Astronomy 101 Course Review and Summary Main Topics The Night Sky History of Astronomy Science Light and Matter The Solar System Structure and Evolution of Stars Structure and Evolution of Galaxies Structure and Evolution of the Universe The Night Sky The Celestial Sphere The sky as seen from Earth is divided into 88 constellations It is convenient to pretend the stars are attached to a celestial sphere The celestial sphere appears to rotate about the celestial poles 1 day The Sun appears to move west to east relative to stars 1 year The Moon appears to move west to east relative to stars 1 month Celestial Sphere A large imaginary sphere centered on Earth Season Calendars The cause of the seasons is the tilt of the Earth s rotation axis relative to its orbit around the Sun The day is based on the time between one noon and the next The year is based on the time between one vernal equinox and the next The moon month is based on the time between one new moon and the next Moon Phases Eclipses Key Concepts Lunar phases change as we see more or less of the Moon s sunlit half The Moon rotates about its axis as it revolves around the Earth The sidereal month 27 3 days the synodic month 29 5 days A lunar eclipse occurs when the Moon passes through the Earth s shadow A solar eclipse occurs when the Earth passes through the Moon s shadow Solar eclipses occur when Moon is between Sun and Earth Solar eclipses occur at NEW MOON A lunar eclipse occurs when the Moon passes through the Earth s shadow Lunar eclipses occur when Earth is between Sun and Moon Lunar eclipses occur at FULL MOON From Ptolemy to Copernicus Key Concepts Aristotle 4th cent BC showed that the Earth is round Greek astronomers developed a geocentric model for the universe Ptolemy 2nd cent used epicycles to explain retrograde motion of planers Copernicus 16th cent proposed a heliocentric model for the universe In the model of Copernicus retrograde motion is easily explained The combination of small and large circles produces loop the loop motion Tycho Kepler Galileo Key Concepts Tycho Brahe made accurate measurements of planetary motion Planetary orbits are ellipses with the Sun at one focus A line between planet Sun sweeps out equal areas in equal times The square of a planet s orbital period is proportional to the cube of its average distance from the Sun Galileo made telescopic observations supporting the heliocentric model Kepler s First Law of planetary motion The orbits of planets around the Sun are ellipses with the Sun at one focus Kepler s Second Law of planetary motion A line from the Sun to a planet sweeps out equal areas in equal time intervals Kepler s Third Law of planetary motion The square of a planet s orbital period is proportional to the cube of its average distance from the Sun A planet s average distance from the Sun is equal to the semimajor axis of its orbit Newton s Laws Three Laws of Motion 1 An object remains at rest or moves in a straight line at constant speed unless acted on by an outside force 2 The acceleration of an object is directly proportional to force and inversely proportional to mass 3 For every action there is an equal and opposite reaction Law of Gravity The gravitational force between masses M and m separated by distance r is proportional to the product of the masses divided by the square of the separation Applying Newton s Laws Newton modified and expanded Kepler s Laws of Planetary Motion Kepler described how planets move Newton explained why they move Tides are caused by the difference between the Moon s gravitational force on different sides of the Earth Tidal forces are slowing the Earth s rotation enlarging the Moon s orbit Newton s First Law of Motion An object remains at rest or moves in a straight line at constant speed unless acted on by an outside force Precise mathematical laws require precise definitions of terms SPEED rate at which an object changes its position Example 65 miles hour VELOCITY speed plus direction of travel Example 65 miles hour to the north Newton s Second Law of Motion The acceleration of an object is directly proportional to the force acting on it and inversely proportional to its mass In mathematical form Or alternatively Newton s Third Law of Motion For every action there is an equal and opposite reaction Whenever A exerts a force on B B exerts a force on A that s equal in size and opposite in direction All forces come in pairs Kepler s Third Law Light Visible light is just one form of electromagnetic radiation Light can be though of as a wave or as a particle Light forms a spectrum from short to long wavelengths A hot opaque object produces a continuous blackbody spectrum Light forms a spectrum from short to long wavelength Visible light has wavelengths from 400 to 700 nanometers 1 nanometer nm 10 9 meter Color is determined by wavelength Blue 480 nm Green 530 nm Red 660 nm Visible light occupies only a tiny sliver of the full spectrum Matter and Forces Matter can come in various forms that are composed of fundamental particles An element is known by it number of protons Isotopes of an element contain different number of neutrons Isotopes can be radioactive and spontaneously decay There are four fundamental forces Gravity Electromagnetism Strong and Weak Hydrogen 1 proton 1H 2H 3He 4He 6Li 7Li Helium 2 protons Lithium 3 protons Proton Neutron 3H Spectra A hot transparent gas produces an emission spectrum A cool transparent gas produces an absorption spectrum Every type of atom ion and molecule has a unique spectrum The most abundant elements in the universe are hydrogen and helium The radial velocity of an object is found from its Doppler shift Continuum Source Cloud Solar System Solar System Constituents The terrestrial planets are made primarily of rock and metal The Jovian planets are made primarily of hydrogen and helium also have large amounts of water methane and ammonia Moons a k a satellites orbit the planets some moons are large The terrestrial planets are made primarily of rock and metal Mercury Venus Earth Mars The terrestrial planets are low in mass Earth mass high in density 3900 kg m3 Water 1000 kg m3 Air 1 kg m3 Rock 3000 kg m3 The Earth The study of seismic waves tells us about the Earth s interior The Earth is layered into crust mantle inner core and outer core The Earth is layered because it underwent differentiation when molten The crust is broken into plates that move relative to each other Seismic waves radiating through the
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