Astronomy 101 Exam 1 Study Guide
Astronomy 101 Exam 1 Study Guide Astronomy 101
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This 10 page Study Guide was uploaded by himechan15 on Saturday January 23, 2016. The Study Guide belongs to Astronomy 101 at Texas A&M University taught by Macri in Spring 2016. Since its upload, it has received 60 views. For similar materials see Basic Astronomy in Astronomy at Texas A&M University.
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Date Created: 01/23/16
Chapter 1 • Circumpolar: Stars that are always visible in the sky • Seasons: The tilt of Earth's axis causes different portions of Earth to receive more or less direct sunlight at different times of year. • If we have a new moon today we will have a full moon in 2 weeks • Can not see a new moon in the sky because it is too near the Sun • Lunar eclipse occurs only during a full moon • Atypical galaxy is a collection of a few million to a trillion or more stars, bound together by gravity • Universe: The sum total of all matter and energy • Big Bang: The event that marked the beginning of the expansion of the universe • The universe is expanding means the average distances between galaxies are increasing with time. • Age of the Universe is approx. 14 billion years old • Light year is a unit of distance • Observable universe: that portion of the universe that we can see in principle, given the current age of the universe • Distance of the Earth to the Sun is about 15 meters • Distance of the Earth to the Moon is small enough to fit on your hand • Using a scale of 1 to 10 billion, on which Earth is about the size of a ballpoint and located about 15 meters from a grapefruit-size Sun, the distance to the nearest stars is more than 4,000 kilometers (about 2,500 miles). • The number of stars in the Milky Way Galaxy is approximately The number of stars in the Milky Way Galaxy is approximately a few hundred billion • AU: the average distance between the sun and moon, about 150 million KM • Ecliptic Plane: The plane of Earth's orbit around the Sun • The universe is about 14 billion-years old, so we cannot observe objects more than about 14 billion light-years away. • The total number of observable stars is about the number of grains of sand on Earth • According to present understanding, the early universe contained only the elements hydrogen and helium. Stars produced all other elements -including carbon, oxygen, and other major ingredients of life. • Galaxies recycle material from one generation of stars to the next, and without this recycling we could not exist. • Except for hydrogen, all the elements of life were produced by stars and scattered into space when the stars died. Without galactic recycling, these elements would have dispersed into the expanding universe rather than being available to build planets and life around later generations of stars. • Our solar system is about 1/3 the age of the universe • Our solar system is located about half way between the center and the edge of the visible disk of the galaxy • Astronomers infer that the universe is expanding because distant galaxies all appear to be moving away from us and the more distant ones are moving faster, (Hubble’s great discovery) • Dark matter move with galaxies and clusters but they do not move faster than them • Your speed of rotation is faster if you live near the equator than if you live near the north pole • The fact that the universe was expanding lead us to the idea of a Big Bang • Elements such as carbon and oxygen came from nuclear fusion in massive stars • The Sun began to shine as a full-fledged star when nuclear fusion began inside of it when our solar system was born 4.5 billion years ago • The earth was formed around early September • Life arose in September • Alpha Centauri: the nearest star besides our Sun about 2.5 million light years away • Orion Nebula: a star-forming cloud located outside our solar system • The disk of the milky ways is about 100,000 light years in diameter • The distance to theAndromeda Galaxy is about 2.5 million light years • The distance between the center of the Milky Way to us is about 28,000 light years away • From farthest to nearest:Andromeda, star on the far side of the Milky way, star on the near side of the Milky way, Orion nebula, alpha centauri, Pluto, Sun • The distance betweenAlpha Centauri and Earth is about 4 light years • Alight year is 60,000 times bigger than 1AU Chapter 2 • Ecliptic: the path the Sun appears to trace around the celestial sphere • Meridian: a half circle extending from your horizon due north, through your zenith to your horizon due south, divides the sky into eastern and western halves • Zenith: the point directly over your head • We describe a location on Earth’s surface by stating its latitude and longitude • The altitude of the north celestial pole equals your latitude • The cycle of Moon’s phases takes about a month • Earth’s rotation defines a day • Earth’s orbit defines a year • Earth’s cycle of axis precession takes about 26,000 years • The moon always show the same face to Earth bc the moon rotates once in the same amount of time it takes the moon to orbit Earth once • Lunar eclipses can occur only during a full moon • Saros cycle: the 18 year period over which the pattern of eclipses repeats • Apparent Retrograde Motion: Over many days or weeks the planet moves westward relative to the stars, rather than the usual eastward relative to the stars • Stellar Parallax: the slight back-forth shifting of star positions that occurs as we view the stars form different positions in Earths orbit of the Sun, direct proof that Earth orbits the Sun • An angle of 1 arc second is less than the thickness of a human hair held at arms length • The altitude of the north celestial pole in the local sky is equal to your latitude so traveling north (to a higher altitude) means the North Star will move higher in your sky • The altitude of the south celestial pole is equal to your latitude, therefor if you see the south celestial pole at altitude 40 degrees then you must be located at latitude 40 degrees south • Facing north the sky appears to turn counterclockwise, in 6 hours the sky will turn ¼ of the way around a full circle, so all the stars will appear to have moved 90 degrees counterclockwise around the north celestial pole • If the big dipper is close to the northern horizon and the north star is above it in 6 hours the big dipper will appear to the right of the north star which is 90 degrees counterclockwise from its current position • Some constellations are only visible at certain times in the year because our view depends on where the Earth is located in its orbit around the Sun • The Sun’s path as viewed from the equator is highest in the sky on the spring and fall equinoxes • At the equator the Sun passes through the zenith just twice each year, at noon on the spring and fall equinoxes • If Earths axis tilt was greater than its current 23.5 degrees then Polaris will not be our north star and the region where the Sun does not rise on the winter solstice will be larger (farther south), summer and winters would be more severe, the length of the seasons are unaffected • The length of the seasons is determined by Earth’s orbital period • If our year was twice as long then the 4 seasons would each be twice as long as they are now • Earth’s varying distance from the Sun does not affect our seasons • If it’s a full moon then someone on the Moon will see a new Earth • The 3 quarter moon rises around midnight, reaches its highest point around 6am and sets around noon, you can see it during the morning • The full moon rises around sunset and reaches its highest point around midnight before setting around dawn • There is not a solar eclipse at every new moon because the orbital plane of the Moon is tilted by about 5 degrees • When Mars is undergoing retrograde motion, the Earth is catching up with and passing by Mars in their respective orbits • The Galileo spacecraft orbited Jupiter in the 1990s • Geocentric: Earth is the center of the Solar System, a model designed to explain what we see in the sky while having Earth located in the center of the Universe • Epicycles: in order to explain retrograde motion the geocentric model had to resort to epicycles • Astronomy allowed the ancient civilizations to keep track of time and seasons and was used in some cultures for navigation • Scientific Thinking: based on everyday ideas of observation and trial/error experiments • Greeks were the first people known to try and explain nature with models based on reason and mathematics without resort to the supernatural • Ptolemaic Model: a Earth centered model of planetary motion published by Ptolemy • About a couple of thousand stars are visible to the naked eye in a sky away from the city • Constellation: region in the sky as seen from Earth • Latitude= altitude in the northern celestial sphere • The altitude of the north celestial pole is the observer's latitude • In the months June/July the Sun’s north pole is tipped towards the sun and six months later (Dec/ Jan) it is pointed away • In the Northern hemispheres summer the Sun takes a higher path in the sky because the path is longer • Precession: the gradual change of Earth’s axis tilt • New Moon: moon between earth and sun, travels with the sun, up during the day, rises at sunrise and sets at sunset • Waxing: means growing • First Quarter: • Gibbous: swollen/ convex, rises in late afternoon and is up most of the night • Full moon: Earth is between the moon and sun, rises at sunset and sets at sunrise • Solar eclipse: can only happen during new moons, moon between Earth and Sun, blocking the sun • Umbra: in Latin means shadow, able to get a total solar eclipse • Penumbra: • Lunar eclipse: can only happen during full moons, anyone on Earth can see it, the moon changes color due to the Earths atmosphere that only