Lectures 3-7 Astronomy 154
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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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