Ast1002 Chapters 1 18 Notes Review for Final Chapter1 Astronomy Patterns among stars H W Q There are 6 000 stars in the entire sky that can be seen by unaided eye How many stars would be seen at a particular instant on a given dark night from a single location with an uninterrupted distant horizon ANS 3 000 because if you have an uninterrupted view of the sky you see a hemisphere or half a sphere and only one half of 6 000 stars will be visible to you The basics Astronomical Unit 1 A U Distance between Earth and the Sun 1 5x10 8 km Light Year Distance light travels in a year Measuring Distances Light takes 4 2 years to get to the nearest star 100 000 years to cross the galaxy 2 5 million years to get to nearest big galaxy 10 billion years to come from distant galaxies Almost all atoms in body are made of stars Stars generate energy by making heavier elements out of lighter ones Stars then eject those elements into space in massive explosions and new stars planets and humans form Scientific Theories Must be testable Simple Occam s Razor and elegant Can be proven wrong but can never with 100 certainty be proven right H W Q Our early universe was originally composed of hydrogen and helium H W Q Cosmological Principle Universe looks the same everywhere as long as you look on larger enough spatial scales H W Q Major Structures we reside in Earth Solar System Milky Way Local Group Virgo Superclustor Universe Every Sad Man Looks Very Ugly Big Bang speed satellite hypothesis scientific method theory H W Q The nightly motion of objects across our sky is caused by the rotation of Earth on its axis H W Q The overall rational for scientific investigation Reality is comprehensible and a limited number of fundamental principles governs nature and behavior of universe Important Definitions astronomy The scientific study of planets stars galaxies and the universe as a whole The event that occurred 13 7 billion years ago that marks the beginning of time and the universe The rate of change of an object s position with time without regard to the direction of movement Units meters per second m s or kilometers per hour km h Compare velocity astrobiology An interdisciplinary science combining astronomy biology chemistry geology and physics to study life in the cosmos 1 An object in orbit about a more massive body 2 A moon A well thought out idea based on scientific principles and knowledge that leads to testable predictions Compare theory The formal procedure including hypothesis prediction and experiment or observation used to test attempt to falsify the validity of scientific hypotheses and theories A well developed idea or group of ideas that are tied solidly to known physical laws and make testable predictions about the world A very well tested theory may be called a physical law or simply a fact Compare hypothesis cosmological principle The testable assumption that the same physical laws that apply here and now also apply everywhere and at all times and that there are no special locations or directions in the universe Occam s razor The principle that the simplest hypothesis is the most likely named after William of Occam circa 1285 1349 the medieval English cleric to whom the idea is attributed physical law A broad statement that predicts a particular aspect of how the physical universe behaves and that is supported by many empirical tests See also theory principle A general idea or sense about how the universe is that guides us in constructing new scientific theories Principles can be testable theories Chapter 2 Patterns in the Sky Motions of the Earth and Moon From the Northern Hemisphere the sun moon and constellations appear to rise in the East set in the West Constellations shift over the course of a year When viewed from above the North Pole Earth rotates counter clockwise on it s axis 24 hour rotation Earth s orbit is nearly circular Average distance to the sun 1 AU 150 million km and Study this Picture Celestial Sphere Projection of Earth s axis and equator into space The points on the sphere do not correspond to actual distances Rotates around North and South poles once each day Ecliptic Sun s apparent annual path around the celestial sphere Inclined 23 5 degrees to the equator At Earth s North Pole we would see half the celestial sphere The north celestial pole is directly overhead Stars rotate counterclockwise in 24 hours No star rises or sets all are circumpolar Observers can never see the south celestial pole Example View from Earth s Equator At Earth s equator all stars rise and set The celestial poles are on the northern and southern horizons Observers can see the whole celestial sphere as it rotates Example View from an Intermediate Latitude At an intermediate latitude on Earth one pole is above the horizon The angle from the pole to the horizon equals the latitude Some stars are circumpolar some stars rise and set some stars are never visible Example The Earth is Tilted on it s side and gives four special days each year Study this Picture Vernal Equinox March 21 Equal hours day and night Summer Solstice June 21 longest day in Northern Hemisphere Autumnal Equinox September 23 Equal hours day and night Winter Solstice December 22 shortest day in Northern Hemisphere There are two reasons for the seasons 1 Angle of sunlight is more direct in summer Energy concentrated 2 Sun is in the sky longer The angles and hours change throughout the year and this is why there are seasons Moon Makes 1 orbit around Earth and spins 1 time on axis in same amount of time Synchronous rotation Solar Eclipse Moon passes between Earth and the Sun 3 Types Total Moon completely blocks sun s light Partial Only part is blocked Annular Sun appears as bright ring surrounding moon Lunar eclipse Earth is between Sun and Moon Happens at a full moon Lunar phases Memorize Chart We don t have solar and lunar eclipses every month because the moon s orbit around Earth is tilted about 5 degrees out of the ecliptic so most of the time the sun earth moon are not on the same plane H W Q Shortest period of daylight occurs near which solstice or equinox Winter Solstice H W Q Sun sets furthest North or West on what date Summer solstice H W Q Beginning of Spring occurs on what date Vernal Equinox H W Q Altitude of the Sun at noon is greatest on what date Summer Solstice Important Definitions celestial equator The imaginary great circle that is the projection of Earth s equator onto the celestial sphere apparent retrograde