lets red in, it is red during the penumbra but In the umbra the moon completely darkens • Annular solar eclipse: Because the moons orbit around earth is an eclipse, sometimes the moon is further away so it looks smaller and is not able to completely cover the sun • If the Moon orbited Earth in the same plane that Earth orbits the Sun (the ecliptic plane), we would have a solar eclipse at every new moon. Because there are about twelve new moons in a year, we would have twelve solar eclipses • There are generally two periods of time each year when eclipses are possible — the “eclipse seasons” — and there is a solar eclipse of some type (partial, total, or annular) during each of these eclipse seasons. (Because the time between eclipse seasons is less than six months, in some years there is a third eclipse season and hence a third solar eclipse.) • Conditions for a solar eclipse is that (1) it must be new moon, and (2) the Moon must be at or quite near one of the nodes of its orbit. • Conditions for a lunar eclipse is that (1) it must be full moon, and (2) the Moon must be at or quite near one of the nodes of its orbit. • The greater the axis tilt, the more extreme the seasonal temperature differences. Northern Hemisphere • Sidereal day:Asidereal day lasts from when a distant star is on the meridian at a point on Earth until it is next on the meridian.Asidereal day lasts 23 hours and 56 minutes (of solar time), about 4 minutes less than a solar day.Any star can be used to measure the length of the sidereal day, because the daily motion of stars across the sky is due only to Earth's daily rotation. • ASolar day is longer than a Sidereal day by about 1/360 of 24 hours which is about 4 minutes • To have a shorter Solar day one would have to reverse the direction of Earth’s orbit, like Venus • If Earth orbited 10 times as fast, it would move 10 times as far around its orbit each day, making the solar day longer than the sidereal day by about 10 × 4 minutes = 40minutes • Just as the solar day is longer than the sidereal day because Earth rotates and orbits in the same direction, the fact that the Moon orbits in the same direction as Earth means that its synodic period (from new moon to new moon) must be longer than its actual orbital period. The difference is 1/12 of the 27-day orbital period, which is just over 2 days. That is why the Moon's cycle of phases lasts about 29 1/2 days on average, or about 2 days longer than the Moon's orbital period of 27 1/3 days. • The Sun's path, as viewed from the equator, is highest in the sky on the spring/fall equinoxes • If our year were twice as long (that is, if Earth took twice as many days to complete each orbit around the Sun), but Earth's rotation period and axis tilt were unchanged, then the four seasons would each be twice as long as they are now Chapter 3 • Copernicus: proposed a heliocentric model, greatest contribution was to create a detailed model of our solar system with the Sun rather than Earth at the center • Tycho: tried to find stellar parallax but couldn’t and proposed a geocentric model, greatest contribution was to observe planetary positions with sufficient accuracy so that Kepler could later use the data to discover the laws of planetary motion • Galileo: observed mountains and valleys on the moon and Jupiter’s 4 moons and the phases of Venus • The semi major axis of an ellipse is half the length of the longest line that you can draw across an ellipse • Acircle is considered to be a special type of ellipse • Theory: a broad idea that has been repeatedly verified so as to give scientists great confidence that it represents reality • Acceleration: change of velocity over time. • Angular momentum: the rotational momentum of a spinning or orbiting object. • Sir Isaac Newton: formulated the laws of motion and gravity • 2 2 The acceleration of gravity on Earth is approximately 10 m/s (more precisely, 9.8 m/s ). If you drop a rock from a tall building, it will be falling 30 m/s in 3 seconds • Speed described by an astronomer is 10 light years per 1 million years • Newton’s laws of Motion: 1) In the absence of a net force acting upon it, an object moves with constant velocity, 2) The rate of change of momentum of an object is equal to the net force applied to the object, 3) For any force, there always is an equal and opposite reaction force. • A12 month lunar calendar would have 11 less days than a 12 month solar calendar • Scientific models are used to make specific predictions that can be tested through observations or experiments • Ptolemaic model: explained retrograde motion by stating that planets moved in small circles that moved on larger circles around Earth,An Earth-centered model of planetary motion published by Ptolemy • Copernicus’s Sun centered model was not any better than the Ptolemaic model because he used perfect circles for the orbits of the planets • Kepler's third law makes no allowance for planetary mass, and in fact the