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Lectures 3-7

by: Lauren Price
Lauren Price
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About this Document

This covers half of week 2, week 3, and week 4
Stars/Galax/Cosmology Lecture
Sean Lindsay
Class Notes
Science, Physics, astronomy




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This 10 page Class Notes was uploaded by Lauren Price on Monday September 19, 2016. The Class Notes belongs to Astronomy 154 at University of Tennessee - Knoxville taught by Sean Lindsay in Fall 2016. Since its upload, it has received 5 views. For similar materials see Stars/Galax/Cosmology Lecture in Physics and Astronomy at University of Tennessee - Knoxville.

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Date Created: 09/19/16
Astronomy: Chapter 1 8/22­31/16 Earth’s Motions­ General  ­Motions in the night sky as observed from the earth  ­Daily, or Diurnal, motions ­Rise in the east, set in the west ­Because of the earth’s rotation about its spin axis  ­Yearly, or annual, motion ­What stars/constellations are viewable at night changes from month to month  over the course of a year and then repeats  Because of the earth’s revolution, or orbital motion, around the sun ­The term sidereal means in reference to the background stars and so corresponds to a  rotation of revolution of 360 ­The stars are so very far away that they appear to not change their relative  positions (fixed background of stars) ­However, what stars are up when depend on the earth’s position in its orbit  around the sun.  Earth’s rotation: diurnal motions ­Celestial objects rise in the east and set in the west ­Due to earth’s rotation ­Earth’s rotation also defines a day  Earth’s rotation: defining days ­Daily or diurnal, noon to noon defines the solar day (24 hours)  ­Single 360 rotation is slightly shorter  ­Defined as the Sidereal Day (23 hours 56 mins)  ­Reason: Earth is not only rotating by revolving  Earth’s Motions ­The result of this motion is a slow 12 month progression through the  constellations/night sky objects, such that the winter sky is entirely different from the  summer sky Understanding the Night Sky ­From any vantage point on the earth, you can see roughly 3,000 stars ­Only half of the Celestial Sphere, indicating that about 6,000 stars are visible  from earth with the naked eye ­The constellations that are visible depend on  ­Your location on earth (latitude) ­The time of year ­For the Northern hemisphere observer the depicted Southern sky stars are forver  below the horizon  The Celestial Sphere ­Stars seem to be held at a fixed distance on the inner surface of the sphere (Hung in  the sky)  ­Projects Earth’s coordinate system (Poles, equator, latitude, and longitude) ­North and south celestial poles and the celestial equator  ­Right ascension is analogous to longitude ­For the curious: an angular measurement, given as if it was a measurement of  the hour hand around the 24 hour clock ­Declination is analogous to latitude  ­For the curious: traditional angular measurement, but with additional sub­degree unit  View from Earth ­Zenith: The point directly overhead ­Meridian: Line going through south, zenith, and north ­Altitude­Azimuth  ­Altitude: angular measure of how far above horizon ­Azimuth: direction in number of degrees away from North  ­Celestial coordinates use right ascension and declination  ­Fixed coordinate system, such that every star and deep space object has a  specific RA and Dec.  ­Does not change ­Horizontal coordinates use altitude and azimuth  ­Convenient to point out a point of the local sky ­Because everything is moving in the sky, objects Alt.­Az. coordinates are  constantly changing Earth’s Motions­ Axial Tilt  ­Definition: The Ecliptic is the path of the sun on the celestial sphere over the course of  a year ­Because of Earth’s axial tilt of 23.5 degrees the ecliptic is also inclined 23.5 degrees  with respect to the sun­earth orbital plane (defines the plane of the solar system) The reason for the seasons ­Includes two main effects ­Sunlight is more/less concentrated in summer/winter, respectively  ­Days are longer/shorter in summer/winter respectively  ­Solstice: means “Sun Stands” ­Ecliptic at northern most point in the sky ­ summer solstice ­Sunlight most direct in the N. Hemisphere  ­Ecliptic at Southern most point in the sky ­ winter solstice  ­Sunlight most direct in southern hemisphere  ­Equinox: “Equal Night” Axial tilt point in a different direction from the earth­sun line ­Ecliptic at southern most point in the sky  ­Sun moving above celestial equator: vernal/spring equinox  ­Sun moving below celestial equator: autumnal/fall equinox  Earth’s Motions­ Axial Tilt  ­In summer, the sun rises north of east, travels through the southern sky, and sets north of west.  ­Travels a longer and higher path through the sky (longer days) ­in winter, the sun rises south of east, travels through southern sky, and sets south of  west ­Travels a shorter and lower path through the sky (shorter days)  Earth’s Motions ­ Precession  ­A 26,000 year cycle. The Earth’s axis also rotates. It wobbles around. 23.5 degree tilt ­Time for earth to complete one orbit around the sun relative to the fixed stars is the  Sidereal Year (365.256 days) ­Time between two Vernal (spring) equinoxes is defined as the Tropical year (365.2422  days), and it happens slightly sooner because Earth’s axis has rotated toward the Earth­ Sun line.  ­The Tropical Year is our calendar year  ­Keeps seasons at same time of year  ­Very slow change of what season a constellation is.  ­In 13,000 years Orion will be a summer constellation ­If we use the Sidereal Year, in 13,000 years Orion would still be a Winter constellation,  but Northern summer would be in December  ­i.e., the Spring Equinox would drift through the calendar, occurring about 20  minutes later each year  Motion of the Moon  ­Orbital Period, or Sidereal Period, of the moon is 27.3 days  ­That means the amount of time to complete one 360 degree rotation  ­Synodic Period, or period for a full cycle of lunar phases, is longer at 29.5 days Synodic Versus Sidereal Month  ­While the Moon orbits the earth, the earth still continues to orbit the sun  ­The moon needs to continue its revolution around the sun to get back to a new moon  phase ­Full cycle of phases is called a synodic month and is 29.5 days long ­Longer than a sidereal month due to the motion of the earth Phases of the Moon  ­Moon is bright because of reflected sunlight ­Half of the moon is always illuminated  ­The phase we see is due to our view of the illuminated portion  ­Takes about 4 weeks to complete synodic period  Quiz Hint:  The time period that it takes to rotate exactly 360 degrees around rotation axis is known as? answer: sidereal day  What a noon to noon time period is: solar day  which is longer, solar or sidereal day: solar day is longer  what is analogous to latitude on earth: declination analogous to longitude: right ascension  he will give the celestial coordinate and i give the earth analog A. Today is third quarter moon, draw the moon in the appropriate box B. what phase will the moon be a week from today? One week is two moons later Draw phases of the moon Moon phases ­Make sure you know how to draw the graph on the right. The inside part. ­Waxing on bottom, waning on top Eclipses  ­Due to the alignment of sun­earth­moon  ­Occur when the sun, moon, and earth form a straight line ­Caused by the shadow of the Earth/Moon falling on the Moon/Earth ­This does not always happen because the Moon’s orbit is inclined by 5.2 degrees Lunar Eclipses ­Only occur during a full moon ­Call it a partial eclipse when only partially shadowed (in penumbra) ­Total eclipse when entirely shadowed (in umbra) ­When earth is between the sun and moon Quiz hints: 3. if the earth were to orbit the sun twice as fast as it currently does, then the solar day  would be: longer  Same question but sidereal day: stays the same  Be able to write out numbers in scientific notation Which is smaller/bigger if using the metric system: 13.8Gyr or 13.8Myr (Gyr) Solar Eclipses  ­Moon’s orbit is not perfectly round, and therefore sometimes closer and sometimes  farther away ­If the umbra of a solar eclipse when Moon is further away, the disk of the Moon is too  small to entirely block the sun, and an Annular eclipse happens  Quiz hints:  1. for a total solar eclipse to occur what phase must the moon be in? New moon  2. for a total lunar eclipse to occur what must the moon phase be? full moon  Remember the 5 degree tilt  Eclipse Seasons  ­The precise alignment requirement for eclipses means that eclipses are favorable twice per year, so called, eclipse seasons  Measuring Distance  ­Triangulation  ­Can calculate distance to object given that you have measured: ­The baseline distance ­and the angle ­You won’t have to work out the geometry/trigonometry, but you do need to know the  method ­Parallax ­Same principle as Triangulation, but here we measure the angle via the  apparent motion of an object against a distance background using two  different  vantage points.  ­Baseline: one point on earth and another point on earth  Angular Measurement ­Full circle contains 360 degrees  ­Each degree contains 60’ (arc­minutes) ­Each arc­minute contains 60” (arc­seconds) ­Angular size, measured in degrees, arcmin, arcsec, of an object depends on its actual  size and distance from the viewer  Local Cosmic Distances  ­Parallax is the apparent displacement (change in position) a distant object has when  observed from two different points  ­Measure in angular distance (radians, degrees, arcminutes, arcseconds) ­Parallax is inversely proportional to distance, i.e., if distance increases, then parallax  decreases. ­Extremely hard to measure with small angles ­Angle increases with increasing baseline Distance and Size  ­We can measure the distances to the planets using parallax ­Once the distance is known, we can measure the angular size of a planet to determine  it’s true size (diameter or radius) ­Diameter is size of spherical object ­Radius is half the diameter, or center of object to surface distance  Science  ­Science is a system of knowing that provides explanations for HOW (not why) natural  phenomenon and the universe work ­The sciences are NOT in the business of proving things ­We are in business of providing explanations that:  ­Are testable with clear hypotheses/predictions ­Are falsifiable (even by a single new piece of evidence) ­Are repeatable  ­Rely upon evidence/data and are flexible in the light of new evidence gathered  by observations, experimentation, and modeling  ­Provide a high level of confidence with their predictions and results  ­Qualities of a scientific theory  ­Must be testable  ­Must be continually tested ­Must be simple ­Must be elegant  ­Scientific theories can be proven wrong (falsifiability)  ` ­More so, they can never be proven right with 100 percent certainty  ­Scientists: “We aren’t in the business of proving things” Scientific Theory and Methods  ­Let’s start with some limited observations of a natural phenomenon  ­Recording those observations, wether that be descriptive or numerical, is our  initial evidence or data  ­This evidence allows for the formation of a hypothesis attempting to explain  some aspect of that phenomenon  ­A hypothesis is a predicted outcome that can be tested via further  observation and experimentation ­What do we do with a hypothesis? ­Design and perform further experiments to test the predicted outcome ­If the prediction is fulfilled (within confidence bounds) then we have gained some knowledge (albeit limited) about a natural phenomenon ­Confirmation or rejection of hypothesis leads to… ­The design of further experiments to confirm the results ­The generation of new hypotheses and new experiments with predicted  outcomes to test those hypotheses ­As more and more evidence is gathered, and more and more hypotheses tested, and  repeatedly confirmed, a more full explanation for the observed phenomenon emerges.  The collection of knowledge is what we call a scientific theory  ­Observation leads to theory explaining a natural phenomenon ­Theory leads to predictions consistent with previous observations ­Predictions of new phenomena are observed. If the observations agree with the  prediction, more predictions can be made. If not, a new theory should be made. 


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