe began from an extremely hot and dense
• This was a explosion (Big Bang) that caused the universe to expand (and cool) from a point that expands away with greater distancing is galaxies move away quickly

Redshift and CMBR

• Evidences for the Big Bang Theory include multiple sources such as Galactic red-shift and Cosmic Microwave Background Radiation or CMBR
• These observations imply that the universe is expanding and therefore support the Big Bang Theory

Cosmic Microwave Background Radiation (CMB Radiation)

• discovery of radiation in the microwave regions from all directiosn and at a uniform temp is radiation is result of high energy events that has redshifted over time which provides a unform spectrum of space even at differnt densities
• According to the Big Bang theory, the early Universe was an extremely hot and dense environment therefore emission suggests thermal radiation

Hubble's Law

• Edwin Hubble showed in 1929 that galaxies were expanding, so he then observed the spectra from distant light and found light was shifted on the red
• Hubble also recorded that the further distant, the further away an object moves
• v = Hod
• where Ho = Hubble constant, v = velocity of light from an object, d = distance between the object and the Earth
• Ho or ratio defines the speed at distance of a galaxy and the Earth which is calculated by the time that universe is expanding