planet's mass has virtually no effect on its orbit of the Sun. (The Sun's mass has a major effect, however.) • Kepler's third law tells us that orbital speed declines with distance, so Venus must orbit the Sun at a slower speed than Mercury. • Kepler's third law tells us that any object with the same average distance as Earth will orbit in the same time of 1 year. • Galileo's observed that Venus goes through all the phases, which cannot be explained unless Venus is orbiting the Sun. (In the Ptolemaic system, Venus's phases vary only from new to crescent and back.), this offered direct proof of a planet orbiting the Sun • Scientific theories can never be proven true beyond all doubt; they can only be supported by a wide body of evidence. • If Earth is rotating from west-to-east, then all celestial objects must move from east to west across our sky. (The only exception is satellites in low-Earth orbit, where they orbit faster than Earth rotates.) So a planet going in the opposite direction across the sky would pose a direct challenge to our view of Earth as a rotating planet. • Scientific thinking is based on everyday ideas of observation and trial-and-error experiments • The names of the seven days of the week are based on the seven naked-eye objects that appear to move among the constellations, Sun (Sunday), the Moon (Monday), Mars (Tuesday), Mercury (Wednesday), Jupiter (Thursday), Venus (Friday), and Saturn (Saturday). • Metonic cycle: used to keep lunar calendars approximately synchronized with solar calendars, the 19-year period over which the lunar phases occur on about the same dates, so using it ensures that a lunar calendar begins on the same solar date every 19 years. • Planets rise in the east and set in the west each day because Earth rotates from west to east each day • Venus is the highest in the sky at noon (both full and new), same cycle as the Sun • In the Ptolemaic system, we should never see more than a crescent in Venus. Because we do in fact see more, the Ptolemaic model must be wrong. The full range of phases that we see for Venus is consistent only with the idea that Venus orbits the Sun. Galileo was the first to observe the phases of Venus — and hence to find this evidence in support of the Sun-centered system — because he was the first to observe Venus through a telescope. Without a telescope, we cannot tell that Venus goes through phases. • Aphelion: travels slower around sun • Perihelion: travels faster around sun • Kepler’s 1st law: The orbit of a planet is an ellipse with the Sun at one of the two foci. • nd Kepler’s 2 law: equal areas in equal times, faster near the sun, object covers a greater distance during any particular time period when it is closer to the Sun than when it is farther away. • Kepler’s third law: Planets with larger average orbital distances have slower average speeds. The orbital period of the planet is related to its average distance from the star, but not to the planet’s mass Chapter 4 • Spectrum- refer to a plot that has wavelength/frequency in the x axis, usually a combo of 3 basic types (emission, continuous, absorption) • Intensity of light in the y axis • Red- greatest intensity/ strongest wavelength • Continuous spectrum- spectrum of common incandescent light, changes as the temp of its sources increase, intensity increases at all wavelengths, the wavelength at which most light is emitted becomes smaller, solid obj • Emission line spectrum- low density, emits light only at specific wavelength that depend on its composition/ temp • Any dense obj heated up means light will be emitted • Absorption spectrum- dark lines, some wavelengths are absorbed • Neon signs are emission light spectrum • Electrons are in constant fuzzy motion around the nucleus, most of the time within a specific distance, can only move to an outer ring by absorbing a proton, closer the orbit the tighter the bond and the more energy needed to move to a lower orbit or a higher one, electrons want to stay in a tighter orbit therefore they will release a photon with the right wavelength in order to jump to a smaller orbit • The fingerprints in a spectrum tells us the type of elements present • Each type of atom has a unique set of energy levels • Each transition corresponds to a unique photon energy • Upward transitions = absorption of energy, pattern of absorption lines at the same wavelength • Downward transitions produce a unique pattern of emission lines • Wiens Law: particular shape of an obj due to different temps • People do not glow in the dark because they only emit light that is invisible to our eyes • Hotter obj emit more light at all wavelengths and have peak intensity at shorter wavelengths • Generally measure the Doppler effect from the shifts in the wavelengths of spectral lines • Shifting to shorter wavelengths is also called blue shifted • Doppler shift only applies to light moving away and to you
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