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Astronomy 101 Course Review and SummaryMain 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 UniverseThe Night SkyThe 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 EarthSeason & 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 separationApplying 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 nmVisible 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 Helium 2 protons Lithium 3 protons Proton: Neutron: 1H 3He 2H 3H 4He 6Li 7LiSpectra 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 CloudSolar SystemSolar 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
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