AST1002 Week 1-3
AST1002 Week 1-3 AST1002
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This 5 page Class Notes was uploaded by Hugo Notetaker on Saturday September 10, 2016. The Class Notes belongs to AST1002 at University of Florida taught by Vicki Sarajedini in Fall 2016. Since its upload, it has received 4 views. For similar materials see Discovering the Universe in Astronomy at University of Florida.
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Date Created: 09/10/16
AST1002 Week 1-3 Chapter 1: The Copernican Revolution The Sun, Moon, and Stars all moved throughout the sky, there would also be slight changes of these movements over time Planets have a different type of movement and at times would go backwards with their direction (Retrograde Motion) Ptolemy created an explanation for the planets bizarre movement which used deferents and epicycles based on the geocentric model (which was ultimately wrong) Copernicus formalized the heliocentric universe He explained the retrograde motion of the planets as optical illusions Galileo Galilei adopted Copernicus' heliocentric model and used observations with a telescope (that he built) He discovered Moon craters Sunspots Moons of Jupiter Phases of Venus Galileo used the Scientific Method when studying the sky and its objects (stars, planets, moons, etc.) Tycho Brahe Recorded superb nakedeye positions of the planets Became an employer of Kepler who eventually inherited all of his data Johannes Kepler Used Brahe's data to describe the shapes and speeds of planetary orbits Produced a theoretically derived Heliocentric model that marched the data Planets on elliptical orbits NOT circular orbits 1. Planetary orbits are ellipses, with the Sun at one focus eccentricity = major axis minor axis / major axis Describes Moon and all orbiting bodies 2. Planets sweep out equal areas in equal intervals of time They move fastest at perihelion and slowest at aphelion 3. The square of a planet's orbital period is proportional to the cube of its semimajor axis P^2 = a^3 Period (P in years) = time for one orbit Semimajor (a in AU) = average distance (radius if circular) Isaac Newton developed what is now known as "Newtonian mechanics" 3 laws of motion + gravity + calculus sufficiently explains virtually all motion Only in extreme cases do these laws of motion break down 1st Law A body of motion or at rest will stay that way until some external force changes it. 2nd Law The acceleration of an object is directly proportional to the applied force and inversely proportional to the object's mass Force = mass x acceleration Two objects pulled with the same force causes the one with greater mass to accelerate less 3rd Law Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first or For every action (force), there is an equal and opposite reaction (force) Gravity Newton realized that any object having mass exerts an attractive gravitational force on all other objects having mass. Mass does not equal weight Kepler's 3rd Law revisdted The Sun and both orbit their mutual center of mass, which is inside the Sun. The Sun moves very little (BIG mass), while the Earth moves a lot (less massive). Check Powerpoint Chapter 4 Solar System 1 Star 8 Planets Several dwarf planets Many moons Asteroids Meteoroids Comets The distance to the planets can be determined from Kepler's Laws plus solar system scale from radar ranging distance to Venus Measure the angular size (A in degrees) Know the distance (d in kilometers) Get actual diameter of the planet (D in kilometers) A planet’s mass is determined through observing the gravitational effect of the planet on some nearby object (moons, nearby planets, satellites) The density of a planet can then be determined by dividing the mass by volume (determined from the size) Planets orbit the sun counterclockwise as seen from the North Celestial Pole. All planets are roughly in the same orbital plane EXCEPT Mercury (and the dwarf planet Pluto). Comets are sometimes called dirty snowballs dust and rock in methane, ammonia and ice All light is reflected from the Sun the comet makes no light of its own The nucleus is a few km in diameter Meteoroids interplanetary rocky material smaller than 100m (down to grain size). called a meteor as it burns in the Earth’s atmosphere if it makes it to the ground, it is a meteorite Meteors are rocky mainly iron and nickel Some contain carbonaceous material rich in organic material Meteors are old 4.5 billion years based on carbon dating Asteroids – rocks between 100m and 1000km in size Most asteroids remain in the Asteroid belt between Mars and Jupiter but about 2000 have orbits that cross Earth’s path. They are composed of carbon or iron and other rocky material. The Asteroid Belt is a group of rocks that appear to have never joined to make a planet (as opposed to having once been a planet that was later destroyed). Why do we think this? Too little mass Different chemical compositions Gravitational effects of Jupiter may have prevented planet forming at this location… Chapter 4 Continued Nebular Theory for Solar System formation Our Sun and the planets began from a cloud of dust and gas (nebula) As the cloud contracts under its own gravity, the Sun is formed at the center. The cloud starts to spin and the smaller it contracts, the faster it spins. Conservation of angular momentum The cloud forms a flattened, pancake shape Dust grains in the flattened cloud form condensation nuclei as nearby atoms collapse onto them to begin forming planets. As mentioned before, the solar nebula contracts and flattens into a disk. Condensation nuclei form clumps that grow into moonsize planetesimals. • The strong solar wind from the newly formed star (the Sun) blows out the rest of the gas. • Planetesimals collide and grow. • Growing planetesimals form the planets over about 100 million years. • The more massive protoplanets are also able to sweep up large amounts of gas to become the Jovian planets. It is the temperature gradient (change in temperature across the solar system) in the early solar system that is largely responsible for the primary differences between the terrestrial and Jovian planets Rocky inner planets: The type of the material that condensed out of the nebular cloud at these higher temperatures was rocky in nature Giant gaseous, outer planets: Both rock and gas could condense out of the cloud at the lower temperatures where these planets formed. Also, accretion onto the planet could last longer since the outer part of the nebula was less effected by the solar wind. Less than 20 years ago, the only known planets were those in our own solar system. Although most believed planet systems should be common around other stars, none had been detected. The first extrasolar planets (or exoplanets) were identified using the radial velocity method to detect the “wobble” of a star as it is gravitationally tugged by the orbiting planet(s). The Kepler Space Mission is targeting a small region of our galaxy to identify 1000’s of planets using the transit method – observing small changes in the brightness of a star as a planet orbits in front of it. To find planets where life as we know it could develop, the planet must orbit its host star at a distance where the planet temperature is just right for liquid water to exist. This is known as the habitable zone. Definition to the term planet and dwarf planet 1.A planet is a celestial body that (a) orbits the Sun, (b) has enough mass to form a spherical shape, and (c) has cleared the neighborhood around its orbit. 2. A dwarf planet is a celestial body that (a) orbits the Sun, (b) has enough mass to form a spherical shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. Chapter 5: Earth and its Moon The Earth Solid inner core, liquid outer core Thick mantle and thin crust hydrosphere – liquid water atmosphere 50km thick magnetosphere charged particles caught in Earth’s magnetic field The Moon Moon has no hydrosphere, atmosphere or magnetosphere same basic interior regions as Earth but no liquid core Moon’s mass is just 1.2% that of the Earth’s (7 x 1022 kg) One of the ways in which the Moon effects the Earth is observed in the TIDES of the hydrosphere Every mass in the Universe is gravitational attracted to all other masses. Newton’s law of Gravity describes the relationship between the strength of that force and the distance between two objects with mass Force of gravity between two objects (Fg) is inversely proportional to the square of the distance between them (R). Earth is the only planet with large quantities of liquid water on the surface so the tidal effects of gravity can be observed. The moon pulls the water... The moon also pulls the Earth... This causes two bulges one on the side facing the moon and one on the opposite side where the water is “left behind”. Over time, tides have the following effects on the Earth and the Moon: 1. Slowing the Earth’s rotation the day is increasing by 0.002 sec/century. 2. Increasing the size of the Moon’s orbit its distance from the Earth is increasing by 4 cm/year (2 inch/year) The moon is tidally locked to the Earth the same side of the moon is always facing us (moon rotation period is the same as its orbital period) Earth's Atmosphere Protects the surface Regulates temperature nitrogen (78%) oxygen (21%) argon (0.9%) carbon dioxide (0.03%) Convection: Warmer air travels up and cooler air comes down to take its place Results in convection cells which heat the atmosphere creates surface winds and is responsible for most types of weather The Greenhouse Effect Sunlight not reflected by clouds reaches the Earths surface (about 50%) The heated earth reradiates this light in the form of infrared radiation Infrared light (heat) is partially blocked by the Earth’s carbon dioxide (and water vapor) So, only part of the light (heat) goes into space, part goes back to earth The average Earth temperature is about 40K hotter because of greenhouse effect than it would be without it. Why is this important? It keeps our water supply in liquid form Why does Earth have an atmosphere (while the Moon has none)? Gravity Gas molecules are in constant motion hotter gas, faster motion The fact that the atmosphere is heated keeps it from falling onto Earth Escape speed is the speed an object (in this case, a molecule) must travel to escape another object’s surface (like Earth or the Moon) If a planet’s escape speed is at least 6 times greater than the molecules’ velocity, molecules of that type will not “escape” in significant quantities Earth’s escape speed = 11.2 km/s Oxygen and nitrogen molecular speed = 0.6 km/s Moon escape speed = 2.4 km/s Seismology Earth’s interior structure is probed by studying how waves travel through it (we can only drill so far! 10km) Earthquakes generate seismic waves Certain types of waves reflect off different materials and travel through these materials at different speeds (e.g. waves travel faster through higher density material). Crust 15 km thick (8 km under ocean 2050 km under continents) Mantle 3000 km thick (80% of planet volume) Core (3500 km outer core and 1300 km inner core) High central density suggests the core is mostly nickel and iron Density and temperature increase with depth Density “jumps” between mantle and core but smoothly increases between inner and outer core Why? Due to the fact that we are switching from the liquids to the solids
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